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Diffstat (limited to 'doc/format.txt')
| -rw-r--r-- | doc/format.txt | 59 |
1 files changed, 54 insertions, 5 deletions
diff --git a/doc/format.txt b/doc/format.txt index ae5a81b..1e059c2 100644 --- a/doc/format.txt +++ b/doc/format.txt @@ -61,8 +61,9 @@ 2) Overview *********** - A SquashFS archive consists of a maximum of nine parts, packed together on - a byte alignment: + SquashFS always stores integers in little endian format. + + A SquashFS archive consists of a maximum of nine parts: _______________ | | Important information about the archive, including @@ -100,6 +101,19 @@ |_______________| + Although the super block details the exact positions of each section, most + implementations, including the one in the Linux kernel, insist on this exact + order. + + The archive is usually padded with null bytes to make the size a multiple of + 1024 or 4096 bytes, called the "device block size". Some implementations + insists on the size to be a multiple of the device block size (particularly + the one in the Linux kernel where the device block size is a configure option). + + The individual parts don't have to be aligned and it is perfectly fine to + cram them together at single byte alignment. + + 2.1) Packing File Data The file data is packed into the archive after the super block (and optional @@ -192,8 +206,33 @@ worst a single metadata block read (at most 8194 bytes). + 2.4) Supported Compressors + + The SquashFS format supports the following compressors: + + - zlib deflate (referred to as "gzip" but only uses raw deflate streams) + - lzo + - lzma 1 + - lzma 2 (referred to as "xz") + - lz4 + - zstd + + The archive can only specify one compressor in the super block and has to use + it for both file data and metadata compression. Using one compressor for data + and switching to a different compressor for e.g. inodes is not supported. - 2) The superblock + A data or metadata block is only stored compressed, if compressing actually + shrinks the input data. If not, the original uncompressed block is stored. + So while it is technically not possible to pick a "null" compressor in the + super block, an implementation can still deliberately write only uncompressed + blocks to a SquashFS file. + + If compatibility with the Linux implementation is desired, the lzma 2 aka xz + compressor should only use CRC32 checksums. The decompressor in the kernel + cannot process the data if checksummed with SHA-256. + + + 3) The superblock ***************** The superblock is the first section of a SquashFS archive. It is always @@ -332,6 +371,11 @@ | | | 0x0010 | Fixed. | +------+-------------------+--------+----------------------------------------+ + Note: If multiple strategies are selected, the SquashFS writer tries all of + them (including not setting any and letting zlib work with defaults) and + selects the resulting block that has the smallest size. + + 3.1.2) XZ +======+===================+=================================================+ @@ -355,6 +399,11 @@ | | | 0x0020 | SPARC | +------+-------------------+--------+----------------------------------------+ + Note: If multiple filters are selected, the SquashFS writer tries all of + them (including not setting any and letting libxz work with defaults) and + selects the resulting block that has the smallest size. + + 3.1.3) LZ4 +======+===================+=================================================+ @@ -435,7 +484,7 @@ In Figure 1, file A consists of 3 blocks and a single tail end, file B has - 2 blocks and one tail end while file 3 is smaller than block size. + 2 blocks and one tail end while file C is smaller than block size. For each file, the blocks are compressed in sequence and stored on disk. @@ -879,7 +928,7 @@ Also note, that the inode type is stored in the entry, but always as a basic type! - + 6.1) Directory Index |