Redundancy

Two-disk failures are common enough that raidz1 vdevs should not be used for data storage in production. 2-disk mirrors are also at risk, but not quite as much as raidz1 vdevs unless only two disks are used. This is because the failure of all disks in a 2-disk mirror is statistically less likely than the failure of only 2 disks in a parity configuration such as raidz1, unless only 2 disks are used, where the failure risk is the same. The same risks also apply to RAID 1 and RAID 5.

Single disk vdevs (excluding log and cache devices) should never be used in production because they are not redundant. However, pools containing single disk vdevs can be made redundant by attaching disks via zpool attach to convert the disks into mirrors. For production pools, it is important not to create top level vdevs that are not raidz, mirror, log, cache or spare vdevs.