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/Documentation/filesystems/cramfs.txt

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 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.