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SQUASHFS 1.3r3 - A squashed read-only filesystem for Linux Copyright 2004 Phillip Lougher (phillip@lougher.demon.co.uk) Released under the GPL licence (version 2 or later). Squashfs is currently at version 1.3 release 3. Please see the CHANGES file for recent changes to squashfs. Squashfs is a highly compressed read-only filesystem for Linux. It uses zlib compression to compress both 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 32K. 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. The section 'mksquashfs' gives information on using the mksquashfs tool to create and append to squashfs filesystems. The 'using squashfs' section gives information on mounting and using squashfs filesystems stored on block devices and as normal files using the loopback device. 1. Squashfs overview -------------------- 1. Data, inodes and directories are compressed. 2. Squashfs stores full uid/gids (32 bits), and file creation time. 3. Files up to 2^32 bytes are supported. Filesystems can be up to 2^32 bytes. 4. 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). 5. Squashfs can use block sizes up to 32K (the default size is 32K). Using 32K blocks achieves greater compression ratios than the normal 4K block size. 6. File duplicates are detected and removed. 7. Both big and little endian architectures are supported. Squashfs can mount filesystems created on different byte order machines. 2. mksquashfs ------------- As squashfs is a read-only filesystem, the mksquashfs program must be used to create populated squashfs filesystems. Beginning with Squashfs 1.2, mksquashfs will also append directories and files to pre-existing squashfs filesystems, see the following 'appending to squashfs filesystems' subsection. SYNTAX:mksquashfs source1 source2 ... dest [options] [-e list of exclude dirs/files] Options are -info print files written to filesystem -b block size size of blocks in filesystem, default 32768 -noappend Do not append to existing filesystem on dest, write a new filesystem This is the default action if dest does not exist, or if no filesystem is on it -keep-as-directory If one source directory is specified, create a root directory containing that directory, rather than the contents of the directory -root-becomes name When appending source files/directories, make the original root become a subdirectory in the new root called name, rather than adding the new source items to the original root -noI -noInodeCompression do not compress inode table -noD -noDataCompression do not compress data blocks -nopad do not pad filesystem to a multiple of 4K -check_data add checkdata for greater filesystem checks -le create a little endian filesystem -be create a big endian filesystem -ef exclude file file is a list of exclude dirs/files - one per line -version print version, licence and copyright message Source1 source2 ... are the source directories/files containing the files/directories that will form the squashfs filesystem. If a single directory is specified (i.e. mksquashfs source output_fs) the squashfs filesystem will consist of that directory, with the top-level root directory corresponding to the source directory. If multiple source directories or files are specified, mksquashfs will merge the specified sources into a single filesystem, with the root directory containing each of the source files/directories. The name of each directory entry will be the basename of the source path. If more than one source entry maps to the same name, the conflicts are named xxx_1, xxx_2, etc. where xxx is the original name. To make this clear, take two example directories. Source directory "/home/phillip/test" contains "file1", "file2" and "dir1". Source directory "goodies" contains "goodies1", "goodies2" and "goodies3". usage example 1: %mksquashfs /home/phillip/test output_fs This will generate a squashfs filesystem with root entries "file1", "file2" and "dir1". example 2: %mksquashfs /home/phillip/test goodies output_fs This will create a squashfs filesystem with the root containing entries "test" and "goodies" corresponding to the source directories "/home/phillip/test" and "goodies". example 3: %mksquashfs /home/phillip/test goodies test output_fs This is the same as the previous example, except a third source directory "test" has been specified. This conflicts with the first directory named "test" and will be renamed "test_1". Multiple sources allow filesystems to be generated without needing to copy all source files into a common directory. This simplifies creating filesystems. The -keep-as-directory option can be used when only one source directory is specified, and you wish the root to contain that directory, rather than the contents of the directory. For example: example 4: %mksquashfs /home/phillip/test output_fs -keep-as-directory This is the same as example 1, except for -keep-as-directory. This will generate a root directory containing directory "test", rather than the "test" directory contents "file1", "file2" and "dir1". The Dest argument is the destination where the squashfs filesystem will be written. This can either be a conventional file or a block device. If the file doesn't exist it will be created, if it does exist and a squashfs filesystem exists on it, mksquashfs will append. The -noappend option will write a new filesystem irrespective of whether an existing filesystem is present. The -e and -ef options allow files/directories to be specified which are excluded from the output filesystem. The -e option takes the exclude files/directories from the command line, the -ef option takes the exlude files/directories from the specified exclude file, one file/directory per line. If an exclude file/directory is absolute (i.e. prefixed with /, ../, or ./) the entry is treated as absolute, however, if an exclude file/directory is relative, it is treated as being relative to each of the sources in turn, i.e. %mksquashfs /tmp/source1 source2 output_fs -e ex1 /tmp/source1/ex2 out/ex3 Will generate exclude files /tmp/source1/ex2, /tmp/source1/ex1, source2/ex1, /tmp/source1/out/ex3 and source2/out/ex3. The -e and -ef exclude options are usefully used in archiving the entire filesystem, where it is wished to avoid archiving /proc, and the filesystem being generated, i.e. %mksquashfs / /tmp/root.sqsh -e proc /tmp/root.sqsh Multiple -ef options can be specified on the command line, and the -ef option can be used in conjuction with the -e option. The -info option displays the files/directories as they are compressed and added to the filesystem. The compression percentage achieved is printed, with the original uncompressed size. If the compression percentage is listed as 0% it means the file is a duplicate. The -b option allows the block size to be selected, this can be either 512, 1024, 2048, 4096, 8192, 16384, or 32768 bytes. The -noI and -noD options (also -noInodeCompression and -noDataCompression) can be used to force mksquashfs to not compress inodes/directories and data respectively. Giving both options generates an uncompressed filesystem. The -le and -be options can be used to force mksquashfs to generate a little endian or big endian filesystem. Normally mksquashfs will generate a filesystem in the host byte order. Squashfs, for portability, will mount different ordered filesystems (i.e. it can mount big endian filesystems running on a little endian machine), but these options can be used for greater optimisation. The -nopad option informs mksquashfs to not pad the filesystem to a 4K multiple. This is performed by default to enable the output filesystem file to be mounted by loopback, which requires files to be a 4K multiple. If the filesystem is being written to a block device, or is to be stored in a bootimage, the extra pad bytes are not needed. 2.1 appending to squashfs filesystems ------------------------------------- Beginning with squashfs1.2, mksquashfs can append to existing squashfs filesystems. Three extra options "-noappend", "-keep-as-directory", and "root-becomes" have been added. Running squashfs with the destination directory containing an existing filesystem, will add the source items to the existing filesystem. By default, the source items are added to the existing root directory. To make this clear... An existing filesystem "image" contains root entries "old1", and "old2". Source directory "/home/phillip/test" contains "file1", "file2" and "dir1". example 1: %mksquashfs /home/phillip/test image Will create a new "image" with root entries "old1", "old2", "file1", "file2" and "dir1" example 2: %mksquashfs /home/phillip/test image -keep-as-directory Will create a new "image" with root entries "old1", "old2", and "test". As shown in the previous section, for single source directories '-keep-as-directory' adds the source directory rather than the contents of the directory. example 3: %mksquashfs /home/phillip/test image -keep-as-directory -root-becomes original-root Will create a new "image" with root entries "original-root", and "test". The '-root-becomes' option specifies that the original root becomes a subdirectory in the new root, with the specified name. The append option with file duplicate detection, means squashfs can be used as a simple versioning archiving filesystem. A squashfs filesystem can be created with for example the linux-2.4.19 source. Appending the linux-2.4.20 source will create a filesystem with the two source trees, but only the changed files will take extra room, the unchanged files will be detected as duplicates. 3. Using squashfs ----------------- Squashfs filesystems should be mounted with 'mount' with the filesystem type 'squashfs'. If the filesystem is on a block device, the filesystem can be mounted directly, e.g. %mount -t squashfs /dev/sda1 /mnt Will mount the squashfs filesystem on "/dev/sda1" under the directory "/mnt". If the squashfs filesystem has been written to a file, the loopback device can be used to mount it (loopback support must be in the kernel), e.g. %mount -t squashfs image /mnt -o loop Will mount the squashfs filesystem in the file "image" under the directory "/mnt". 4. Filesystem layout -------------------- Brief filesystem design notes follow. A squashfs filesystem consists of five parts, packed together on a byte alignment: --------------- | superblock | |---------------| | data | | blocks | |---------------| | inodes | |---------------| | directories | |---------------| | uid/gid | | lookup 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 and uid/gid lookup tables are written. 4.1 Metadata ------------ 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. An inode is identified by a two field tuple . Inode contents vary depending on the file type. The base inode consists of: base inode: Inode type Mode uid index gid index The inode type is 4 bits in size, and the mode is 12 bits. The uid and gid indexes are 4 bits in length. Ordinarily, this will allow 16 unique indexes into the uid table. To minimise overhead, the uid index is used in conjunction with the spare bit in the file type to form a 48 entry index as follows: inode type 1 - 5: uid index = uid inode type 5 -10: uid index = 16 + uid inode type 11 - 15: uid index = 32 + uid In this way 48 unique uids are supported using 4 bits, minimising data inode overhead. The 4 bit gid index is used to index into a 15 entry gid table. Gid index 15 is used to indicate that the gid is the same as the uid. This prevents the 15 entry gid table filling up with the common case where the uid/gid is the same. The data contents of symbolic links are stored immediately after the symbolic link inode, inside the inode table. This allows the normally small symbolic link to be compressed as part of the inode table, achieving much greater compression than if the symbolic link was compressed individually. Similarly, the block index for regular files is stored immediately after the regular file inode. The block index is a list of block lengths (two bytes each), rather than block addresses, saving two bytes per block. The block address for a given block is computed by the summation of the previous block lengths. This takes advantage of the fact that the blocks making up a file are stored contiguously in the filesystem. The top bit of each block length is set if the block is uncompressed, either because the -noD option is set, or if the compressed block was larger than the uncompressed block. 4.2 Directories --------------- Like inodes, directories are packed into the metadata blocks, and are not aligned on block boundaries, therefore directories can overlap compressed blocks. A directory is, again, identified by a two field tuple . Directories are organised in a slightly complex way, and are not simply a list of file names and inode tuples. The organisation takes advantage of the observation that in most cases, the inodes of the files in the directory will be in the same compressed metadata block, and therefore, the inode tuples will have the same 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. The organisation is as follows: directory_header: count (8 bits) inode start block (24 bits) directory entry: * count inode offset (13 bits) inode type (3 bits) filename size (8 bits) filename This organisation saves on average 3 bytes per filename. 4.3 File data ------------- File data is compressed on a block by block basis and written to the filesystem. The filesystem supports up to 32K blocks, which achieves greater compression ratios than the Linux 4K page size. The disadvantage with using greater than 4K blocks (and the reason why most filesystems do not), is that the VFS reads data in 4K pages. The filesystem reads and decompresses a larger block containing that page (e.g. 32K). However, only 4K can be returned to the VFS, resulting in a very inefficient filesystem, as 28K must be thrown away. Squashfs, solves this problem by explicitly pushing the extra pages into the page cache. 5. Author info -------------- Squashfs was written by Phillip Lougher, email phillip@lougher.demon.co.uk, in Chepstow, Wales, UK. If you like the program, or have any problems, then please email me, as it's nice to get feedback!