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