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Linux/Documentation/filesystems/cramfs.txt

  1 
  2         Cramfs - cram a filesystem onto a small ROM
  3 
  4 cramfs is designed to be simple and small, and to compress things well. 
  5 
  6 It uses the zlib routines to compress a file one page at a time, and
  7 allows random page access.  The meta-data is not compressed, but is
  8 expressed in a very terse representation to make it use much less
  9 diskspace than traditional filesystems. 
 10 
 11 You can't write to a cramfs filesystem (making it compressible and
 12 compact also makes it _very_ hard to update on-the-fly), so you have to
 13 create the disk image with the "mkcramfs" utility.
 14 
 15 
 16 Usage Notes
 17 -----------
 18 
 19 File sizes are limited to less than 16MB.
 20 
 21 Maximum filesystem size is a little over 256MB.  (The last file on the
 22 filesystem is allowed to extend past 256MB.)
 23 
 24 Only the low 8 bits of gid are stored.  The current version of
 25 mkcramfs simply truncates to 8 bits, which is a potential security
 26 issue.
 27 
 28 Hard links are supported, but hard linked files
 29 will still have a link count of 1 in the cramfs image.
 30 
 31 Cramfs directories have no `.' or `..' entries.  Directories (like
 32 every other file on cramfs) always have a link count of 1.  (There's
 33 no need to use -noleaf in `find', btw.)
 34 
 35 No timestamps are stored in a cramfs, so these default to the epoch
 36 (1970 GMT).  Recently-accessed files may have updated timestamps, but
 37 the update lasts only as long as the inode is cached in memory, after
 38 which the timestamp reverts to 1970, i.e. moves backwards in time.
 39 
 40 Currently, cramfs must be written and read with architectures of the
 41 same 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
 43 decided what the best fix is.  For the moment if you have larger pages
 44 you can just change the #define in mkcramfs.c, so long as you don't
 45 mind the filesystem becoming unreadable to future kernels.
 46 
 47 
 48 For /usr/share/magic
 49 --------------------
 50 
 51 0       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"
 61 512     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 
 73 Hacker Notes
 74 ------------
 75 
 76 See fs/cramfs/README for filesystem layout and implementation notes.

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