156 lines
6.6 KiB
ReStructuredText
156 lines
6.6 KiB
ReStructuredText
Btrfs supports transparent file compression. There are three algorithms
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available: ZLIB, LZO and ZSTD (since v4.14), with various levels.
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The compression happens on the level of file extents and the algorithm is
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selected by file property, mount option or by a defrag command.
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You can have a single btrfs mount point that has some files that are
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uncompressed, some that are compressed with LZO, some with ZLIB, for instance
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(though you may not want it that way, it is supported).
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Once the compression is set, all newly written data will be compressed, ie.
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existing data are untouched. Data are split into smaller chunks (128KiB) before
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compression to make random rewrites possible without a high performance hit. Due
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to the increased number of extents the metadata consumption is higher. The
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chunks are compressed in parallel.
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The algorithms can be characterized as follows regarding the speed/ratio
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trade-offs:
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ZLIB
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* slower, higher compression ratio
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* levels: 1 to 9, mapped directly, default level is 3
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* good backward compatibility
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LZO
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* faster compression and decompression than zlib, worse compression ratio, designed to be fast
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* no levels
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* good backward compatibility
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ZSTD
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* compression comparable to zlib with higher compression/decompression speeds and different ratio
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* levels: 1 to 15, mapped directly (higher levels are not available)
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* since 4.14, levels since 5.1
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The differences depend on the actual data set and cannot be expressed by a
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single number or recommendation. Higher levels consume more CPU time and may
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not bring a significant improvement, lower levels are close to real time.
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How to enable compression
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-------------------------
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Typically the compression can be enabled on the whole filesystem, specified for
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the mount point. Note that the compression mount options are shared among all
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mounts of the same filesystem, either bind mounts or subvolume mounts.
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Please refer to section *MOUNT OPTIONS*.
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.. code-block:: shell
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$ mount -o compress=zstd /dev/sdx /mnt
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This will enable the ``zstd`` algorithm on the default level (which is 3).
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The level can be specified manually too like ``zstd:3``. Higher levels compress
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better at the cost of time. This in turn may cause increased write latency, low
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levels are suitable for real-time compression and on reasonably fast CPU don't
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cause noticeable performance drops.
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.. code-block:: shell
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$ btrfs filesystem defrag -czstd file
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The command above will start defragmentation of the whole *file* and apply
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the compression, regardless of the mount option. (Note: specifying level is not
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yet implemented). The compression algorithm is not persistent and applies only
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to the defragmentation command, for any other writes other compression settings
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apply.
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Persistent settings on a per-file basis can be set in two ways:
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.. code-block:: shell
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$ chattr +c file
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$ btrfs property set file compression zstd
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The first command is using legacy interface of file attributes inherited from
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ext2 filesystem and is not flexible, so by default the *zlib* compression is
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set. The other command sets a property on the file with the given algorithm.
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(Note: setting level that way is not yet implemented.)
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Compression levels
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------------------
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The level support of ZLIB has been added in v4.14, LZO does not support levels
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(the kernel implementation provides only one), ZSTD level support has been added
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in v5.1.
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There are 9 levels of ZLIB supported (1 to 9), mapping 1:1 from the mount option
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to the algorithm defined level. The default is level 3, which provides the
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reasonably good compression ratio and is still reasonably fast. The difference
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in compression gain of levels 7, 8 and 9 is comparable but the higher levels
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take longer.
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The ZSTD support includes levels 1 to 15, a subset of full range of what ZSTD
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provides. Levels 1-3 are real-time, 4-8 slower with improved compression and
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9-15 try even harder though the resulting size may not be significantly improved.
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Level 0 always maps to the default. The compression level does not affect
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compatibility.
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Incompressible data
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-------------------
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Files with already compressed data or with data that won't compress well with
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the CPU and memory constraints of the kernel implementations are using a simple
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decision logic. If the first portion of data being compressed is not smaller
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than the original, the compression of the file is disabled -- unless the
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filesystem is mounted with *compress-force*. In that case compression will
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always be attempted on the file only to be later discarded. This is not optimal
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and subject to optimizations and further development.
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If a file is identified as incompressible, a flag is set (*NOCOMPRESS*) and it's
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sticky. On that file compression won't be performed unless forced. The flag
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can be also set by **chattr +m** (since e2fsprogs 1.46.2) or by properties with
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value *no* or *none*. Empty value will reset it to the default that's currently
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applicable on the mounted filesystem.
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There are two ways to detect incompressible data:
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* actual compression attempt - data are compressed, if the result is not smaller,
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it's discarded, so this depends on the algorithm and level
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* pre-compression heuristics - a quick statistical evaluation on the data is
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performed and based on the result either compression is performed or skipped,
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the NOCOMPRESS bit is not set just by the heuristic, only if the compression
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algorithm does not make an improvement
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.. code-block:: shell
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$ lsattr file
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---------------------m file
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Using the forcing compression is not recommended, the heuristics are
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supposed to decide that and compression algorithms internally detect
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incompressible data too.
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Pre-compression heuristics
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--------------------------
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The heuristics aim to do a few quick statistical tests on the compressed data
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in order to avoid probably costly compression that would turn out to be
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inefficient. Compression algorithms could have internal detection of
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incompressible data too but this leads to more overhead as the compression is
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done in another thread and has to write the data anyway. The heuristic is
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read-only and can utilize cached memory.
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The tests performed based on the following: data sampling, long repeated
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pattern detection, byte frequency, Shannon entropy.
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Compatibility
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-------------
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Compression is done using the COW mechanism so it's incompatible with
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*nodatacow*. Direct IO works on compressed files but will fall back to buffered
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writes and leads to recompression. Currently *nodatasum* and compression don't
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work together.
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The compression algorithms have been added over time so the version
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compatibility should be also considered, together with other tools that may
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access the compressed data like bootloaders.
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