patch-2.3.40 linux/fs/cramfs/README

• Lines: 167
• Date: Tue Jan 11 10:24:58 2000
• Orig file: v2.3.39/linux/fs/cramfs/README
• Orig date: Wed Dec 31 16:00:00 1969

diff -u --recursive --new-file v2.3.39/linux/fs/cramfs/README linux/fs/cramfs/README
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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 in host-endian format; neither mkcramfs nor the kernel
+ever do swabbing.  (See section `Block Size' below.)
+
+<filesystem>:
+	<superblock>
+	<directory_structure>
+	<data>
+
+<superblock>: struct cramfs_super (see cramfs.h).
+
+<directory_structure>:
+	For each file:
+		struct cramfs_inode (see cramfs.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 {} \;'.
+
+<data>:
+	One <file_data> for each file that's either a symlink or a
+	 regular file of non-zero st_size.
+
+<file_data>:
+	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 \( -type f -o -type l \)
+-print'.
+
+
+<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.
+
+
+Holes
+-----
+
+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.  Compile mkcramfs
+with -DDO_HOLES if you want it to create files that can have holes in
+them.
+
+
+Tools
+-----
+
+If you're hacking on cramfs, you might find useful some tools for
+testing cramfs at <http://cvs.bofh.asn.au/cramfs/>, including a
+rudimentary fsck for 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
+correctly).
+
+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
+changed.
+
+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.
+
+
+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 >
+     PAGE_CACHE_SIZE.
+
+  3. Writer chooses blocksize; kernel adapts even to blocksize >
+     PAGE_CACHE_SIZE.
+
+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
+e2compr.)
+
+
+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
+bytes.