ZDB(8)

📦
version 2.4.4 · 10 versions
, in alpine 3.23, alpine 3.24, nixos 26.05, nixpkgs unstable, ubuntu resolute, ubuntu stonking

zdb(8) — display ZFS storage pool debugging and consistency information 💖

okay bestie, buckle UP, because we're about to read the entire on-disk soul of your ZFS pool together and it's giving forensics. 🕵️♀️✨

✨ NAME ✨

zdb — display ZFS storage pool debugging and consistency information 🫶

🌸 SYNOPSIS 🌸

zdb [-AbcdDFGhikLMNPsTvXYy] [-e [-V] [-p path]…] [-I inflight-I/O-ops]
    [-o var=value]… [-t txg] [-U cache] [-x dumpdir] [-K key]
    [poolname[/dataset|objset-ID]] [object|range…]
zdb [-AdiPv] [-e [-V] [-p path]…] [-U cache] [-K key]
    poolname[/dataset|objset-ID] [object|range…]
zdb -B [-e [-V] [-p path]…] [-U cache] [-K key] poolname/objset-ID
    [backup-flags]
zdb -C [-A] [-U cache] [poolname]
zdb -E [-A] word0:word1:…:word15
zdb -l [-Aqu] device
zdb -m [-AFLPXY] [-e [-V] [-p path]…] [-t txg] [-U cache] poolname [vdev
    [metaslab]…]
zdb --allocated-map [-mAFLPXY] [-e [-V] [-p path]…] [-t txg] [-U cache]
    poolname [vdev [metaslab]…]
zdb -O [-K key] dataset path
zdb -r [-K key] dataset path destination
zdb -r -O [-K key] dataset object-id destination
zdb -R [-A] [-e [-V] [-p path]…] [-U cache] poolname
    vdev:offset:[lsize/]psize[:flags]
zdb -S [-AP] [-e [-V] [-p path]…] [-U cache] poolname

💅 DESCRIPTION 💅

okay so the tea is: zdb shows you info about a ZFS pool that's super useful for debugging, and it also does a lil bit of consistency checking while it's at it. ✨ But listen — this is not a general purpose tool, and the options (and the facilities) can literally change on you. It is not a fsck(8) utility. periodt. 💀

The output of this command, generally speaking, reflects the on-disk structure of a ZFS pool, and it is inherently unstable. The precise output of most invocations is not documented, and a knowledge of ZFS internals is assumed. iykyk. 🧠

If the dataset argument does not contain any / or @ characters, it gets interpreted as a pool name. The root dataset can be specified as "pool/". cute, right? 🎀

zdb is an "offline" tool — it accesses the block devices underneath the pools directly from userspace and does not care if the pool is imported or datasets are mounted (or even if the system understands ZFS at all 😭). When operating on an imported and active pool it is possible, though unlikely, that zdb may interpret inconsistent pool data and behave erratically. so like, vibe check your expectations first. 🌈

fr fr: it is not a fsck(8) utility, and the output format is unstable. do not build your whole personality around parsing it.

👑 OPTIONS 👑

🌟 Display options 🌟

  • --allocated-map — Prints out a list of all the allocated regions in the pool. Primarily intended for use with the zhack metaslab leak subcommand. 💖
  • -b, --block-stats — Display statistics regarding the number, size (logical, physical and allocated) and deduplication of blocks. numbers go brrr. 📊
  • --bin=(lsize|psize|asize) — When used with -bb, sort blocks into all three bins according to the given size (instead of binning a block for each size separately).

For instance, with --bin=lsize, a block with lsize of 16K and psize of 4K will be added to the 16K bin in all three columns. slay, that's the whole bit. ✨

  • --class=(normal|special|dedup|other)[,…] — When used with -bb, only consider blocks from these allocation classes. pick your faves. 💅
  • -B, --backup — Generate a backup stream, similar to zfs send, but for the numeric objset ID, and without opening the dataset. This can be useful in recovery scenarios if dataset metadata has become corrupted but the dataset itself is readable. The optional flags argument is a string of one or more of the letters e, L, c, and w, which correspond to the same flags in zfs-send(8). 🫶
  • -c, --checksum — Verify the checksum of all metadata blocks while printing block statistics (see -b).

If specified multiple times, verify the checksums of all blocks. 👑

  • -C, --config — Display information about the configuration. If specified with no other options, instead display information about the cache file (/etc/zfs/zpool.cache). To specify the cache file to display, see -U.

If specified multiple times, and a pool name is also specified, display both the cached configuration and the on-disk configuration. If specified multiple times with -e also display the configuration that would be used were the pool to be imported. so many configs, so little time. 🌸

  • -d, --datasets — Display information about datasets. Specified once, displays basic dataset information: ID, create transaction, size, and object count. See -N for determining if poolname[/dataset|objset-ID] is to use the specified dataset|objset-ID as a string (dataset name) or a number (objset ID) when datasets have numeric names.

If specified multiple times provides greater and greater verbosity. 🔥

If object IDs or object ID ranges are specified, display information about those specific objects or ranges only.

An object ID range is specified in terms of a colon-separated tuple of the form ⟨start⟩:⟨end⟩[:⟨flags⟩]. The fields start and end are integer object identifiers that denote the upper and lower bounds of the range. An end value of -1 specifies a range with no upper bound. The flags field optionally specifies a set of flags, described below, that control which object types are dumped. By default, all object types are dumped. A minus sign (-) negates the effect of the flag that follows it and has no effect unless preceded by the A flag. For example, the range 0:-1:A-d will dump all object types except for directories. 🦋

  • A — Dump all objects (this is the default)
  • d — Dump ZFS directory objects
  • f — Dump ZFS plain file objects
  • m — Dump SPA space map objects
  • z — Dump ZAP objects
  • - — Negate the effect of next flag
  • -D, --dedup-stats — Display deduplication statistics, including the deduplication ratio (dedup), compression ratio (compress), inflation due to the zfs copies property (copies), and an overall effective ratio (dedup × compress / copies). math ate, honestly. 🧮
  • -DD — Display a histogram of deduplication statistics, showing the allocated (physically present on disk) and referenced (logically referenced in the pool) block counts and sizes by reference count. 📈
  • -DDD — Display the statistics independently for each deduplication table. 💜
  • -DDDD — Dump the contents of the deduplication tables describing duplicate blocks. 😭
  • -DDDDD — Also dump the contents of the deduplication tables describing unique blocks. not me adding one more D like it's nothing 💀
  • -E, --embedded-block-pointer=word0:word1:…:word15 — Decode and display block from an embedded block pointer specified by the word arguments. 🔍
  • -h, --history — Display pool history similar to zpool history, but include internal changes, transaction, and dataset information. the receipts, but internal. 🧾
  • -i, --intent-logs — Display information about intent log (ZIL) entries relating to each dataset. If specified multiple times, display counts of each intent log transaction type. 🎀
  • -k, --checkpointed-state — Examine the checkpointed state of the pool. Note, the on disk format of the pool is not reverted to the checkpointed state. ⚠️ look only, no touch.
  • -l, --label=device — Read the vdev labels and L2ARC header from the specified device. zdb -l will return 0 if valid label was found, 1 if error occurred, and 2 if no valid labels were found. The presence of L2ARC header is indicated by a specific sequence (L2ARC_DEV_HDR_MAGIC). If there is an accounting error in the size or the number of L2ARC log blocks zdb -l will return 1. Each unique configuration is displayed only once. ✨
  • -ll device — In addition display label space usage stats. If a valid L2ARC header was found also display the properties of log blocks used for restoring L2ARC contents (persistent L2ARC). 💖
  • -lll device — Display every configuration, unique or not. If a valid L2ARC header was found also display the properties of log entries in log blocks used for restoring L2ARC contents (persistent L2ARC). 👑

If the -q option is also specified, don't print the labels or the L2ARC header.

If the -u option is also specified, also display the uberblocks on this device. Specify multiple times to increase verbosity. 🔥

  • -L, --disable-leak-tracking — Disable leak detection and the loading of space maps. By default, zdb verifies that all non-free blocks are referenced, which can be very expensive. 😮💨 your wallet-of-time will thank you.
  • -m, --metaslabs — Display the offset, spacemap, free space of each metaslab, all the log spacemaps and their obsolete entry statistics. 🌈
  • -mm — Also display information about the on-disk free space histogram associated with each metaslab. 📊
  • -mmm — Display the maximum contiguous free space, the in-core free space histogram, and the percentage of free space in each space map. 💅
  • -mmmm — Display every spacemap record. literally every single one. 💀
  • -M, --metaslab-groups — Display all "normal" vdev metaslab group information — per-vdev metaslab count, fragmentation, and free space histogram, as well as overall pool fragmentation and histogram. 🫶
  • -MM — "Special" vdevs are added to -M's normal output. Also display information about the maximum contiguous free space and the percentage of free space in each space map. ⭐
  • -MMM — Display every spacemap record. again. every. one. 😭
  • -N — Same as -d but force zdb to interpret the [dataset|objset-ID] in [poolname[/dataset|objset-ID]] as a numeric objset ID. no guessing games. 🎯
  • -O, --object-lookups=dataset path — Look up the specified path inside of the dataset and display its metadata and indirect blocks. Specified path must be relative to the root of dataset. This option can be combined with -v for increasing verbosity. 🔍
  • -r, --copy-object=dataset path destination — Copy the specified path inside of the dataset to the specified destination. Specified path must be relative to the root of dataset. When used with -O, the path argument is interpreted as an object identifier, not a path. This option can be combined with -v for increasing verbosity. copy that, bestie. 💖
  • -R, --read-block=poolname vdev:offset:[lsize/]psize[:flags] — Read and display a block from the specified device. By default the block is displayed as a hex dump, but see the description of the r flag, below. 🧚

…and that's only part 1, bestie — more is coming, don't go anywhere 💜

The block is specified in terms of a colon-separated tuple: vdev (an integer vdev identifier), offset (the offset within the vdev), size (the physical size, or logical size / physical size) of the block to read and, optionally, flags (a set of flags, described below). Bestie it's literally just vdev:offset:size[:flags] and you're cooking 💅

  • b offset — Print block pointer at hex offset 🎯
  • c — Calculate and display checksums ✅
  • d — Decompress the block. Set the environment variable ZDB_NO_ZLE to skip zle when guessing 🫧
  • e — Byte swap the block 🔄
  • g — Dump gang block header 👯
  • i — Dump indirect block 🪆
  • r — Dump raw uninterpreted block data 🍖
  • v — Verbose output for guessing compression algorithm 🗣️
  • -s, --io-stats — Report statistics on zdb I/O. It displays operation counts, bandwidth, and error counts of I/O to the pool from zdb 📊
  • -S, --simulate-dedup — Simulate the effects of deduplication, constructing a DDT and then displaying that DDT just like -DD does 🧪
  • -T, --brt-stats — Display block reference table (BRT) statistics, including the size of unique blocks cloned, the space saving as a result of cloning, and the saving ratio 👑
  • -TT — Display the per-vdev BRT statistics, including total references 📈
  • -TTT — Display histograms of per-vdev BRT refcounts 📉
  • -TTTT — Dump the contents of the block reference tables 🗂️
  • -u, --uberblock — Display the current uberblock ✨

🌸 Other options 🌸

  • -A, --ignore-assertions — Do not abort should any assertion fail 🙈
  • -AA — Enable panic recovery; certain errors which would otherwise be fatal are demoted to warnings 🩹
  • -AAA — Do not abort if asserts fail and also enable panic recovery 🛟
  • -e, --exported=[-p path]… — Operate on an exported pool, not present in /etc/zfs/zpool.cache. The -p flag specifies the path under which devices are to be searched 🔍
  • -x, --dump-blocks=dumpdir — All blocks accessed will be copied to files in the specified directory. The blocks will be placed in sparse files whose name is the same as that of the file or device read. zdb can then be run on the generated files. Note that the -bbc flags are sufficient to access (and thus copy) all metadata on the pool 💾
  • -F, --automatic-rewind — Attempt to make an unreadable pool readable by trying progressively older transactions ⏪
  • -G, --dump-debug-msg — Dump the contents of the zfs_dbgmsg buffer before exiting zdb. zfs_dbgmsg is a buffer used by ZFS to dump advanced debug information 🐛
  • -I, --inflight=inflight-I/O-ops — Limit the number of outstanding checksum I/O operations to the specified value. The default value is 200. This option affects the performance of the -c option 🚦
  • -K, --key=key — Decryption key needed to access an encrypted dataset. This will cause zdb to attempt to unlock the dataset using the encryption root, key format and other encryption parameters on the given dataset. zdb can still inspect pool and dataset structures on encrypted datasets without unlocking them, but will not be able to access file names and attributes and object contents 🔐
WARNING ⚠️ — The raw decryption key and any decrypted data will be in user memory while zdb is running. Other user programs may be able to extract it by inspecting zdb as it runs. Exercise extreme caution when using this option in shared or uncontrolled environments. Not to be dramatic but this is giving "don't do this on a shared box" energy fr fr 💀
  • -o, --option=var=value… — Set the given tunable to the provided value 🎛️
  • -o, --option=var… — Show the value of the given tunable 👀
  • -o, --option=show — Show all tunables and their values 📋
  • -o, --option=info=value… — Show info about a tunable, including their name, type and description 📖
  • -o, --option=info — Show info about all tunables 📚
  • -P, --parseable — Print numbers in an unscaled form more amenable to parsing, e.g. 1000000 rather than 1M 🔢
  • -t, --txg=transaction — Specify the highest transaction to use when searching for uberblocks. See also the -u and -l options for a means to see the available uberblocks and their associated transaction numbers 🔎
  • -U, --cachefile=cachefile — Use a cache file other than /etc/zfs/zpool.cache 🗃️
  • -v, --verbose — Enable verbosity. Specify multiple times for increased verbosity 🗣️🗣️🗣️
  • -V, --verbatim — Attempt verbatim import. This mimics the behavior of the kernel when loading a pool from a cachefile. Only usable with -e 🧬
  • -X, --extreme-rewind — Attempt "extreme" transaction rewind, that is attempt the same recovery as -F but read transactions otherwise deemed too old 🦖
  • -Y, --all-reconstruction — Attempt all possible combinations when reconstructing indirect split blocks. This flag disables the individual I/O deadman timer in order to allow as much time as required for the attempted reconstruction ⏳
  • -y, --livelist — Perform validation for livelists that are being deleted. Scans through the livelist and metaslabs, checking for duplicate entries and compares the two, checking for potential double frees. If it encounters issues, warnings will be printed, but the command will not necessarily fail 🧹

Specifying a display option more than once enables verbosity for only that option, with more occurrences enabling more verbosity 🔁

If no options are specified, all information about the named pool will be displayed at default verbosity 🌈

🏁 EXIT STATUS 🏁

The zdb utility exits 0 on success, 1 if a fatal error occurs, 2 if invalid command line options were specified, or 3 if on-disk corruption was detected, but was not fatal. Know your numbers bestie, they're the whole plot 🎬

🍓 EXAMPLES 🍓

💖 Example 1: Display the configuration of imported pool rpool

# zdb -C rpool
MOS Configuration:
        version: 28
        name: 'rpool'
 …

💖 Example 2: Display basic dataset information about rpool

# zdb -d rpool
Dataset mos [META], ID 0, cr_txg 4, 26.9M, 1051 objects
Dataset rpool/swap [ZVOL], ID 59, cr_txg 356, 486M, 2 objects
 …

💖 Example 3: Display basic information about object 0 in rpool/export/home

# zdb -d rpool/export/home 0
Dataset rpool/export/home [ZPL], ID 137, cr_txg 1546, 32K, 8 objects

    Object  lvl   iblk   dblk  dsize  lsize   %full  type
         0    7    16K    16K  15.0K    16K   25.00  DMU dnode

💖 Example 4: Display the predicted effect of enabling deduplication on rpool

# zdb -S rpool
Simulated DDT histogram:

bucket              allocated                       referenced
______   ______________________________   ______________________________
refcnt   blocks   LSIZE   PSIZE   DSIZE   blocks   LSIZE   PSIZE   DSIZE
------   ------   -----   -----   -----   ------   -----   -----   -----
     1     694K   27.1G   15.0G   15.0G     694K   27.1G   15.0G   15.0G
     2    35.0K   1.33G    699M    699M    74.7K   2.79G   1.45G   1.45G
 …
dedup = 1.11, compress = 1.80, copies = 1.00, dedup * compress / copies = 2.00

👀 SEE ALSO 👀

zfs(8), zpool(8) — go read the besties next, they're iconic 🫶

And that's the whole page, bestie!! This one came to you from OpenZFS, dated August 12, 2025 — ZDB(8) in the house 🎀 No cap, you now know zdb better than most people who've been running it for years. Go forth and inspect some blocks ✨💜

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