Over time, single bit errors in stored data occur more often than you’d expect. The industry calls this “bit-rot”. Traditional volume managers won’t notice when bit-rot occurs, but TrueNAS will proactively detect and repair data corruption.
During a read, If the data is corrupted, TrueNAS will read from redundant storage and automatically repair the bad data. If the data is infrequently read then when TrueNAS performs an automatic disk scrub, the corrupted data will be fixed. Under no circumstances will TrueNAS return bad data as though it were good.
During a write, changed data is written to a completely new location on disk, thereby preserving the original data. This makes advanced snapshot features possible and prevents data loss in the case of an incomplete write.