Checksums

Checksums and Their Use in ZFS

End-to-end checksums are a key feature of ZFS and an important differentiator for ZFS over other RAID implementations and filesystems. Advantages of end-to-end checksums include:

• detects data corruption upon reading from media
• blocks that are detected as corrupt are automatically repaired if possible, by using the RAID protection in suitably configured pools, or redundant copies (see the zfs copies property)
• periodic scrubs can check data to detect and repair latent media degradation (bit rot) and corruption from other sources
• checksums on ZFS replication streams, zfs send and zfs receive, ensure the data received is not corrupted by intervening storage or transport mechanisms

Checksum Algorithms

The checksum algorithms in ZFS can be changed for datasets (filesystems or volumes). The checksum algorithm used for each block is stored in the block pointer (metadata). The block checksum is calculated when the block is written, so changing the algorithm only affects writes occurring after the change.

The checksum algorithm for a dataset can be changed by setting the checksum property:

zfs set checksum=sha256 pool_name/dataset_name

Checksum	Ok for dedup and nopwrite?	Compatible with other ZFS implementations?	Notes
on	see notes	yes	on is a short hand for fletcher4 for non-deduped datasets and sha256 for deduped datasets
off	no	yes	Do not do use off
fletcher2	no	yes	Deprecated implementation of Fletcher checksum, use fletcher4 instead
fletcher4	no	yes	Fletcher algorithm, also used for zfs send streams
sha256	yes	yes	Default for deduped datasets
noparity	no	yes	Do not use noparity
sha512	yes	requires pool feature org.illumos:sha512	salted sha512 currently not supported for any filesystem on the boot pools
skein	yes	requires pool feature org.illumos:skein	salted skein currently not supported for any filesystem on the boot pools
edonr	yes	requires pool feature org.illumos:edonr	salted edonr currently not supported for any filesystem on the boot pools

Checksum Accelerators

ZFS has the ability to offload checksum operations to the Intel QuickAssist Technology (QAT) adapters.

Checksum Microbenchmarks

Some ZFS features use microbenchmarks when the zfs.ko kernel module is loaded to determine the optimal algorithm for checksums. The results of the microbenchmarks are observable in the /proc/spl/kstat/zfs directory. The winning algorithm is reported as the "fastest" and becomes the default. The default can be overridden by setting zfs module parameters.

Checksum	Results Filename	zfs module parameter
Fletcher4	/proc/spl/kstat/zfs/fletcher_4_bench	zfs_fletcher_4_impl

Disabling Checksums

While it may be tempting to disable checksums to improve CPU performance, it is widely considered by the ZFS community to be an extrodinarily bad idea. Don't disable checksums.