mirror of
https://github.com/AuxXxilium/linux_dsm_epyc7002.git
synced 2024-11-24 22:40:53 +07:00
89cab5b572
One paragraph was not updated when xattrs were added to Squashfs. Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
260 lines
10 KiB
Plaintext
260 lines
10 KiB
Plaintext
SQUASHFS 4.0 FILESYSTEM
|
|
=======================
|
|
|
|
Squashfs is a compressed read-only filesystem for Linux.
|
|
It uses zlib/lzo/xz compression to compress files, inodes and directories.
|
|
Inodes in the system are very small and all blocks are packed to minimise
|
|
data overhead. Block sizes greater than 4K are supported up to a maximum
|
|
of 1Mbytes (default block size 128K).
|
|
|
|
Squashfs is intended for general read-only filesystem use, for archival
|
|
use (i.e. in cases where a .tar.gz file may be used), and in constrained
|
|
block device/memory systems (e.g. embedded systems) where low overhead is
|
|
needed.
|
|
|
|
Mailing list: squashfs-devel@lists.sourceforge.net
|
|
Web site: www.squashfs.org
|
|
|
|
1. FILESYSTEM FEATURES
|
|
----------------------
|
|
|
|
Squashfs filesystem features versus Cramfs:
|
|
|
|
Squashfs Cramfs
|
|
|
|
Max filesystem size: 2^64 256 MiB
|
|
Max file size: ~ 2 TiB 16 MiB
|
|
Max files: unlimited unlimited
|
|
Max directories: unlimited unlimited
|
|
Max entries per directory: unlimited unlimited
|
|
Max block size: 1 MiB 4 KiB
|
|
Metadata compression: yes no
|
|
Directory indexes: yes no
|
|
Sparse file support: yes no
|
|
Tail-end packing (fragments): yes no
|
|
Exportable (NFS etc.): yes no
|
|
Hard link support: yes no
|
|
"." and ".." in readdir: yes no
|
|
Real inode numbers: yes no
|
|
32-bit uids/gids: yes no
|
|
File creation time: yes no
|
|
Xattr support: yes no
|
|
ACL support: no no
|
|
|
|
Squashfs compresses data, inodes and directories. In addition, inode and
|
|
directory data are highly compacted, and packed on byte boundaries. Each
|
|
compressed inode is on average 8 bytes in length (the exact length varies on
|
|
file type, i.e. regular file, directory, symbolic link, and block/char device
|
|
inodes have different sizes).
|
|
|
|
2. USING SQUASHFS
|
|
-----------------
|
|
|
|
As squashfs is a read-only filesystem, the mksquashfs program must be used to
|
|
create populated squashfs filesystems. This and other squashfs utilities
|
|
can be obtained from http://www.squashfs.org. Usage instructions can be
|
|
obtained from this site also.
|
|
|
|
The squashfs-tools development tree is now located on kernel.org
|
|
git://git.kernel.org/pub/scm/fs/squashfs/squashfs-tools.git
|
|
|
|
3. SQUASHFS FILESYSTEM DESIGN
|
|
-----------------------------
|
|
|
|
A squashfs filesystem consists of a maximum of nine parts, packed together on a
|
|
byte alignment:
|
|
|
|
---------------
|
|
| superblock |
|
|
|---------------|
|
|
| compression |
|
|
| options |
|
|
|---------------|
|
|
| datablocks |
|
|
| & fragments |
|
|
|---------------|
|
|
| inode table |
|
|
|---------------|
|
|
| directory |
|
|
| table |
|
|
|---------------|
|
|
| fragment |
|
|
| table |
|
|
|---------------|
|
|
| export |
|
|
| table |
|
|
|---------------|
|
|
| uid/gid |
|
|
| lookup table |
|
|
|---------------|
|
|
| xattr |
|
|
| table |
|
|
---------------
|
|
|
|
Compressed data blocks are written to the filesystem as files are read from
|
|
the source directory, and checked for duplicates. Once all file data has been
|
|
written the completed inode, directory, fragment, export, uid/gid lookup and
|
|
xattr tables are written.
|
|
|
|
3.1 Compression options
|
|
-----------------------
|
|
|
|
Compressors can optionally support compression specific options (e.g.
|
|
dictionary size). If non-default compression options have been used, then
|
|
these are stored here.
|
|
|
|
3.2 Inodes
|
|
----------
|
|
|
|
Metadata (inodes and directories) are compressed in 8Kbyte blocks. Each
|
|
compressed block is prefixed by a two byte length, the top bit is set if the
|
|
block is uncompressed. A block will be uncompressed if the -noI option is set,
|
|
or if the compressed block was larger than the uncompressed block.
|
|
|
|
Inodes are packed into the metadata blocks, and are not aligned to block
|
|
boundaries, therefore inodes overlap compressed blocks. Inodes are identified
|
|
by a 48-bit number which encodes the location of the compressed metadata block
|
|
containing the inode, and the byte offset into that block where the inode is
|
|
placed (<block, offset>).
|
|
|
|
To maximise compression there are different inodes for each file type
|
|
(regular file, directory, device, etc.), the inode contents and length
|
|
varying with the type.
|
|
|
|
To further maximise compression, two types of regular file inode and
|
|
directory inode are defined: inodes optimised for frequently occurring
|
|
regular files and directories, and extended types where extra
|
|
information has to be stored.
|
|
|
|
3.3 Directories
|
|
---------------
|
|
|
|
Like inodes, directories are packed into compressed metadata blocks, stored
|
|
in a directory table. Directories are accessed using the start address of
|
|
the metablock containing the directory and the offset into the
|
|
decompressed block (<block, offset>).
|
|
|
|
Directories are organised in a slightly complex way, and are not simply
|
|
a list of file names. The organisation takes advantage of the
|
|
fact that (in most cases) the inodes of the files will be in the same
|
|
compressed metadata block, and therefore, can share the start block.
|
|
Directories are therefore organised in a two level list, a directory
|
|
header containing the shared start block value, and a sequence of directory
|
|
entries, each of which share the shared start block. A new directory header
|
|
is written once/if the inode start block changes. The directory
|
|
header/directory entry list is repeated as many times as necessary.
|
|
|
|
Directories are sorted, and can contain a directory index to speed up
|
|
file lookup. Directory indexes store one entry per metablock, each entry
|
|
storing the index/filename mapping to the first directory header
|
|
in each metadata block. Directories are sorted in alphabetical order,
|
|
and at lookup the index is scanned linearly looking for the first filename
|
|
alphabetically larger than the filename being looked up. At this point the
|
|
location of the metadata block the filename is in has been found.
|
|
The general idea of the index is to ensure only one metadata block needs to be
|
|
decompressed to do a lookup irrespective of the length of the directory.
|
|
This scheme has the advantage that it doesn't require extra memory overhead
|
|
and doesn't require much extra storage on disk.
|
|
|
|
3.4 File data
|
|
-------------
|
|
|
|
Regular files consist of a sequence of contiguous compressed blocks, and/or a
|
|
compressed fragment block (tail-end packed block). The compressed size
|
|
of each datablock is stored in a block list contained within the
|
|
file inode.
|
|
|
|
To speed up access to datablocks when reading 'large' files (256 Mbytes or
|
|
larger), the code implements an index cache that caches the mapping from
|
|
block index to datablock location on disk.
|
|
|
|
The index cache allows Squashfs to handle large files (up to 1.75 TiB) while
|
|
retaining a simple and space-efficient block list on disk. The cache
|
|
is split into slots, caching up to eight 224 GiB files (128 KiB blocks).
|
|
Larger files use multiple slots, with 1.75 TiB files using all 8 slots.
|
|
The index cache is designed to be memory efficient, and by default uses
|
|
16 KiB.
|
|
|
|
3.5 Fragment lookup table
|
|
-------------------------
|
|
|
|
Regular files can contain a fragment index which is mapped to a fragment
|
|
location on disk and compressed size using a fragment lookup table. This
|
|
fragment lookup table is itself stored compressed into metadata blocks.
|
|
A second index table is used to locate these. This second index table for
|
|
speed of access (and because it is small) is read at mount time and cached
|
|
in memory.
|
|
|
|
3.6 Uid/gid lookup table
|
|
------------------------
|
|
|
|
For space efficiency regular files store uid and gid indexes, which are
|
|
converted to 32-bit uids/gids using an id look up table. This table is
|
|
stored compressed into metadata blocks. A second index table is used to
|
|
locate these. This second index table for speed of access (and because it
|
|
is small) is read at mount time and cached in memory.
|
|
|
|
3.7 Export table
|
|
----------------
|
|
|
|
To enable Squashfs filesystems to be exportable (via NFS etc.) filesystems
|
|
can optionally (disabled with the -no-exports Mksquashfs option) contain
|
|
an inode number to inode disk location lookup table. This is required to
|
|
enable Squashfs to map inode numbers passed in filehandles to the inode
|
|
location on disk, which is necessary when the export code reinstantiates
|
|
expired/flushed inodes.
|
|
|
|
This table is stored compressed into metadata blocks. A second index table is
|
|
used to locate these. This second index table for speed of access (and because
|
|
it is small) is read at mount time and cached in memory.
|
|
|
|
3.8 Xattr table
|
|
---------------
|
|
|
|
The xattr table contains extended attributes for each inode. The xattrs
|
|
for each inode are stored in a list, each list entry containing a type,
|
|
name and value field. The type field encodes the xattr prefix
|
|
("user.", "trusted." etc) and it also encodes how the name/value fields
|
|
should be interpreted. Currently the type indicates whether the value
|
|
is stored inline (in which case the value field contains the xattr value),
|
|
or if it is stored out of line (in which case the value field stores a
|
|
reference to where the actual value is stored). This allows large values
|
|
to be stored out of line improving scanning and lookup performance and it
|
|
also allows values to be de-duplicated, the value being stored once, and
|
|
all other occurrences holding an out of line reference to that value.
|
|
|
|
The xattr lists are packed into compressed 8K metadata blocks.
|
|
To reduce overhead in inodes, rather than storing the on-disk
|
|
location of the xattr list inside each inode, a 32-bit xattr id
|
|
is stored. This xattr id is mapped into the location of the xattr
|
|
list using a second xattr id lookup table.
|
|
|
|
4. TODOS AND OUTSTANDING ISSUES
|
|
-------------------------------
|
|
|
|
4.1 Todo list
|
|
-------------
|
|
|
|
Implement ACL support.
|
|
|
|
4.2 Squashfs internal cache
|
|
---------------------------
|
|
|
|
Blocks in Squashfs are compressed. To avoid repeatedly decompressing
|
|
recently accessed data Squashfs uses two small metadata and fragment caches.
|
|
|
|
The cache is not used for file datablocks, these are decompressed and cached in
|
|
the page-cache in the normal way. The cache is used to temporarily cache
|
|
fragment and metadata blocks which have been read as a result of a metadata
|
|
(i.e. inode or directory) or fragment access. Because metadata and fragments
|
|
are packed together into blocks (to gain greater compression) the read of a
|
|
particular piece of metadata or fragment will retrieve other metadata/fragments
|
|
which have been packed with it, these because of locality-of-reference may be
|
|
read in the near future. Temporarily caching them ensures they are available
|
|
for near future access without requiring an additional read and decompress.
|
|
|
|
In the future this internal cache may be replaced with an implementation which
|
|
uses the kernel page cache. Because the page cache operates on page sized
|
|
units this may introduce additional complexity in terms of locking and
|
|
associated race conditions.
|