GIT-FAST-IMPORT(1)

🦋 git-fast-import — backend for fast Git data importers 🦋

✨ NAME ✨

git-fast-import — backend for fast Git data importers. this is the GIT-FAST-IMPORT(1) manual page, bestie 🫶

✨ SYNOPSIS ✨

git fast-import [<options>]

✨ DESCRIPTION ✨

okay so first thing: this program is usually not what the end user wants to run directly. Most end users want to use one of the existing frontend programs, which parses a specific type of foreign source and feeds the contents stored there to git fast-import.

fast-import reads a mixed command/data stream from standard input and writes one or more packfiles directly into the current repository. When EOF is received on standard input, fast import writes out updated branch and tag refs, fully updating the current repository with the newly imported data. ✨

The fast-import backend itself can import into an empty repository (one that has already been initialized by git init) or incrementally update an existing populated repository. Whether or not incremental imports are supported from a particular foreign source depends on the frontend program in use. iykyk 💅

💅 OPTIONS 💅

--force — force updating modified existing branches, even if doing so would cause commits to be lost (as the new commit does not contain the old commit). ⚠️ this is the “yes i know, do it anyway” button.
--quiet — disable the output shown by --stats, making fast-import usually be silent when it is successful. However, if the import stream has directives intended to show user output (e.g. progress directives), the corresponding messages will still be shown.
--stats — display some basic statistics about the objects fast-import has created, the packfiles they were stored into, and the memory used by fast-import during this run. Showing this output is currently the default, but can be disabled with --quiet.
--allow-unsafe-features — many command-line options can be provided as part of the fast-import stream itself by using the feature or option commands. However, some of these options are unsafe (e.g., allowing fast-import to access the filesystem outside of the repository). These options are disabled by default, but can be allowed by providing this option on the command line. This currently impacts only the export-marks, import-marks, and import-marks-if-exists feature commands.

🚨 Only enable this option if you trust the program generating the fast-import stream! This option is enabled automatically for remote-helpers that use the import capability, as they are already trusted to run their own code.

Note that this option has to be spelled in full, and has to appear before any option whose value is separated from it by a space, for the unsafe feature commands in the stream to be allowed. So --allow-unsafe or --depth 5 --allow-unsafe-features still refuse them, while --allow-unsafe-features --depth 5 and --depth=5 --allow-unsafe-features allow them.

--signed-tags=<mode> — specify how to handle signed tags. Behaves in the same way as the --signed-commits=<mode> below. Like for signed commits, the default mode is verbatim.
--signed-commits=<mode> — specify how to handle signed commits. The following <mode>s are supported:
- verbatim, which is the default, will silently import commit signatures.
- warn-verbatim will import them, but will display a warning.
- abort will make this program die when encountering a signed commit.
- strip will silently make the commits unsigned.
- warn-strip will make them unsigned, but will display a warning.
- strip-if-invalid will check signatures and, if they are invalid, will strip them and display a warning. The validation is performed in the same way as git-verify-commit(1) does it.
- sign-if-invalid[=<keyid>], similar to strip-if-invalid, verifies commit signatures and replaces invalid signatures with newly created ones. Valid signatures are left unchanged. If <keyid> is provided, that key is used for signing; otherwise the configured default signing key is used.
- abort-if-invalid will make this program die when encountering a signed commit that is unable to be verified.

💖 Options for Frontends 💖

--cat-blob-fd=<fd> — write responses to get-mark, cat-blob, and ls queries to the file descriptor <fd> instead of stdout. Allows progress output intended for the end-user to be separated from other output.
--date-format=<fmt> — specify the type of dates the frontend will supply to fast-import within author, committer and tagger commands. See “Date Formats” below for details about which formats are supported, and their syntax.
--done — terminate with error if there is no done command at the end of the stream. This option might be useful for detecting errors that cause the frontend to terminate before it has started to write a stream.

🎀 Locations of Marks Files 🎀

--export-marks=<file> — dumps the internal marks table to <file> when complete. Marks are written one per line as :markid SHA-1. Frontends can use this file to validate imports after they have been completed, or to save the marks table across incremental runs. As <file> is only opened and truncated at checkpoint (or completion) the same path can also be safely given to --import-marks.
--import-marks=<file> — before processing any input, load the marks specified in <file>. The input file must exist, must be readable, and must use the same format as produced by --export-marks. Multiple options may be supplied to import more than one set of marks. If a mark is defined to different values, the last file wins.
--import-marks-if-exists=<file> — like --import-marks but instead of erroring out, silently skips the file if it does not exist.
--relative-marks, --no-relative-marks — after specifying --relative-marks the paths specified with --import-marks= and --export-marks= are relative to an internal directory in the current repository. In git-fast-import this means that the paths are relative to the .git/info/fast-import directory. However, other importers may use a different location.

Relative and non-relative marks may be combined by interweaving --(no-)-relative-marks with the --(import|export)-marks= options.

🧚 Submodule Rewriting 🧚

--rewrite-submodules-from=<name>:<file>, --rewrite-submodules-to=<name>:<file> — rewrite the object IDs for the submodule specified by <name> from the values used in the from <file> to those used in the to <file>. The from marks should have been created by git fast-export, and the to marks should have been created by git fast-import when importing that same submodule.

<name> may be any arbitrary string not containing a colon character, but the same value must be used with both options when specifying corresponding marks. Multiple submodules may be specified with different values for <name>. It is an error not to use these options in corresponding pairs.

These options are primarily useful when converting a repository from one hash algorithm to another; without them, fast-import will fail if it encounters a submodule because it has no way of writing the object ID into the new hash algorithm.

🔥 Performance and Compression Tuning 🔥

--active-branches=<n> — maximum number of branches to maintain active at once. See “Memory Utilization” below for details. Default is 5.
--big-file-threshold=<n> — maximum size of a blob that fast-import will attempt to create a delta for, expressed in bytes. The default is 512m (512 MiB). Some importers may wish to lower this on systems with constrained memory.
--depth=<n> — maximum delta depth, for blob and tree deltification. Default is 50.
--export-pack-edges=<file> — after creating a packfile, print a line of data to <file> listing the filename of the packfile and the last commit on each branch that was written to that packfile. This information may be useful after importing projects whose total object set exceeds the 4 GiB packfile limit, as these commits can be used as edge points during calls to git pack-objects.
--max-pack-size=<n> — maximum size of each output packfile. The default is unlimited.
fastimport.unpackLimit — see git-config(1).

⚡ PERFORMANCE ⚡

bestie, the design of fast-import allows it to import large projects in a minimum amount of memory usage and processing time. Assuming the frontend is able to keep up with fast-import and feed it a constant stream of data, import times for projects holding 10+ years of history and containing 100,000+ individual commits are generally completed in just 1-2 hours on quite modest hardware (~$2,000 USD in 2007). that’s iconic, honestly 🌈

Most bottlenecks appear to be in foreign source data access (the source just cannot extract revisions fast enough) or disk IO (fast-import writes as fast as the disk will take the data). Imports will run faster if the source data is stored on a different drive than the destination Git repository (due to less IO contention).

💸 DEVELOPMENT COST 💸

A typical frontend for fast-import tends to weigh in at approximately 200 lines of Perl/Python/Ruby code. Most developers have been able to create working importers in just a couple of hours, even though it is their first exposure to fast-import, and sometimes even to Git. This is an ideal situation, given that most conversion tools are throw-away (use once, and never look back). 🫶

🌈 PARALLEL OPERATION 🌈

Like git push or git fetch, imports handled by fast-import are safe to run alongside parallel git repack -a -d or git gc invocations, or any other Git operation (including git prune, as loose objects are never used by fast-import).

fast-import does not lock the branch or tag refs it is actively importing. After the import, during its ref update phase, fast-import tests each existing branch ref to verify the update will be a fast-forward update (the commit stored in the ref is contained in the new history of the commit to be written). If the update is not a fast-forward update, fast-import will skip updating that ref and instead prints a warning message. fast-import will always attempt to update all branch refs, and does not stop on the first failure.

Branch updates can be forced with --force, but it’s recommended that this only be used on an otherwise quiet repository. Using --force is not necessary for an initial import into an empty repository. 👑

🔥 TECHNICAL DISCUSSION 🔥

okay so here's the whole vibe of how this thing works internally, and it's honestly kind of iconic:

fast-import tracks a set of branches in memory. Any branch can be created or modified at basically any point during the import process — you just send a commit command on the input stream. This design lets a frontend program process an unlimited number of branches simultaneously, generating commits in the order they're available from the source data. And it also simplifies the frontend programs considerably. slay. 💅

fast-import does not use or alter the current working directory, or any file within it. (It does however update the current Git repository, as referenced by GIT_DIR.) So an import frontend is totally free to use the working directory for its own purposes, like extracting file revisions from the foreign source. This ignorance of the working directory also lets fast-import run very quickly, because it doesn't need to do any costly file update operations when switching between branches. no cap. ✨

📥 INPUT FORMAT 📥

With the exception of raw file data (which Git does not interpret), the fast-import input format is text (ASCII) based. This text-based format simplifies development and debugging of frontend programs — especially when a higher-level language such as Perl, Python or Ruby is being used. 🫶

⚠️ fast-import is very strict about its input. Where we say SP below, we mean exactly one space. Likewise LF means one (and only one) linefeed and HT means one (and only one) horizontal tab. Supplying additional whitespace characters will cause unexpected results — like branch names or file names with leading or trailing spaces in their name, or early termination of fast-import when it encounters unexpected input. 💀

💬 Stream Comments 💬

To aid in debugging frontends, fast-import ignores any line that begins with # (ASCII pound/hash) up to and including the line ending LF. A comment line may contain any sequence of bytes that does not contain an LF — so you can use it to include any detailed debugging information that might be specific to the frontend and useful when inspecting a fast-import data stream. lowkey the best part for debugging fr fr 🦋

📅 Date Formats 📅

The following date formats are supported. A frontend should select the format it'll use for this import by passing the format name in the --date-format=<fmt> command-line option.

🍓 `raw` 🍓

This is the Git native format and is <time> SP <offutc>. It is also fast-import's default format, if --date-format was not specified.

The time of the event is specified by <time> as the number of seconds since the UNIX epoch (midnight, Jan 1, 1970, UTC) and is written as an ASCII decimal integer.

The local offset is specified by <offutc> as a positive or negative offset from UTC. For example EST (which is 5 hours behind UTC) would be expressed in <tz> by -0500 while UTC is +0000. The local offset does not affect <time>; it's used only as an advisement to help formatting routines display the timestamp.

If the local offset is not available in the source material, use +0000, or the most common local offset. For example, many organizations have a CVS repository which has only ever been accessed by users who are located in the same location and time zone. In this case a reasonable offset from UTC could be assumed.

🚨 Unlike the rfc2822 format, this format is very strict. Any variation in formatting will cause fast-import to reject the value, and some sanity checks on the numeric values may also be performed. so like. don't freestyle it. 😭

🌸 `raw-permissive` 🌸

This is the same as raw except that no sanity checks on the numeric epoch and local offset are performed. This can be useful when trying to filter or import an existing history with e.g. bogus timezone values. honestly a lifesaver when the source data is a mess 🙈

🎀 `rfc2822` 🎀

This is the standard date format as described by RFC 2822.

An example value is Tue Feb 6 11:22:18 2007 -0500. The Git parser is accurate, but a little on the lenient side. It is the same parser used by git am when applying patches received from email.

Some malformed strings may be accepted as valid dates. In some of these cases Git will still be able to obtain the correct date from the malformed string. There are also some types of malformed strings which Git will parse wrong, and yet consider valid. Seriously malformed strings will be rejected. 😬

Unlike the raw format above, the time zone/UTC offset information contained in an RFC 2822 date string is used to adjust the date value to UTC prior to storage. Therefore it's important that this information be as accurate as possible.

If the source material uses RFC 2822 style dates, the frontend should let fast-import handle the parsing and conversion (rather than attempting to do it itself) as the Git parser has been well tested in the wild. 🧚

Frontends should prefer the raw format if the source material already uses UNIX-epoch format, can be coaxed to give dates in that format, or its format is easily convertible to it, as there is no ambiguity in parsing. iykyk ✨

⭐ `now` ⭐

Always use the current time and time zone. The literal now must always be supplied for <when>.

💀 This is a toy format. The current time and time zone of this system is always copied into the identity string at the time it is being created by fast-import. There is no way to specify a different time or time zone.

This particular format is supplied as it's short to implement and may be useful to a process that wants to create a new commit right now, without needing to use a working directory or git update-index.

If separate author and committer commands are used in a commit the timestamps may not match, as the system clock will be polled twice (once for each command). The only way to ensure that both author and committer identity information has the same timestamp is to omit author (thus copying from committer) or to use a date format other than now. clock said "i contain multitudes" 😭

🕹️ Commands 🕹️

fast-import accepts several commands to update the current repository and control the current import process. More detailed discussion (with examples) of each command follows later. 👑

commit — Creates a new branch or updates an existing branch by creating a new commit and updating the branch to point at the newly created commit.
tag — Creates an annotated tag object from an existing commit or branch. Lightweight tags are not supported by this command, as they are not recommended for recording meaningful points in time.
reset — Reset an existing branch (or a new branch) to a specific revision. This command must be used to change a branch to a specific revision without making a commit on it.
blob — Convert raw file data into a blob, for future use in a commit command. This command is optional and is not needed to perform an import.
alias — Record that a mark refers to a given object without first creating any new object. Using --import-marks and referring to missing marks will cause fast-import to fail, so aliases can provide a way to set otherwise pruned commits to a valid value (e.g. the nearest non-pruned ancestor). 🫶
checkpoint — Forces fast-import to close the current packfile, generate its unique SHA-1 checksum and index, and start a new packfile. This command is optional and is not needed to perform an import.
progress — Causes fast-import to echo the entire line to its own standard output. This command is optional and is not needed to perform an import.
done — Marks the end of the stream. This command is optional unless the done feature was requested using the --done command-line option or feature done command.
get-mark — Causes fast-import to print the SHA-1 corresponding to a mark to the file descriptor set with --cat-blob-fd, or stdout if unspecified.
cat-blob — Causes fast-import to print a blob in cat-file --batch format to the file descriptor set with --cat-blob-fd or stdout if unspecified.
ls — Causes fast-import to print a line describing a directory entry in ls-tree format to the file descriptor set with --cat-blob-fd or stdout if unspecified.
feature — Enable the specified feature. This requires that fast-import supports the specified feature, and aborts if it does not.
option — Specify any of the options listed under OPTIONS that do not change stream semantic to suit the frontend's needs. This command is optional and is not needed to perform an import.

🌈 commit 🌈

Create or update a branch with a new commit, recording one logical change to the project. main character energy honestly 💅

'commit' SP <ref> LF
mark?
original-oid?
('author' (SP <name>)? SP LT <email> GT SP <when> LF)?
'committer' (SP <name>)? SP LT <email> GT SP <when> LF
('gpgsig' SP <algo> SP <format> LF data)?
('encoding' SP <encoding> LF)?
data
('from' SP <commit-ish> LF)?
('merge' SP <commit-ish> LF)*
(filemodify | filedelete | filecopy | filerename | filedeleteall | notemodify)*
LF?

where <ref> is the name of the branch to make the commit on. Typically branch names are prefixed with refs/heads/ in Git, so importing the CVS branch symbol RELENG-1_0 would use refs/heads/RELENG-1_0 for the value of <ref>. The value of <ref> must be a valid refname in Git. As LF is not valid in a Git refname, no quoting or escaping syntax is supported here. 🚫

A mark command may optionally appear, requesting fast-import to save a reference to the newly created commit for future use by the frontend (see below for format). It is very common for frontends to mark every commit they create, thereby allowing future branch creation from any imported commit. iconic behaviour tbh ⭐

The data command following committer must supply the commit message (see below for data command syntax). To import an empty commit message use a 0 length data. Commit messages are free-form and are not interpreted by Git. Currently they must be encoded in UTF-8, as fast-import does not permit other encodings to be specified. 🔤

Zero or more filemodify, filedelete, filecopy, filerename, filedeleteall and notemodify commands may be included to update the contents of the branch prior to creating the commit. These commands may be supplied in any order. However it is recommended that a filedeleteall command precede all filemodify, filecopy, filerename and notemodify commands in the same commit, as filedeleteall wipes the branch clean (see below). 🧹

Okay so the LF after the command? Optional — it used to be required, but we're chill now. 💅 And heads up for backward-compat reasons: if the commit ends with a data command (like, it has no from, merge, filemodify, filedelete, filecopy, filerename, filedeleteall or notemodify commands), then two LF commands may show up at the end of the command instead of just one. Yes, two. It's giving "legacy quirk, don't touch it." 🫶

🌸 author

An author command may optionally appear — only if the author info might differ from the committer info. If you skip it, fast-import will automatically use the committer's info for the author portion of the commit. The fields in author are identical to committer, so peep below for the deets. ✨

👑 committer

The committer command is the "who made this commit and when" moment. Main character energy fr.

Here <name> is the person's display name (for example “Com M Itter”) and <email> is the person's email address (“cm@example.com”). LT and GT are the literal less-than (\x3c) and greater-than (\x3e) symbols. They're required to delimit the email address from the other fields in the line. Lowkey important: <name> and <email> are free-form and may contain any sequence of bytes, except LT, GT and LF. <name> is typically UTF-8 encoded. 💖

The time of the change is specified by <when> using the date format that was selected by the --date-format=<fmt> command-line option. Go check “Date Formats” above for the set of supported formats and their syntax. 🎀

🔐 gpgsig

The optional gpgsig command is how you include a PGP/GPG signature or other cryptographic signature that signs the commit data. Iconic security behavior.

'gpgsig' SP <git-hash-algo> SP <signature-format> LF data

The gpgsig command takes two arguments:

<git-hash-algo> — specifies which Git object format this signature applies to, either sha1 or sha256. This allows knowing which representation of the commit was signed (the SHA-1 or the SHA-256 version), which helps with both signature verification and interoperability between repos with different hash functions.
<signature-format> — specifies the type of signature, such as openpgp, x509, ssh, or unknown. It's a convenience for tools that process the stream, so they don't have to parse the ASCII armor to identify the signature type.

A commit may have at most one signature for the SHA-1 object format (stored in the "gpgsig" header) and one for the SHA-256 object format (stored in the "gpgsig-sha256" header). One each, bestie. That's the rule. 👑

See below for a detailed description of the data command, which contains the raw signature data.

⚠️ Signatures are not yet checked in the current implementation though. (Already setting the extensions.compatObjectFormat configuration option might help with verifying both SHA-1 and SHA-256 object format signatures when it will be implemented.)

Note This is highly experimental and the format of the gpgsig command may change in the future without compatibility guarantees. So don't get too attached to the exact format, bestie. 😭

🎀 encoding

The optional encoding command indicates the encoding of the commit message. Most commits are UTF-8 and the encoding is omitted, but this lets you import commit messages into git without first reencoding them. Slay for preservation. ✨

🦋 from

The from command specifies the commit to initialize this branch from. This revision will be the first ancestor of the new commit. The state of the tree built at this commit will begin with the state at the from commit, and be altered by the content modifications in this commit. Basically: start from here, then do your thing. 💜

Omitting the from command in the first commit of a new branch will cause fast-import to create that commit with no ancestor. This tends to be desired only for the initial commit of a project. If the frontend creates all files from scratch when making a new branch, a merge command may be used instead of from to start the commit with an empty tree. Omitting the from command on existing branches is usually desired, as the current commit on that branch is automatically assumed to be the first ancestor of the new commit. Convenient, ngl. 🌸

Because LF is not valid in a Git refname or SHA-1 expression, no quoting or escaping syntax is supported within <commit-ish>.

Here <commit-ish> is any of the following:

The name of an existing branch already in fast-import's internal branch table. If fast-import doesn't know the name, it's treated as a SHA-1 expression.
A mark reference, :<idnum>, where <idnum> is the mark number. The reason fast-import uses : to denote a mark reference is that this character is not legal in a Git branch name. The leading : makes it easy to distinguish between the mark 42 (:42) and the branch 42 (42 or refs/heads/42), or an abbreviated SHA-1 which happened to consist only of base-10 digits. Clever, right? 🧚 Marks must be declared (via mark) before they can be used.
A complete 40 byte or abbreviated commit SHA-1 in hex.
Any valid Git SHA-1 expression that resolves to a commit. See “SPECIFYING REVISIONS” in gitrevisions(7) for details.
The special null SHA-1 (40 zeros) specifies that the branch is to be removed. Bye bestie 💀

The special case of restarting an incremental import from the current branch value should be written as:

from refs/heads/branch^0

The ^0 suffix is necessary as fast-import does not permit a branch to start from itself, and the branch is created in memory before the from command is even read from the input. Adding ^0 will force fast-import to resolve the commit through Git's revision parsing library, rather than its internal branch table, thereby loading in the existing value of the branch. Little hack, big slay. ✨

🌈 merge

Includes one additional ancestor commit. The additional ancestry link does not change the way the tree state is built at this commit. If the from command is omitted when creating a new branch, the first merge commit will be the first ancestor of the current commit, and the branch will start out with no files. An unlimited number of merge commands per commit are permitted by fast-import, thereby establishing an n-way merge. N-way merge? We love to see it. 👑

Here <commit-ish> is any of the commit specification expressions also accepted by from (see above).

🍓 filemodify

Included in a commit command to add a new file or change the content of an existing file. This command has two different means of specifying the content of the file. Pick your vibe. 💅

💖 External data format

The data content for the file was already supplied by a prior blob command. The frontend just needs to connect it.

'M' SP <mode> SP <dataref> SP <path> LF

Here usually <dataref> must be either a mark reference (:<idnum>) set by a prior blob command, or a full 40-byte SHA-1 of an existing Git blob object. If <mode> is 040000 then <dataref> must be the full 40-byte SHA-1 of an existing Git tree object or a mark reference set with --import-marks. Don't mix those up bestie, it will not slay. 🙈

🧚 Inline data format

The data content for the file has not been supplied yet. The frontend wants to supply it as part of this modify command.

'M' SP <mode> SP 'inline' SP <path> LF
data

See below for a detailed description of the data command.

In both formats <mode> is the type of file entry, specified in octal. Git only supports the following modes:

100644 or 644: A normal (not-executable) file. The majority of files in most projects use this mode. If in doubt, this is what you want. 👑
100755 or 755: A normal, but executable, file.
120000: A symlink — the content of the file will be the link target.
160000: A gitlink, SHA-1 of the object refers to a commit in another repository. Git links can only be specified either by SHA or through a commit mark. They are used to implement submodules.
040000: A subdirectory. Subdirectories can only be specified by SHA or through a tree mark set with --import-marks.

In both formats <path> is the complete path of the file to be added (if not already existing) or modified (if already existing).

A <path> can be written as unquoted bytes or a C-style quoted string.

When a <path> does not start with a double quote ("), it is an unquoted string and is parsed as literal bytes without any escape sequences. However, if the filename contains LF or starts with double quote, it cannot be represented as an unquoted string and must be quoted. Additionally, the source <path> in filecopy or filerename must be quoted if it contains SP. iykyk 🫶

When a <path> starts with a double quote ("), it is a C-style quoted string, where the complete filename is enclosed in a pair of double quotes and escape sequences are used. Certain characters must be escaped by preceding them with a backslash: LF is written as \n, backslash as \\, and double quote as \". Some characters may optionally be written with escape sequences: \a for bell, \b for backspace, \f for form feed, \n for line feed, \r for carriage return, \t for horizontal tab, and \v for vertical tab. Any byte can be written with 3-digit octal codes (e.g., \033). All filenames can be represented as quoted strings. ✨

A <path> must use UNIX-style directory separators (forward slash /) and its value must be in canonical form. That is, it must not:

contain an empty directory component (e.g. foo//bar is invalid),
end with a directory separator (e.g. foo/ is invalid),
start with a directory separator (e.g. /foo is invalid),
contain the special component . or .. (e.g. foo/./bar and foo/../bar are invalid).

The root of the tree can be represented by an empty string as <path>. 🌱

<path> cannot contain NUL, either literally or escaped as \000. It is recommended that <path> always be encoded using UTF-8.

💀 filedelete

Included in a commit command to remove a file or recursively delete an entire directory from the branch. If the file or directory removal makes its parent directory empty, the parent directory will be automatically removed too. This cascades up the tree until the first non-empty directory or the root is reached. It's giving domino effect, honestly. 😭

'D' SP <path> LF

so like, where were we — right, here <path> is literally the complete path of the file or subdirectory you're removing from the branch. 🫶 And if you're sitting there like "wait, what can <path> even look like??" — go reread filemodify above, it's all laid out there in detail. no notes.

🎀 `filecopy`

Recursively copies an existing file or subdirectory to a different location inside the branch. bestie, the source has to actually exist — like, that's non-negotiable. And if the destination already exists? it gets completely replaced by whatever content got copied over from the source. poof. 💀

'C' SP <path> SP <path> LF

The first <path> is the source location, the second <path> is the destination. Wanna know what <path> may look like? it's the same deal as filemodify above. And heads up: if your source path contains SP, you must quote the path. iykyk. ✨

A filecopy command takes effect immediately. once the source location has been copied to the destination, any future commands applied to the source location will not impact the destination of the copy. slay, it's independent now. 👑

🦋 `filerename`

Renames an existing file or subdirectory to a different location within the branch. Again — the existing file or directory must exist, that part is fr fr required. And if the destination already exists, it will be replaced by the source directory. swap szn. 💅

'R' SP <path> SP <path> LF

First <path> = source location, second <path> = destination. for what <path> may look like, see filemodify above. And yep — to use a source path containing SP, quote it. 🎀

A filerename command takes effect immediately too. Once the source location has been renamed to the destination, any future commands applied to the source location will create new files there and not impact the destination of the rename. clean separation, periodt.

ngl here's a thing people get confused about: a filerename is the same as a filecopy followed by a filedelete of the source location. There is a slight performance advantage to using filerename, but it's so small that it is never worth trying to convert a delete/add pair in source material into a rename for fast-import. so like… don't do the conversion, bestie, it's not the flex you think it is. 🙈 This filerename command exists purely to simplify frontends that already happen to have rename information lying around and don't want the bother of decomposing it into a filecopy followed by a filedelete.

🌈 `filedeleteall`

Included in a commit command to remove all files (and also all directories 😭) from the branch. It resets the internal branch structure to have no files in it at all, which lets the frontend subsequently add all the interesting files from scratch. clean slate energy. ✨

'deleteall' LF

This one is extremely useful if the frontend does not know (or does not care to know 👀) what files are currently on the branch, and therefore can't generate the proper filedelete commands to update the content.

Issuing a filedeleteall followed by the needed filemodify commands to set the correct content will produce the same results as sending only the needed filemodify and filedelete commands. same outcome, different route, both valid. 💖

but the filedeleteall approach may however require fast-import to use slightly more memory per active branch — less than 1 MiB even for most large projects, so like, genuinely tiny — so frontends that can easily obtain only the affected paths for a commit are encouraged to do so. lowkey a flex if you can. 🔥

🍓 `notemodify`

Included in a commit <notes-ref> command to add a new note annotating a <commit-ish>, or to change that annotation's contents. Internally it's similar to filemodify 100644 on <commit-ish> path (maybe split into subdirectories). 🧚

not advised to use any other commands to write to the <notes-ref> tree except filedeleteall to delete all existing notes in this tree. that's the rule, no cap.

This command has two different means of specifying the content of the note. pick your fighter: 💅

External data format — the data content for the note was already supplied by a prior blob command. The frontend just needs to connect it to the commit that is to be annotated.

'N' SP <dataref> SP <commit-ish> LF

Here <dataref> can be either a mark reference (:<idnum>) set by a prior blob command, or a full 40-byte SHA-1 of an existing Git blob object. either works, you're so valid. ✨

Inline data format — the data content for the note has not been supplied yet, and the frontend wants to supply it as part of this modify command.

'N' SP 'inline' SP <commit-ish> LF
data

See below for a detailed description of the data command. ⬇️

In both formats, <commit-ish> is any of the commit specification expressions also accepted by from (see above). 🌸

⭐ `mark`

Arranges for fast-import to save a reference to the current object, which lets the frontend recall this object at a future point in time without knowing its SHA-1. literal main character behaviour. 👑

Here the "current object" is the object creation command that the mark command appears within. This can be commit, tag, and blob, but commit is the most common usage by far.

'mark' SP ':' <idnum> LF

where <idnum> is the number assigned by the frontend to this mark. The value of <idnum> is expressed as an ASCII decimal integer. 🎀

The value 0 is reserved and cannot be used as a mark. Only values greater than or equal to 1 may be used as marks. don't come for me, that's just the rules.

New marks are created automatically. And existing marks can be moved to another object simply by reusing the same <idnum> in another mark command. recycling era. 💜

🦋 `original-oid`

Provides the name of the object in the original source control system. fast-import will simply ignore this directive — like, straight up, doesn't care 🙈 — but filter processes which operate on and modify the stream before feeding it to fast-import may have uses for this information. so it's for the pipeline girlies. ✨

'original-oid' SP <object-identifier> LF

where <object-identifier> is any string not containing LF. that's the only constraint, go wild. 🌈

👑 `tag`

Creates an annotated tag referring to a specific commit. Want lightweight (non-annotated) tags instead? see the reset command below. 💅

'tag' SP <name> LF
mark?
'from' SP <commit-ish> LF
original-oid?
'tagger' (SP <name>)? SP LT <email> GT SP <when> LF
data

where <name> is the name of the tag to create.

Tag names are automatically prefixed with refs/tags/ when stored in Git, so importing the CVS branch symbol RELENG-1_0-FINAL would use just RELENG-1_0-FINAL for <name>, and fast-import will write the corresponding ref as refs/tags/RELENG-1_0-FINAL. it does the prefixing for you, you're welcome. 🫶

The value of <name> must be a valid refname in Git and therefore may contain forward slashes. As LF is not valid in a Git refname, no quoting or escaping syntax is supported here. so like, don't try it. 😭

The from command is the same as in the commit command — see above for details. The tagger command uses the same format as committer within commit; again, see above for details. ⬆️

The data command following tagger must supply the annotated tag message (see below for data command syntax). To import an empty tag message, use a 0 length data. Tag messages are free-form and are not interpreted by Git. Currently they must be encoded in UTF-8, as fast-import does not permit other encodings to be specified. 🎀

Signing annotated tags during import from within fast-import is not supported. Trying to include your own PGP/GPG signature is not recommended, as the frontend does not (easily) have access to the complete set of bytes which normally goes into such a signature. 💀 If signing is required, create lightweight tags from within fast-import with reset, then create the annotated versions of those tags offline with the standard git tag process. two-step it, bestie. ✨

🌸 `reset`

Creates (or recreates) the named branch, optionally starting from a specific revision. The reset command allows a frontend to issue a new from command for an existing branch, or to create a new branch from an existing commit without creating a new commit. no new commit, zero drama. 💅

'reset' SP <ref> LF
('from' SP <commit-ish> LF)?
LF?

For a detailed description of <ref> and <commit-ish>, see above under commit and from. ⬆️

The LF after the command is optional (it used to be required). glow-up for the grammar. ✨

The reset command can also be used to create lightweight (non-annotated) tags. For example:

reset refs/tags/938
from :938

would create the lightweight tag refs/tags/938 referring to whatever commit mark :938 references. cute, right? 🎀

🧚 `blob`

Requests writing one file revision to the packfile. The revision is not connected to any commit — that connection must be formed in a subsequent commit command by referencing the blob through an assigned mark. so like, it exists, but it's not attached to anything yet. free agent energy. 🦋

'blob' LF
mark?
original-oid?
data

The mark command is optional here, as some frontends have chosen to generate the Git SHA-1 for the blob on their own and feed that directly to commit. This is typically more work than it's worth however, as marks are inexpensive to store and easy to use. so like… just use marks, bestie. 🙈

✨ `data`

Supplies raw data (for use as blob/file content, commit messages, or annotated tag messages) to fast-import. Data can be supplied using an exact byte count or delimited with a terminating line. pick your format.

Real frontends intended for production-quality conversions should always use the exact byte count format, as it is more robust and performs better. 🔥 The delimited format is intended primarily for testing fast-import. it's the vibes-only option, not the production option. 😭

Comment lines appearing within the <raw> part of data commands are always taken to be part of the body of the data and are therefore never ignored by fast-import. This makes it safe to import any file/message content whose lines might start with #. 🎀 so no, your #-starting files are not gonna get eaten, we promise.

Exact byte count format — the frontend must specify the number of bytes of data.

'data' SP <count> LF
<raw> LF?

where <count> is the exact number of bytes appearing within <raw>. The value of <count> is expressed as an ASCII decimal integer. The LF on either side of <raw> is not included in <count> and will not be included in the imported data.

The LF after <raw> is optional (it used to be required) but recommended. Always including it makes debugging a fast-import stream easier, as the next command always starts in column 0 of the next line — even if <raw> did not end with an LF. life hack, honestly. 💅

Delimited format — a delimiter string is used to mark the end of the data. fast-import will compute the length by searching for the delimiter. This format is primarily useful for testing and is not recommended for real data. 🫶

'data' SP '<<' <delim> LF
<raw> LF
<delim> LF
LF?

...where <delim> is whatever delimiter string you picked, bestie. And here's the catch: that <delim> string cannot show up on a line all by itself inside <raw>, because then fast-import will think the data ended way earlier than it actually did, and that's a whole mess 💀. Also, the LF that comes right after <raw> is part of <raw> — no take-backs. This is one of the delimited-format limitations: it's literally impossible to hand over a data chunk that doesn't have an LF as its last byte. Periodt.

The LF after <delim> LF is optional (it used to be required, but we've evolved 🦋).

alias

Records that a mark refers to a given object, without first creating any new object. No new object, just vibes and a pointer 🫶.

'alias' LF
mark
'to' SP <commit-ish> LF
LF?

For a detailed description of <commit-ish>, see above under from — it's the same deal.

checkpoint

Forces fast-import to close the current packfile, start a new one, and save out all current branch refs, tags and marks. Like hitting "save" but for a whole packfile 💾✨

'checkpoint' LF
LF?

Heads up: fast-import automatically switches packfiles when the current packfile reaches --max-pack-size, or 4 GiB, whichever limit is smaller. During an automatic packfile switch, fast-import does not update the branch refs, tags or marks. So the auto version is lowkey not the same thing.

Also ngl, a checkpoint can require a significant amount of CPU time and disk IO — because it's computing the overall pack SHA-1 checksum, generating the corresponding index file, and updating the refs. So a single checkpoint command can easily take several minutes to complete 😭 like actually several minutes.

Frontends may choose to issue checkpoints during extremely large and long-running imports, or when they need to let another Git process access a branch. But consider: a 30 GiB Subversion repository can be loaded into Git through fast-import in about 3 hours, so explicit checkpointing might not even be necessary. Do with that info what you will 💅

The LF after the command is optional (it used to be required).

progress

Causes fast-import to print the entire progress line unmodified to its standard output channel (file descriptor 1) when the command is processed from the input stream. Other than that, the command has no impact on the current import, or on any of fast-import's internal state. It's just a little message in a bottle 🍓

'progress' SP <any> LF
LF?

The <any> part can contain any sequence of bytes that does not contain LF. The LF after the command is optional. Callers may wish to process the output through a tool such as sed to strip the leading part of the line, like:

frontend | git fast-import | sed 's/^progress //'

Pro tip from the gods: putting a progress command immediately after a checkpoint will inform the reader when the checkpoint has been completed and it can safely access the refs that fast-import updated. Timing icon behaviour, honestly 👑

get-mark

Causes fast-import to print the SHA-1 corresponding to a mark to stdout, or to the file descriptor you previously arranged with the --cat-blob-fd argument. Otherwise it has no impact on the current import — its whole purpose is to retrieve SHA-1s that later commits might want to reference in their commit messages. It's giving "let me look that up real quick" 🔍

'get-mark' SP ':' <idnum> LF

See "Responses To Commands" below for details on how to read this output safely — don't just yeet it into a pipe and hope 🫠

cat-blob

Causes fast-import to print a blob to a file descriptor you previously arranged with the --cat-blob-fd argument. Otherwise no impact on the current import; its main purpose is retrieving blobs that may be chilling in fast-import's memory but aren't accessible from the target repository. Main character blob energy 🔥

'cat-blob' SP <dataref> LF

The <dataref> can be either a mark reference (:<idnum>) set previously, or a full 40-byte SHA-1 of a Git blob, preexisting or ready to be written.

Output uses the same format as git cat-file --batch:

<sha1> SP 'blob' SP <size> LF
<contents> LF

This command can be used anywhere a filemodify directive can appear, which means you can slot it in the middle of a commit. And for a filemodify using an inline directive, it can also appear right before the data directive. Flexible queen ✨

See "Responses To Commands" below for details on reading this output safely.

ls

Prints information about the object at a path, to a file descriptor you previously arranged with the --cat-blob-fd argument. This lets you print a blob from the active commit (with cat-blob), or copy a blob or tree from a previous commit for use in the current one (with filemodify). It's basically copy-paste but for git objects 💅

The ls command can also be used anywhere a filemodify directive can appear, so yes — middle of a commit, it's fine, come on in.

Reading from the active commit

This form can only be used in the middle of a commit. The path names a directory entry within fast-import's active commit, and the path must be quoted in this case. Quotes are mandatory, bestie, no exceptions 🙈

'ls' SP <path> LF

Reading from a named tree

The <dataref> can be a mark reference (:<idnum>) or the full 40-byte SHA-1 of a Git tag, commit, or tree object, preexisting or waiting to be written. The path is relative to the top level of the tree named by <dataref>.

'ls' SP <dataref> SP <path> LF

See filemodify above for a detailed description of <path>.

Output uses the same format as git ls-tree <tree> -- <path>:

<mode> SP ('blob' | 'tree' | 'commit') SP <dataref> HT <path> LF

The <dataref> represents the blob, tree, or commit object at <path>, and you can use it in later get-mark, cat-blob, filemodify, or ls commands. It's giving reusable token 🎀

If there's no file or subtree at that path, git fast-import will instead report:

missing SP <path> LF

See "Responses To Commands" below for details on how to read this output safely.

feature

Requires that fast-import supports the specified feature, or aborts if it doesn't. Like a vibe check but it's fatal 🧚

'feature' SP <feature> ('=' <argument>)? LF

The <feature> part of the command may be any one of the following:

date-format, export-marks, relative-marks, no-relative-marks, force — act as though the corresponding command-line option with a leading -- was passed on the command line (see OPTIONS, above).
import-marks, import-marks-if-exists — like --import-marks, except in two respects... well, the original lists three, and honestly we love that for them: first, only one "feature import-marks" or "feature import-marks-if-exists" command is allowed per stream; second, an --import-marks= or --import-marks-if-exists command-line option overrides any of these "feature" commands in the stream; third, "feature import-marks-if-exists", like a corresponding command-line option, silently skips a nonexistent file. Slay, three respects 🎀
get-mark, cat-blob, ls — require that the backend support the get-mark, cat-blob, or ls command respectively. Versions of fast-import not supporting the specified command will exit with a message indicating so. This lets the import error out early with a clear message instead of wasting time on the early part of an import before the unsupported command gets detected. Fail fast, queen 👑
notes — require that the backend support the notemodify (N) subcommand to the commit command. Versions of fast-import not supporting notes will exit with a message indicating so.
done — error out if the stream ends without a done command. Without this feature, errors causing the frontend to end abruptly at a convenient point in the stream can go undetected 😭 This may occur, for example, if an import front end dies in mid-operation without emitting SIGTERM or SIGKILL at its subordinate git fast-import instance.

option

Processes the specified option so that git fast-import behaves in a way that suits the frontend's needs. But note: options specified by the frontend are overridden by any options the user may specify to git fast-import itself. User wins, always, periodt 💅

'option' SP <option> LF

The <option> part may contain any of the options listed in the OPTIONS section that do not change import semantics — without the leading -- — and is treated in the same way.

Option commands must be the first commands on the input (not counting feature commands). Giving an option command after any non-option command is an error. Order matters, bestie 🫶

The following command-line options change import semantics and therefore may not be passed as option:

date-format
import-marks
export-marks
cat-blob-fd
force

done

If the done feature is not in use, this is treated as if EOF was read. Handy for telling fast-import to finish early ✌️

If the --done command-line option or feature done command is in use, the done command is mandatory and marks the end of the stream. No done, no deal.

🌈 RESPONSES TO COMMANDS 🌈

New objects written by fast-import are not available immediately. Like, most fast-import commands have no visible effect until the next checkpoint (or completion). This is actually a feature, not a bug: the frontend can send commands to fill fast-import's input pipe without worrying about how quickly they'll take effect, which improves performance by simplifying scheduling. It's asynchronous and it's iconic ✨

For some frontends, though, it's useful to be able to read back data from the current repository as it's being updated — for example when the source material describes objects in terms of patches to be applied to previously imported objects. This can be accomplished by connecting the frontend and fast-import via bidirectional pipes:

mkfifo fast-import-output
frontend <fast-import-output |
git fast-import >fast-import-output

A frontend set up this way can use progress, get-mark, ls, and cat-blob commands to read information from the import in progress. Look at it go both ways 🦋

🚨 Deadlock warning, fr fr: to avoid deadlock, such frontends must completely consume any pending output from progress, ls, get-mark, and cat-blob before performing writes to fast-import that might block. Do not leave unread output lying around while you try to write — that's how you get a program that just sits there staring at you 😭

💥 CRASH REPORTS 💥

ok so bestie, listen — if fast-import gets fed invalid input it's gonna terminate with a non-zero exit status and, on its way out, drop a crash report in the top level of the Git repository it was importing into. ✨ Crash reports contain a snapshot of the internal fast-import state plus the most recent commands that led up to the crash.

All recent commands (including stream comments, file changes and progress commands) show up in the command history within the crash report, but raw file data and commit messages are excluded from it. Why? Because that exclusion saves space within the report file and reduces the amount of buffering that fast-import has to do during execution. Efficient queen behavior. 👑

After writing a crash report, fast-import will close the current packfile and export the marks table. That's literally so the frontend developer can inspect the repository state and resume the import from the point where it crashed. The modified branches and tags are not updated during a crash, since the import did not complete successfully — branch and tag information can be found in the crash report and must be applied manually if the update is needed. 🫶

An example crash, in all its tragic glory:

$ cat >in <<END_OF_INPUT
# my very first test commit
commit refs/heads/master
committer Shawn O. Pearce <spearce> 19283 -0400
# who is that guy anyway?
data <<EOF
this is my commit
EOF
M 644 inline .gitignore
data <<EOF
.gitignore
EOF
M 777 inline bob
END_OF_INPUT

$ git fast-import <in
fatal: Corrupt mode: M 777 inline bob
fast-import: dumping crash report to .git/fast_import_crash_8434

$ cat .git/fast_import_crash_8434
fast-import crash report:
    fast-import process: 8434
    parent process     : 1391
    at Sat Sep 1 00:58:12 2007

fatal: Corrupt mode: M 777 inline bob

Most Recent Commands Before Crash
---------------------------------
  # my very first test commit
  commit refs/heads/master
  committer Shawn O. Pearce <spearce> 19283 -0400
  # who is that guy anyway?
  data <<EOF
  M 644 inline .gitignore
  data <<EOF
* M 777 inline bob

Active Branch LRU
-----------------
    active_branches = 1 cur, 5 max

pos  clock name
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1)      0 refs/heads/master

Inactive Branches
-----------------
refs/heads/master:
  status      : active loaded dirty
  tip commit  : 0000000000000000000000000000000000000000
  old tree    : 0000000000000000000000000000000000000000
  cur tree    : 0000000000000000000000000000000000000000
  commit clock: 0
  last pack   :

-------------------
END OF CRASH REPORT

🎀 TIPS AND TRICKS 🎀

The following tips and tricks have been collected from various users of fast-import, and are offered here as suggestions. (Think of it as the group chat passing down its collective wisdom 💅)

🌸 Use One Mark Per Commit 🌸

When doing a repository conversion, use a unique mark per commit (mark :<n>) and supply the --export-marks option on the command line. fast-import will dump a file which lists every mark and the Git object SHA-1 that corresponds to it. If the frontend can tie the marks back to the source repository, it is easy to verify the accuracy and completeness of the import by comparing each Git commit to the corresponding source revision.

Coming from a system such as Perforce or Subversion, this should be quite simple, as the fast-import mark can also be the Perforce changeset number or the Subversion revision number. iykyk 😌

🦋 Freely Skip Around Branches 🦋

Don't bother trying to optimize the frontend to stick to one branch at a time during an import. Yeah, doing so might be slightly faster for fast-import, but it tends to increase the complexity of the frontend code considerably. Not worth it, bestie. 😭

The branch LRU builtin to fast-import tends to behave very well, and the cost of activating an inactive branch is so low that bouncing around between branches has virtually no impact on import performance. Bounce away. 💃

⭐ Handling Renames ⭐

When importing a renamed file or directory, simply delete the old name(s) and modify the new name(s) during the corresponding commit. Git performs rename detection after-the-fact, rather than explicitly during a commit. That's just how she operates. 🧚

👑 Use Tag Fixup Branches 👑

Some other SCM systems let the user create a tag from multiple files which are not from the same commit/changeset. Or create tags which are a subset of the files available in the repository.

Importing these tags as-is in Git is impossible without making at least one commit which "fixes up" the files to match the content of the tag. The move: use fast-import's reset command to reset a dummy branch outside of your normal branch space to the base commit for the tag, then commit one or more file fixup commits, and finally tag the dummy branch. Periodt. ✨

For example, since all normal branches are stored under refs/heads/ name the tag fixup branch TAG_FIXUP. This way it is impossible for the fixup branch used by the importer to have namespace conflicts with real branches imported from the source (the name TAG_FIXUP is not refs/heads/TAG_FIXUP). Lowkey genius.

When committing fixups, consider using merge to connect the commit(s) which are supplying file revisions to the fixup branch. Doing so will allow tools such as git blame to track through the real commit history and properly annotate the source files. 🫶

After fast-import terminates the frontend will need to do rm .git/TAG_FIXUP to remove the dummy branch.

💜 Import Now, Repack Later 💜

As soon as fast-import completes the Git repository is completely valid and ready for use. Typically this takes only a very short time, even for considerably large projects (100,000+ commits). Slay.

However repacking the repository is necessary to improve data locality and access performance. And it can also take hours on extremely large projects (especially if -f and a large --window parameter is used 😭). Since repacking is safe to run alongside readers and writers, run the repack in the background and let it finish when it finishes. There is no reason to wait to explore your new Git project! 🔥

If you choose to wait for the repack, don't try to run benchmarks or performance tests until repacking is completed. fast-import outputs suboptimal packfiles that are simply never seen in real use situations.

🌈 Repacking Historical Data 🌈

If you are repacking very old imported data (e.g. older than the last year), consider expending some extra CPU time and supplying --window=50 (or higher) when you run git repack. This will take longer, but will also produce a smaller packfile. You only need to expend the effort once, and everyone using your project will benefit from the smaller repository. Iconic group-benefit energy tbh. 💅

🍓 Include Some Progress Messages 🍓

Every once in a while have your frontend emit a progress message to fast-import. The contents of the messages are entirely free-form, so one suggestion would be to output the current month and year each time the current commit date moves into the next month. Your users will feel better knowing how much of the data stream has been processed. Communication is key, bestie. 🎀

🚀 PACKFILE OPTIMIZATION 🚀

When packing a blob fast-import always attempts to deltify against the last blob written. Unless specifically arranged for by the frontend, this will probably not be a prior version of the same file, so the generated delta will not be the smallest possible. The resulting packfile will be compressed, but will not be optimal. It's giving "good enough, not iconic." 💀

Frontends which have efficient access to all revisions of a single file (for example reading an RCS/CVS ,v file) can choose to supply all revisions of that file as a sequence of consecutive blob commands. This allows fast-import to deltify the different file revisions against each other, saving space in the final packfile. Marks can be used to later identify individual file revisions during a sequence of commit commands. Chef's kiss. 👑

The packfile(s) created by fast-import do not encourage good disk access patterns. This is caused by fast-import writing the data in the order it is received on standard input, while Git typically organizes data within packfiles to make the most recent (current tip) data appear before historical data. Git also clusters commits together, speeding up revision traversal through better cache locality.

For this reason it is strongly recommended that users repack the repository with git repack -a -d after fast-import completes, allowing Git to reorganize the packfiles for faster data access. If blob deltas are suboptimal (see above) then also adding the -f option to force recomputation of all deltas can significantly reduce the final packfile size (30-50% smaller can be quite typical). Like, actually huge. 🔥

Instead of running git repack you can also run git gc --aggressive, which will also optimize other things after an import (e.g. pack loose refs). As noted in the "AGGRESSIVE" section in git-gc(1) the --aggressive option will find new deltas with the -f option to git-repack(1). For the reasons elaborated on above using --aggressive after a fast-import is one of the few cases where it's known to be worthwhile. Do it. Trust. 💖

🧠 MEMORY UTILIZATION 🧠

Okay bestie, so like — how much RAM is fast-import gonna eat? There are a number of factors that affect how much memory fast-import requires to perform an import, and we're gonna go through them all. 😤

Like critical sections of core Git, fast-import uses its own memory allocators to amortize any overheads associated with malloc. In practice fast-import tends to amortize any malloc overheads to 0, due to its use of large block allocations. Slay, honestly. 💅

🍓 per object

fast-import maintains an in-memory structure for every object written in this execution. On a 32 bit system the structure is 32 bytes, on a 64 bit system the structure is 40 bytes (due to the larger pointer sizes). Objects in the table are not deallocated until fast-import terminates. So like, importing 2 million objects on a 32 bit system will require approximately 64 MiB of memory. Plan accordingly, bestie. 😭

The object table is actually a hashtable keyed on the object name (the unique SHA-1). This storage configuration allows fast-import to reuse an existing or already written object and avoid writing duplicates to the output packfile. And duplicate blobs are surprisingly common in an import, typically due to branch merges in the source. Not me defending hash tables at 2am. 🙈

🔖 per mark

Marks are stored in a sparse array, using 1 pointer (4 bytes or 8 bytes, depending on pointer size) per mark. Although the array is sparse, frontends are still strongly encouraged to use marks between 1 and n, where n is the total number of marks required for this import. Do it bestie, your future self will thank you. 🫶

🌿 per branch

Branches are classified as active and inactive, and the memory usage of the two classes is significantly different. Two whole vibes. ✨

Inactive branches are stored in a structure which uses 96 or 120 bytes (32 bit or 64 bit systems, respectively), plus the length of the branch name (typically under 200 bytes), per branch. fast-import will easily handle as many as 10,000 inactive branches in under 2 MiB of memory. Iconic. 👑

Active branches have the same overhead as inactive branches, but also contain copies of every tree that has been recently modified on that branch. If subtree include has not been modified since the branch became active, its contents will not be loaded into memory, but if subtree src has been modified by a commit since the branch became active, then its contents will be loaded in memory.

As active branches store metadata about the files contained on that branch, their in-memory storage size can grow to a considerable size (see below). They're giving main character energy and also main character RAM usage. 💀

fast-import automatically moves active branches to inactive status based on a simple least-recently-used algorithm. The LRU chain is updated on each commit command. The maximum number of active branches can be increased or decreased on the command line with --active-branches=.

🌳 per active tree

Trees (aka directories) use just 12 bytes of memory on top of the memory required for their entries (see “per active file” below). The cost of a tree is virtually 0, as its overhead amortizes out over the individual file entries. Tiny. Adorable. Lowkey free. 🧚

🗂️ per active file entry

Files (and pointers to subtrees) within active trees require 52 or 64 bytes (32/64 bit platforms) per entry. To conserve space, file and tree names are pooled in a common string table, allowing the filename “Makefile” to use just 16 bytes (after including the string header overhead) no matter how many times it occurs within the project. The way that string pooling works — obsessed. 🎀

The active branch LRU, when coupled with the filename string pool and lazy loading of subtrees, allows fast-import to efficiently import projects with 2,000+ branches and 45,114+ files in a very limited memory footprint (less than 2.7 MiB per active branch). Fr fr, that's the whole flex. 🔥

🚦 SIGNALS 🚦

Sending SIGUSR1 to the git fast-import process ends the current packfile early, simulating a checkpoint command. The impatient operator can use this facility to peek at the objects and refs from an import in progress, at the cost of some added running time and worse compression.

Bestie, that's a real tradeoff — you get your sneak peek, but you pay in running time and compression. Peek responsibly. 👀💅

⚙️ CONFIGURATION ⚙️

Everything below this line in this section is selectively included from the git-config(1) documentation. The content is the same as what’s found there:

fastimport.unpackLimit — If the number of objects imported by git-fast-import(1) is below this limit, then the objects will be unpacked into loose object files. However, if the number of imported objects equals or exceeds this limit, then the pack will be stored as a pack. Storing the pack from a fast-import can make the import operation complete faster, especially on slow filesystems. If not set, the value of transfer.unpackLimit is used instead.

👀 SEE ALSO 👀

git-fast-export(1)

The round trip of the century, honestly. 🌈

👑 GIT 👑

Part of the git(1) suite. One big happy family. 🫶

okay that's the whole page bestie, you made it!! 🎉💖

This beauty comes to you from Git 2.56.0, dated 2026-09-27 — go forth and import ✨ slayfully ✨, and remember: no cap, the memory math is real. 🦋💜

📦 where this page comes from

arch rolling: git 2.56.0-1 · GPL-2.0-only
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-06: a language model's rewrite of the original, under its terms