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9.1.4. Compressed ROM Filesystem

In some cases it is sufficent to have read-only access to some files, and if the files are big enough it becomes desirable to use some method of compression. The Compressed ROM Filesystem CramFs might be a solution here.

ALERT! Please note that CramFs has - beside the fact that it is a read-only filesystem - some severe limitations (like missing support for timestamps, hard links, and 16/32 bit uid/gids), but there are many situations in Embedded Systems where it's still useful.

To create a CramFs filesystem a special tool mkcramfs is used to create a file which contains the CramFs image. Note that the CramFs filesystem can be written and read only by kernels with PAGE_CACHE_SIZE == 4096, and some versions of the mkcramfs program may have other restrictions like that the filesystem must be written and read with architectures of the same endianness. Especially the endianness requirement makes it impossible to build the CramFs image on x86 PC host when you want to use it on a PowerPC target. The endianness problem has been fixed in the version of mkcramfs that comes with the ELDK.

In some cases you can use a target system running with root filesystem mounted over NFS to create the CramFs image on the native system and store it to flash for further use.

ALERT! Note: The normal version of the mkcramfs program tries to initialize some entries in the filesystem's superblock with random numbers by reading /dev/random; this may hang permanently on your target because there is not enough input (like mouse movement) to the entropy pool. You may want to use a modified version of mkcramfs which does not depend on /dev/random.

To create a CramFs image, you put all files you want in the filesystem into one directory, and then use the mkcramfs= program as follows:

$ mkdir /tmp/test
$ cp ... /tmp/test
$ du -sk /tmp/test
64      /tmp/test
$ mkcramfs /tmp/test test.cramfs.img
Super block: 76 bytes
  erase
  eraseall
  mkfs.jffs
  lock
  unlock
Directory data: 176 bytes
-54.96% (-4784 bytes)   erase
-55.46% (-5010 bytes)   eraseall
-51.94% (-8863 bytes)   mkfs.jffs
-58.76% (-4383 bytes)   lock
-59.68% (-4215 bytes)   unlock
Everything: 24 kilobytes
$ ls -l test.cramfs.img
-rw-r--r--    1 wd       users       24576 Nov 10 23:44 test.cramfs.img

As you can see, the CramFs image test.cramfs.img takes just 24 kB, while the input directory contained 64 kB of data. Savings of some 60% like in this case are typical CramFs.

Now we write the CramFs image to a partition in flash and test it:

# cp test.cramfs.img /dev/mtd3
# mount -t cramfs /dev/mtdblock3 /mnt
# mount
/dev/root on / type nfs (rw,v2,rsize=4096,wsize=4096,hard,udp,nolock,addr=10.0.0.2)
proc on /proc type proc (rw)
devpts on /dev/pts type devpts (rw)
/dev/mtdblock3 on /mnt type cramfs (rw)
# ls -l /mnt
total 54
-rwxr-xr-x    1 wd       users        8704 Jan  9 16:32 erase
-rwxr-xr-x    1 wd       users        9034 Jan  1 01:00 eraseall
-rwxr-xr-x    1 wd       users        7459 Jan  1 01:00 lock
-rwxr-xr-x    1 wd       users       17063 Jan  1 01:00 mkfs.jffs
-rwxr-xr-x    1 wd       users        7063 Jan  1 01:00 unlock

Note that all the timestamps in the CramFs filesyste are bogus, and so is for instance the output of the df command for such filesystems:

# df /mnt
Filesystem           1k-blocks      Used Available Use% Mounted on
/dev/mtdblock3               0         0         0   -  /mnt
9.1.3. Second Version of JFFS 1. Abstract 9.2. The TMPFS Virtual Memory Filesystem
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