MKFS.CRAMFS(8)

💾 MKFS.CRAMFS — make compressed ROM file system ✨

okay so bestie, quick vibe check: this is mkfs.cramfs, and she lives in section 8 of the manual, aka System Administration — the big-girl section where all the stuff that can actually touch your filesystem hangs out. buckle up, we're packing a whole directory tree into one compressed little cutie 🎀

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🌸 NAME 🌸

mkfs.cramfs — make compressed ROM file system

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✨ SYNOPSIS ✨

mkfs.cramfs [options] directory file

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🦋 DESCRIPTION 🦋

So here's the tea. On a cramfs file system, files are zlib-compressed one page at a time, which is the whole trick that lets you do random read access. The metadata? Not compressed, bestie. But it is expressed in a terse representation that's more space-efficient than conventional file systems. it's giving minimalism, and honestly? it ate. 💅

The file system is intentionally read-only — like, on purpose — because it simplifies the design. Random write access for compressed files is genuinely difficult to implement, so they simply did not. cramfs ships with a utility (mkcramfs(8)) to pack files into new cramfs images.

File sizes are limited to less than 16 MB. (yes, MB, not GB. she's small and she knows it 🍓)

Maximum file system size is a little under 272 MB. (The last file on the file system must begin before the 256 MB block, but can extend past it.)

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🌈 ARGUMENTS 🌈

• directory — this is simply the root of the directory tree that we want to generate a compressed filesystem out of.
• file — this will contain the cram file system, which later can be mounted.

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👑 OPTIONS 👑

• -v — Enable verbose messaging. she will tell you everything 🗣️
• -E — Treat all warnings as errors, which are reflected as command exit status.
• -b blocksize — Use defined block size, which has to be divisible by page size.
• -e edition — Use defined file system edition number in superblock.
• -N big|little|host — Use the specified endianness. The default is host.
• -i file — Insert a file to cramfs file system.
• -n name — Set name of the cramfs file system.
• -p — Pad by 512 bytes for boot code.
• -s — This option is ignored. Originally the -s turned on directory entry sorting. (she's retired, show some respect 🙈)
• -z — Make explicit holes.
• -l[=mode] — Use an exclusive BSD lock for the device or file that is operated upon. The optional argument mode can be yes (1), no (0), or nonblock. If the mode argument is omitted, it defaults to yes. This option overrides the environment variable $LOCK_BLOCK_DEVICE. The default is to not use any lock at all, but using a lock is recommended to avoid collisions with systemd-udevd(8) or other tools.
• -h, --help — Display help text and exit.
• -V, --version — Display version and exit.

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💀 EXIT STATUS 💀

• 0 — success 🎉
• 8 — operation error, such as unable to allocate memory

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🔥 SEE ALSO 🔥

fsck.cramfs(8), mount(8)

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🫶 REPORTING BUGS 🫶

For bug reports, use the issue tracker <https://github.com/util-linux/util-linux/issues>.

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⭐ AVAILABILITY ⭐

The mkfs.cramfs command is part of the util-linux package which can be downloaded from Linux Kernel Archive <https://www.kernel.org/pub/linux/utils/util-linux/>.

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no cap, if something's weird, file it in the tracker — the maintainers are literally the main characters of this whole story 💜

and that's the whole page, bestie!! this was util-linux 2.42.4, dated 2026-09-21, and the page you just read is MKFS.CRAMFS(8). go forth and compress, icon 💖✨