README
1Notes on Filesystem Layout
2--------------------------
3
4These notes describe what mkcramfs generates. Kernel requirements are
5a bit looser, e.g. it doesn't care if the <file_data> items are
6swapped around (though it does care that directory entries (inodes) in
7a given directory are contiguous, as this is used by readdir).
8
9All data is currently in host-endian format; neither mkcramfs nor the
10kernel 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
25The order of inode traversal is described as "width-first" (not to be
26confused with breadth-first); i.e. like depth-first but listing all of
27a directory's entries before recursing down its subdirectories: the
28same order as `ls -AUR' (but without the /^\..*:$/ directory header
29lines); put another way, the same order as `find -type d -exec
30ls -AU1 {} \;'.
31
32Beginning in 2.4.7, directory entries are sorted. This optimization
33allows cramfs_lookup to return more quickly when a filename does not
34exist, 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
46The 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
48same 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
52When the CRAMFS_FLAG_EXT_BLOCK_POINTERS capability bit is set, each
53<block_pointer>'s top bits may contain special flags as follows:
54
55CRAMFS_BLK_FLAG_UNCOMPRESSED (bit 31):
56 The block data is not compressed and should be copied verbatim.
57
58CRAMFS_BLK_FLAG_DIRECT_PTR (bit 30):
59 The <block_pointer> stores the actual block start offset and not
60 its end, shifted right by 2 bits. The block must therefore be
61 aligned to a 4-byte boundary. The block size is either blksize
62 if CRAMFS_BLK_FLAG_UNCOMPRESSED is also specified, otherwise
63 the compressed data length is included in the first 2 bytes of
64 the block data. This is used to allow discontiguous data layout
65 and specific data block alignments e.g. for XIP applications.
66
67
68The order of <file_data>'s is a depth-first descent of the directory
69tree, i.e. the same order as `find -size +0 \( -type f -o -type l \)
70-print'.
71
72
73<block>: The i'th <block> is the output of zlib's compress function
74applied to the i'th blksize-sized chunk of the input data if the
75corresponding CRAMFS_BLK_FLAG_UNCOMPRESSED <block_ptr> bit is not set,
76otherwise it is the input data directly.
77(For the last <block> of the file, the input may of course be smaller.)
78Each <block> may be a different size. (See <block_pointer> above.)
79
80<block>s are merely byte-aligned, not generally u32-aligned.
81
82When CRAMFS_BLK_FLAG_DIRECT_PTR is specified then the corresponding
83<block> may be located anywhere and not necessarily contiguous with
84the previous/next blocks. In that case it is minimally u32-aligned.
85If CRAMFS_BLK_FLAG_UNCOMPRESSED is also specified then the size is always
86blksize except for the last block which is limited by the file length.
87If CRAMFS_BLK_FLAG_DIRECT_PTR is set and CRAMFS_BLK_FLAG_UNCOMPRESSED
88is not set then the first 2 bytes of the block contains the size of the
89remaining block data as this cannot be determined from the placement of
90logically adjacent blocks.
91
92
93Holes
94-----
95
96This kernel supports cramfs holes (i.e. [efficient representation of]
97blocks in uncompressed data consisting entirely of NUL bytes), but by
98default mkcramfs doesn't test for & create holes, since cramfs in
99kernels up to at least 2.3.39 didn't support holes. Run mkcramfs
100with -z if you want it to create files that can have holes in them.
101
102
103Tools
104-----
105
106The cramfs user-space tools, including mkcramfs and cramfsck, are
107located at <http://sourceforge.net/projects/cramfs/>.
108
109
110Future Development
111==================
112
113Block Size
114----------
115
116(Block size in cramfs refers to the size of input data that is
117compressed at a time. It's intended to be somewhere around
118PAGE_SIZE for cramfs_readpage's convenience.)
119
120The superblock ought to indicate the block size that the fs was
121written for, since comments in <linux/pagemap.h> indicate that
122PAGE_SIZE may grow in future (if I interpret the comment
123correctly).
124
125Currently, mkcramfs #define's PAGE_SIZE as 4096 and uses that
126for blksize, whereas Linux-2.3.39 uses its PAGE_SIZE, which in
127turn is defined as PAGE_SIZE (which can be as large as 32KB on arm).
128This discrepancy is a bug, though it's not clear which should be
129changed.
130
131One option is to change mkcramfs to take its PAGE_SIZE from
132<asm/page.h>. Personally I don't like this option, but it does
133require the least amount of change: just change `#define
134PAGE_SIZE (4096)' to `#include <asm/page.h>'. The disadvantage
135is that the generated cramfs cannot always be shared between different
136kernels, not even necessarily kernels of the same architecture if
137PAGE_SIZE is subject to change between kernel versions
138(currently possible with arm and ia64).
139
140The remaining options try to make cramfs more sharable.
141
142One part of that is addressing endianness. The two options here are
143`always use little-endian' (like ext2fs) or `writer chooses
144endianness; kernel adapts at runtime'. Little-endian wins because of
145code simplicity and little CPU overhead even on big-endian machines.
146
147The cost of swabbing is changing the code to use the le32_to_cpu
148etc. macros as used by ext2fs. We don't need to swab the compressed
149data, only the superblock, inodes and block pointers.
150
151
152The other part of making cramfs more sharable is choosing a block
153size. The options are:
154
155 1. Always 4096 bytes.
156
157 2. Writer chooses blocksize; kernel adapts but rejects blocksize >
158 PAGE_SIZE.
159
160 3. Writer chooses blocksize; kernel adapts even to blocksize >
161 PAGE_SIZE.
162
163It's easy enough to change the kernel to use a smaller value than
164PAGE_SIZE: just make cramfs_readpage read multiple blocks.
165
166The cost of option 1 is that kernels with a larger PAGE_SIZE
167value don't get as good compression as they can.
168
169The cost of option 2 relative to option 1 is that the code uses
170variables instead of #define'd constants. The gain is that people
171with kernels having larger PAGE_SIZE can make use of that if
172they don't mind their cramfs being inaccessible to kernels with
173smaller PAGE_SIZE values.
174
175Option 3 is easy to implement if we don't mind being CPU-inefficient:
176e.g. get readpage to decompress to a buffer of size MAX_BLKSIZE (which
177must be no larger than 32KB) and discard what it doesn't need.
178Getting readpage to read into all the covered pages is harder.
179
180The main advantage of option 3 over 1, 2, is better compression. The
181cost is greater complexity. Probably not worth it, but I hope someone
182will disagree. (If it is implemented, then I'll re-use that code in
183e2compr.)
184
185
186Another cost of 2 and 3 over 1 is making mkcramfs use a different
187block size, but that just means adding and parsing a -b option.
188
189
190Inode Size
191----------
192
193Given that cramfs will probably be used for CDs etc. as well as just
194silicon ROMs, it might make sense to expand the inode a little from
195its current 12 bytes. Inodes other than the root inode are followed
196by filename, so the expansion doesn't even have to be a multiple of 4
197bytes.
198
