btrfs-progs/Documentation/ch-checksumming.rst
David Sterba a0137082de btrfs-progs: docs: formatting updates
- use :file: and :command:
- simplify manual page references
- add more web links
- typo fixes
- more cross-references

Signed-off-by: David Sterba <dsterba@suse.com>
2023-07-26 14:59:10 +02:00

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Data and metadata are checksummed by default, the checksum is calculated before
write and verified after reading the blocks from devices. The whole metadata
block has a checksum stored inline in the b-tree node header, each data block
has a detached checksum stored in the checksum tree.
There are several checksum algorithms supported. The default and backward
compatible is *crc32c*. Since kernel 5.5 there are three more with different
characteristics and trade-offs regarding speed and strength. The following list
may help you to decide which one to select.
CRC32C (32bit digest)
default, best backward compatibility, very fast, modern CPUs have
instruction-level support, not collision-resistant but still good error
detection capabilities
XXHASH (64bit digest)
can be used as CRC32C successor, very fast, optimized for modern CPUs utilizing
instruction pipelining, good collision resistance and error detection
SHA256 (256bit digest)
a cryptographic-strength hash, relatively slow but with possible CPU
instruction acceleration or specialized hardware cards, FIPS certified and
in wide use
BLAKE2b (256bit digest)
a cryptographic-strength hash, relatively fast with possible CPU acceleration
using SIMD extensions, not standardized but based on BLAKE which was a SHA3
finalist, in wide use, the algorithm used is BLAKE2b-256 that's optimized for
64bit platforms
The *digest size* affects overall size of data block checksums stored in the
filesystem. The metadata blocks have a fixed area up to 256 bits (32 bytes), so
there's no increase. Each data block has a separate checksum stored, with
additional overhead of the b-tree leaves.
Approximate relative performance of the algorithms, measured against CRC32C
using reference software implementations on a 3.5GHz intel CPU:
======== ============ ======= ================
Digest Cycles/4KiB Ratio Implementation
======== ============ ======= ================
CRC32C 1700 1.00 CPU instruction
XXHASH 2500 1.44 reference impl.
SHA256 105000 61 reference impl.
SHA256 36000 21 libgcrypt/AVX2
SHA256 63000 37 libsodium/AVX2
BLAKE2b 22000 13 reference impl.
BLAKE2b 19000 11 libgcrypt/AVX2
BLAKE2b 19000 11 libsodium/AVX2
======== ============ ======= ================
Many kernels are configured with SHA256 as built-in and not as a module.
The accelerated versions are however provided by the modules and must be loaded
explicitly (:command:`modprobe sha256`) before mounting the filesystem to make use of
them. You can check in :file:`/sys/fs/btrfs/FSID/checksum` which one is used. If you
see *sha256-generic*, then you may want to unmount and mount the filesystem
again, changing that on a mounted filesystem is not possible.
Check the file :file:`/proc/crypto`, when the implementation is built-in, you'd find
.. code-block:: none
name : sha256
driver : sha256-generic
module : kernel
priority : 100
...
while accelerated implementation is e.g.
.. code-block:: none
name : sha256
driver : sha256-avx2
module : sha256_ssse3
priority : 170
...