Checksums and Self-Healing Data

With ZFS, all data and metadata is checksummed using a user-selectable algorithm. Traditional file systems that do provide checksumming have performed it on a per-block basis, out of necessity due to the volume management layer and traditional file system design. The traditional design means that certain failure modes, such as writing a complete block to an incorrect location, can result in properly checksummed data that is actually incorrect. ZFS checksums are stored in a way such that these failure modes are detected and can be recovered from gracefully. All checksumming and data recovery is done at the file system layer, and is transparent to applications.

In addition, ZFS provides for self-healing data. ZFS supports storage pools with varying levels of data redundancy. When a bad data block is detected, ZFS fetches the correct data from another redundant copy, and repairs the bad data, replacing it with the good copy.