Guide · Reliability
RAID Rebuild Risk and UREs
When a disk in a single-parity array fails, recovery reads every block on every surviving disk to reconstruct the lost one. The bigger and more numerous your disks, the more data that rebuild has to read flawlessly, and the longer the array runs with no remaining redundancy.
What a URE is
Consumer drives are rated for roughly one unrecoverable read error per 10^14 bits read. During a full-array rebuild you may read enough data that the odds of hitting one such error climb from negligible to meaningful. On a single-parity array, a URE mid-rebuild can fail the rebuild and cost the whole array.
Why drive size made this worse
A 2 TB rebuild reads little; a 16 TB rebuild reads eight times as much, over many more hours, all while you have zero redundancy left. That is why single parity (RAID 5, RAIDZ1) is now discouraged for large modern drives.
The fix
Single parity (RAID 5, RAIDZ1) on large modern drives can lose the whole array if a URE or second failure hits mid-rebuild.
Use double parity, RAID 6 or RAIDZ2, so a second failure or a URE during rebuild is survivable. The calculator warns when your configuration falls into the risky single-parity-with-large-drives zone. See also ZFS RAIDZ explained.