linux/Documentation/filesystems/cramfs.txt
<<
>>
Prefs
   1
   2        Cramfs - cram a filesystem onto a small ROM
   3
   4cramfs is designed to be simple and small, and to compress things well. 
   5
   6It uses the zlib routines to compress a file one page at a time, and
   7allows random page access.  The meta-data is not compressed, but is
   8expressed in a very terse representation to make it use much less
   9diskspace than traditional filesystems. 
  10
  11You can't write to a cramfs filesystem (making it compressible and
  12compact also makes it _very_ hard to update on-the-fly), so you have to
  13create the disk image with the "mkcramfs" utility.
  14
  15
  16Usage Notes
  17-----------
  18
  19File sizes are limited to less than 16MB.
  20
  21Maximum filesystem size is a little over 256MB.  (The last file on the
  22filesystem is allowed to extend past 256MB.)
  23
  24Only the low 8 bits of gid are stored.  The current version of
  25mkcramfs simply truncates to 8 bits, which is a potential security
  26issue.
  27
  28Hard links are supported, but hard linked files
  29will still have a link count of 1 in the cramfs image.
  30
  31Cramfs directories have no `.' or `..' entries.  Directories (like
  32every other file on cramfs) always have a link count of 1.  (There's
  33no need to use -noleaf in `find', btw.)
  34
  35No timestamps are stored in a cramfs, so these default to the epoch
  36(1970 GMT).  Recently-accessed files may have updated timestamps, but
  37the update lasts only as long as the inode is cached in memory, after
  38which the timestamp reverts to 1970, i.e. moves backwards in time.
  39
  40Currently, cramfs must be written and read with architectures of the
  41same endianness, and can be read only by kernels with PAGE_CACHE_SIZE
  42== 4096.  At least the latter of these is a bug, but it hasn't been
  43decided what the best fix is.  For the moment if you have larger pages
  44you can just change the #define in mkcramfs.c, so long as you don't
  45mind the filesystem becoming unreadable to future kernels.
  46
  47
  48For /usr/share/magic
  49--------------------
  50
  510       ulelong 0x28cd3d45      Linux cramfs offset 0
  52>4      ulelong x               size %d
  53>8      ulelong x               flags 0x%x
  54>12     ulelong x               future 0x%x
  55>16     string  >\0             signature "%.16s"
  56>32     ulelong x               fsid.crc 0x%x
  57>36     ulelong x               fsid.edition %d
  58>40     ulelong x               fsid.blocks %d
  59>44     ulelong x               fsid.files %d
  60>48     string  >\0             name "%.16s"
  61512     ulelong 0x28cd3d45      Linux cramfs offset 512
  62>516    ulelong x               size %d
  63>520    ulelong x               flags 0x%x
  64>524    ulelong x               future 0x%x
  65>528    string  >\0             signature "%.16s"
  66>544    ulelong x               fsid.crc 0x%x
  67>548    ulelong x               fsid.edition %d
  68>552    ulelong x               fsid.blocks %d
  69>556    ulelong x               fsid.files %d
  70>560    string  >\0             name "%.16s"
  71
  72
  73Hacker Notes
  74------------
  75
  76See fs/cramfs/README for filesystem layout and implementation notes.
  77
lxr.linux.no kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.