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authorDavid Oberhollenzer <david.oberhollenzer@sigma-star.at>2020-01-11 21:24:38 +0100
committerDavid Oberhollenzer <david.oberhollenzer@sigma-star.at>2020-01-11 21:24:38 +0100
commit525d582aea7d42231c8641b19f38881ee43133ea (patch)
treeccc0a2720050d0725cd8bbddd82c5fe53195a380
parentee5424c7e98ffbc7ac094b1c8db61bf8889d0981 (diff)
Some clarifications and fixes for the format specification
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>
-rw-r--r--doc/format.txt59
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