path: root/fs/cramfs/README
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authorLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /fs/cramfs/README
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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+Notes on Filesystem Layout
+These notes describe what mkcramfs generates. Kernel requirements are
+a bit looser, e.g. it doesn't care if the <file_data> items are
+swapped around (though it does care that directory entries (inodes) in
+a given directory are contiguous, as this is used by readdir).
+All data is currently in host-endian format; neither mkcramfs nor the
+kernel ever do swabbing. (See section `Block Size' below.)
+ <superblock>
+ <directory_structure>
+ <data>
+<superblock>: struct cramfs_super (see cramfs_fs.h).
+ For each file:
+ struct cramfs_inode (see cramfs_fs.h).
+ Filename. Not generally null-terminated, but it is
+ null-padded to a multiple of 4 bytes.
+The order of inode traversal is described as "width-first" (not to be
+confused with breadth-first); i.e. like depth-first but listing all of
+a directory's entries before recursing down its subdirectories: the
+same order as `ls -AUR' (but without the /^\..*:$/ directory header
+lines); put another way, the same order as `find -type d -exec
+ls -AU1 {} \;'.
+Beginning in 2.4.7, directory entries are sorted. This optimization
+allows cramfs_lookup to return more quickly when a filename does not
+exist, speeds up user-space directory sorts, etc.
+ One <file_data> for each file that's either a symlink or a
+ regular file of non-zero st_size.
+ nblocks * <block_pointer>
+ (where nblocks = (st_size - 1) / blksize + 1)
+ nblocks * <block>
+ padding to multiple of 4 bytes
+The i'th <block_pointer> for a file stores the byte offset of the
+*end* of the i'th <block> (i.e. one past the last byte, which is the
+same as the start of the (i+1)'th <block> if there is one). The first
+<block> immediately follows the last <block_pointer> for the file.
+<block_pointer>s are each 32 bits long.
+The order of <file_data>'s is a depth-first descent of the directory
+tree, i.e. the same order as `find -size +0 \( -type f -o -type l \)
+<block>: The i'th <block> is the output of zlib's compress function
+applied to the i'th blksize-sized chunk of the input data.
+(For the last <block> of the file, the input may of course be smaller.)
+Each <block> may be a different size. (See <block_pointer> above.)
+<block>s are merely byte-aligned, not generally u32-aligned.
+This kernel supports cramfs holes (i.e. [efficient representation of]
+blocks in uncompressed data consisting entirely of NUL bytes), but by
+default mkcramfs doesn't test for & create holes, since cramfs in
+kernels up to at least 2.3.39 didn't support holes. Run mkcramfs
+with -z if you want it to create files that can have holes in them.
+The cramfs user-space tools, including mkcramfs and cramfsck, are
+located at <http://sourceforge.net/projects/cramfs/>.
+Future Development
+Block Size
+(Block size in cramfs refers to the size of input data that is
+compressed at a time. It's intended to be somewhere around
+PAGE_CACHE_SIZE for cramfs_readpage's convenience.)
+The superblock ought to indicate the block size that the fs was
+written for, since comments in <linux/pagemap.h> indicate that
+PAGE_CACHE_SIZE may grow in future (if I interpret the comment
+Currently, mkcramfs #define's PAGE_CACHE_SIZE as 4096 and uses that
+for blksize, whereas Linux-2.3.39 uses its PAGE_CACHE_SIZE, which in
+turn is defined as PAGE_SIZE (which can be as large as 32KB on arm).
+This discrepancy is a bug, though it's not clear which should be
+One option is to change mkcramfs to take its PAGE_CACHE_SIZE from
+<asm/page.h>. Personally I don't like this option, but it does
+require the least amount of change: just change `#define
+PAGE_CACHE_SIZE (4096)' to `#include <asm/page.h>'. The disadvantage
+is that the generated cramfs cannot always be shared between different
+kernels, not even necessarily kernels of the same architecture if
+PAGE_CACHE_SIZE is subject to change between kernel versions
+(currently possible with arm and ia64).
+The remaining options try to make cramfs more sharable.
+One part of that is addressing endianness. The two options here are
+`always use little-endian' (like ext2fs) or `writer chooses
+endianness; kernel adapts at runtime'. Little-endian wins because of
+code simplicity and little CPU overhead even on big-endian machines.
+The cost of swabbing is changing the code to use the le32_to_cpu
+etc. macros as used by ext2fs. We don't need to swab the compressed
+data, only the superblock, inodes and block pointers.
+The other part of making cramfs more sharable is choosing a block
+size. The options are:
+ 1. Always 4096 bytes.
+ 2. Writer chooses blocksize; kernel adapts but rejects blocksize >
+ 3. Writer chooses blocksize; kernel adapts even to blocksize >
+It's easy enough to change the kernel to use a smaller value than
+PAGE_CACHE_SIZE: just make cramfs_readpage read multiple blocks.
+The cost of option 1 is that kernels with a larger PAGE_CACHE_SIZE
+value don't get as good compression as they can.
+The cost of option 2 relative to option 1 is that the code uses
+variables instead of #define'd constants. The gain is that people
+with kernels having larger PAGE_CACHE_SIZE can make use of that if
+they don't mind their cramfs being inaccessible to kernels with
+smaller PAGE_CACHE_SIZE values.
+Option 3 is easy to implement if we don't mind being CPU-inefficient:
+e.g. get readpage to decompress to a buffer of size MAX_BLKSIZE (which
+must be no larger than 32KB) and discard what it doesn't need.
+Getting readpage to read into all the covered pages is harder.
+The main advantage of option 3 over 1, 2, is better compression. The
+cost is greater complexity. Probably not worth it, but I hope someone
+will disagree. (If it is implemented, then I'll re-use that code in
+Another cost of 2 and 3 over 1 is making mkcramfs use a different
+block size, but that just means adding and parsing a -b option.
+Inode Size
+Given that cramfs will probably be used for CDs etc. as well as just
+silicon ROMs, it might make sense to expand the inode a little from
+its current 12 bytes. Inodes other than the root inode are followed
+by filename, so the expansion doesn't even have to be a multiple of 4