78 lines
3.3 KiB
ReStructuredText
78 lines
3.3 KiB
ReStructuredText
Data and metadata are checksummed by default, the checksum is calculated before
|
|
write and verifed 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 (**modprobe sha256**) before mounting the filesystem to make use of
|
|
them. You can check in */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 */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
|
|
...
|
|
|