Compressed ROM filesystem

Cromfs is a compressed read-only filesystem for Linux. Cromfs is intended for permanently archiving gigabytes of big files that have lots of redundancy.

In terms of compression it is much similar to 7-zip files, except that practical realtime access can be provided for the whole archive contents; the user does not need to launch a program to decompress a single file, nor does he need to wait while the system decompresses 500 files from a 1000-file archive to get him the 1 file he wanted to open.

The creation of cromfs was inspired from Squashfs and Cramfs.

• Data, inodes, directories and block lists are stored compressed
• Duplicate inodes, files and even duplicate file portions are detected and stored only once
  • Especially suitable for gigabyte-class archives of thousands of nearly-identical megabyte-class files.
• Files are stored in solid blocks, meaning that parts of different files are compressed together for effective compression
• All inode types recognized by Linux are supported. This includes also the fifo, devices and symlinks.
• The LZMA compression is used. In the general case, LZMA compresses better than gzip and bzip2.
• As with usual filesystems, the files on a cromfs volume can be accessed in arbitrary order; the waits to open a specific file are small, despite the files being semisolidly archived.

• Filesystems are write-once, read-only. It is not possible to append to a previously-created filesystem, nor it is to mount it read-write.
• Max filesize: 2^64 bytes (16777216 TB), but 256 TB with default settings.
• Max number of files in a directory: 2^30 (smaller if filenames are longer, but still more than 100000 in almost all cases)
• Max number of inodes (all files, dirs etc combined): 2^60, but depends on file sizes
• Max filesystem size: 2^64 bytes (16777216 TB)
• There are no "." and ".." entries in directories.
• mkcromfs is slow. You must be patient.
• The cromfs-driver has a large memory footprint. It is aimed for desktop and server environments, but not embedded.
• Ownerships are not saved.
• Running multiple instances of mkcromfs at same time is not possible, because it uses temporary files in /tmp, filenames of which are always the same.

Development status: Pre-beta. The Cromfs project has been created very recently and it hasn't been yet tested extensively. There is no warranty against data loss or anything else, so use at your own risk.

This is all very biased, hypothetical, and by no means a scientific study, but here goes:

Feature	Cromfs	Cramfs	Squashfs (3.0)
Compression unit	adjustable (1 MB default)	4 kB	adjustable (64 kB max)
Files are compressed	together (up to block limit)	individually	individually
Maximum file size	16 EB	16 MB	16 EB
Duplicate whole file detection	Yes	No (but hardlinks are detected)	Yes
Near-identical file detection	Yes (identical blocks)	No	No
Compression method	LZMA	gzip	gzip
Ownerships	not saved	saved (but gid truncated to 8 bits)	saved
Endianess-safety	works on little-endian only	safe, but not exchangeable	safe
Kernelspace/userspace	user (fuse)	kernel	kernel

1. Install the development requirements: make, gnu-c++, fuse and openssl-dev
2. Compile: Command "make" to build "cromfs-driver" and "util/mkcromfs"
3. Create a sample filesystem:

   $ util/mkcromfs . sample.cromfs

4. Mount the sample filesystem:

   $ mkdir sample
   $ ./cromfs-driver sample.cromfs sample &

5. Observe the sample filesystem:

   $ cd sample
   $ ls -al

6. Unmounting the filesystem:

   $ cd ..
   $ fusermount -u sample

   or, type "fg" and press ctrl-c to terminate the driver.

cromfs has been written by Joel Yliluoma, a.k.a. Bisqwit,
and is distributed under the terms of the General Public License (GPL).

• GNU make and gcc-c++ are required to recompile the source code.
• The openssl development library is required for MD5 calculation.
• The filesystem works under the Fuse user-space filesystem framework. You need to install both the Fuse kernel module and the userspace programs before mounting Cromfs volumes.

The official home page of cromfs is at http://iki.fi/bisqwit/source/cromfs.html.
Check there for new versions.