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