Determining If a Device Can Be Replaced

If the device to be replaced is part of a redundant configuration, sufficient replicas from which to retrieve good data must exist. For example, if two disks in a four-way mirror are UNAVAIL, then either disk can be replaced because healthy replicas are available. However, if two disks in a four-way RAID-Z (raidz1) virtual device are UNAVAIL, then neither disk can be replaced because insufficient replicas from which to retrieve data exist. If the device is damaged but otherwise online, it can be replaced as long as the pool is not in the UNAVAIL state. However, any corrupted data on the device is copied to the new device, unless sufficient replicas with good data exist.

In the following configuration, the c1t1d0 disk can be replaced, and any data in the pool is copied from the healthy replica, c1t0d0:

    mirror            DEGRADED
c1t0d0             ONLINE
c1t1d0             UNAVAIL

The c1t0d0 disk can also be replaced, though no self-healing of data can take place because no good replica is available.

In the following configuration, neither UNAVAIL disk can be replaced. The ONLINE disks cannot be replaced either because the pool itself is UNAVAIL.

    raidz1              UNAVAIL
c1t0d0              ONLINE
c2t0d0              UNAVAIL
c3t0d0              UNAVAIL
c4t0d0              ONLINE

In the following configuration, either top-level disk can be replaced, though any bad data present on the disk is copied to the new disk.

c1t0d0         ONLINE
c1t1d0         ONLINE

If either disk is UNAVAIL, then no replacement can be performed because the pool itself is UNAVAIL.