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Linux/fs/cramfs/

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File Kconfig 880 bytes
File Makefile 117 bytes
File README 6244 bytes
C file inode.c 14837 bytes
C file internal.h 198 bytes
C file uncompress.c 1771 bytes

  1 Notes on Filesystem Layout
  2 --------------------------
  3 
  4 These notes describe what mkcramfs generates.  Kernel requirements are
  5 a bit looser, e.g. it doesn't care if the <file_data> items are
  6 swapped around (though it does care that directory entries (inodes) in
  7 a given directory are contiguous, as this is used by readdir).
  8 
  9 All data is currently in host-endian format; neither mkcramfs nor the
 10 kernel ever do swabbing.  (See section `Block Size' below.)
 11 
 12 <filesystem>:
 13         <superblock>
 14         <directory_structure>
 15         <data>
 16 
 17 <superblock>: struct cramfs_super (see cramfs_fs.h).
 18 
 19 <directory_structure>:
 20         For each file:
 21                 struct cramfs_inode (see cramfs_fs.h).
 22                 Filename.  Not generally null-terminated, but it is
 23                  null-padded to a multiple of 4 bytes.
 24 
 25 The order of inode traversal is described as "width-first" (not to be
 26 confused with breadth-first); i.e. like depth-first but listing all of
 27 a directory's entries before recursing down its subdirectories: the
 28 same order as `ls -AUR' (but without the /^\..*:$/ directory header
 29 lines); put another way, the same order as `find -type d -exec
 30 ls -AU1 {} \;'.
 31 
 32 Beginning in 2.4.7, directory entries are sorted.  This optimization
 33 allows cramfs_lookup to return more quickly when a filename does not
 34 exist, speeds up user-space directory sorts, etc.
 35 
 36 <data>:
 37         One <file_data> for each file that's either a symlink or a
 38          regular file of non-zero st_size.
 39 
 40 <file_data>:
 41         nblocks * <block_pointer>
 42          (where nblocks = (st_size - 1) / blksize + 1)
 43         nblocks * <block>
 44         padding to multiple of 4 bytes
 45 
 46 The i'th <block_pointer> for a file stores the byte offset of the
 47 *end* of the i'th <block> (i.e. one past the last byte, which is the
 48 same as the start of the (i+1)'th <block> if there is one).  The first
 49 <block> immediately follows the last <block_pointer> for the file.
 50 <block_pointer>s are each 32 bits long.
 51 
 52 The order of <file_data>'s is a depth-first descent of the directory
 53 tree, i.e. the same order as `find -size +0 \( -type f -o -type l \)
 54 -print'.
 55 
 56 
 57 <block>: The i'th <block> is the output of zlib's compress function
 58 applied to the i'th blksize-sized chunk of the input data.
 59 (For the last <block> of the file, the input may of course be smaller.)
 60 Each <block> may be a different size.  (See <block_pointer> above.)
 61 <block>s are merely byte-aligned, not generally u32-aligned.
 62 
 63 
 64 Holes
 65 -----
 66 
 67 This kernel supports cramfs holes (i.e. [efficient representation of]
 68 blocks in uncompressed data consisting entirely of NUL bytes), but by
 69 default mkcramfs doesn't test for & create holes, since cramfs in
 70 kernels up to at least 2.3.39 didn't support holes.  Run mkcramfs
 71 with -z if you want it to create files that can have holes in them.
 72 
 73 
 74 Tools
 75 -----
 76 
 77 The cramfs user-space tools, including mkcramfs and cramfsck, are
 78 located at <http://sourceforge.net/projects/cramfs/>.
 79 
 80 
 81 Future Development
 82 ==================
 83 
 84 Block Size
 85 ----------
 86 
 87 (Block size in cramfs refers to the size of input data that is
 88 compressed at a time.  It's intended to be somewhere around
 89 PAGE_SIZE for cramfs_readpage's convenience.)
 90 
 91 The superblock ought to indicate the block size that the fs was
 92 written for, since comments in <linux/pagemap.h> indicate that
 93 PAGE_SIZE may grow in future (if I interpret the comment
 94 correctly).
 95 
 96 Currently, mkcramfs #define's PAGE_SIZE as 4096 and uses that
 97 for blksize, whereas Linux-2.3.39 uses its PAGE_SIZE, which in
 98 turn is defined as PAGE_SIZE (which can be as large as 32KB on arm).
 99 This discrepancy is a bug, though it's not clear which should be
100 changed.
101 
102 One option is to change mkcramfs to take its PAGE_SIZE from
103 <asm/page.h>.  Personally I don't like this option, but it does
104 require the least amount of change: just change `#define
105 PAGE_SIZE (4096)' to `#include <asm/page.h>'.  The disadvantage
106 is that the generated cramfs cannot always be shared between different
107 kernels, not even necessarily kernels of the same architecture if
108 PAGE_SIZE is subject to change between kernel versions
109 (currently possible with arm and ia64).
110 
111 The remaining options try to make cramfs more sharable.
112 
113 One part of that is addressing endianness.  The two options here are
114 `always use little-endian' (like ext2fs) or `writer chooses
115 endianness; kernel adapts at runtime'.  Little-endian wins because of
116 code simplicity and little CPU overhead even on big-endian machines.
117 
118 The cost of swabbing is changing the code to use the le32_to_cpu
119 etc. macros as used by ext2fs.  We don't need to swab the compressed
120 data, only the superblock, inodes and block pointers.
121 
122 
123 The other part of making cramfs more sharable is choosing a block
124 size.  The options are:
125 
126   1. Always 4096 bytes.
127 
128   2. Writer chooses blocksize; kernel adapts but rejects blocksize >
129      PAGE_SIZE.
130 
131   3. Writer chooses blocksize; kernel adapts even to blocksize >
132      PAGE_SIZE.
133 
134 It's easy enough to change the kernel to use a smaller value than
135 PAGE_SIZE: just make cramfs_readpage read multiple blocks.
136 
137 The cost of option 1 is that kernels with a larger PAGE_SIZE
138 value don't get as good compression as they can.
139 
140 The cost of option 2 relative to option 1 is that the code uses
141 variables instead of #define'd constants.  The gain is that people
142 with kernels having larger PAGE_SIZE can make use of that if
143 they don't mind their cramfs being inaccessible to kernels with
144 smaller PAGE_SIZE values.
145 
146 Option 3 is easy to implement if we don't mind being CPU-inefficient:
147 e.g. get readpage to decompress to a buffer of size MAX_BLKSIZE (which
148 must be no larger than 32KB) and discard what it doesn't need.
149 Getting readpage to read into all the covered pages is harder.
150 
151 The main advantage of option 3 over 1, 2, is better compression.  The
152 cost is greater complexity.  Probably not worth it, but I hope someone
153 will disagree.  (If it is implemented, then I'll re-use that code in
154 e2compr.)
155 
156 
157 Another cost of 2 and 3 over 1 is making mkcramfs use a different
158 block size, but that just means adding and parsing a -b option.
159 
160 
161 Inode Size
162 ----------
163 
164 Given that cramfs will probably be used for CDs etc. as well as just
165 silicon ROMs, it might make sense to expand the inode a little from
166 its current 12 bytes.  Inodes other than the root inode are followed
167 by filename, so the expansion doesn't even have to be a multiple of 4
168 bytes.

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