Specifications
# >file
bash: file: No space left on device
# mv file foo
mv: cannot create regular file `foo': No space left on device
You will have to use rm to actually delete a file in this situation.
This is especially critical when you are using the flash filesystem to store log files: when your application
detects some abnormal condition and produces lots of log messages (which usually are especially important in
this situation) the filesystem may fill up and cause extreme long delays - if your system crashes, the most
important messages may never be logged at all.
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.
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 Power
Architecture® 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.
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
9.1.4. Compressed ROM Filesystem 100