Maintenance vs. Last Erred States

When a slice in a mirror or RAID5 metadevice device experiences errors, DiskSuite puts the slice in the "Maintenance" state. No further reads or writes are performed to a slice in the "Maintenance" state. Subsequent errors on other slices in the same metadevice are handled differently, depending on the type of metadevice. A mirror may be able to tolerate many slices in the "Maintenance" state and still be read from and written to. A RAID5 metadevice, by definition, can only tolerate a single slice in the "Maintenance" state. When either a mirror or RAID5 metadevice has a slice in the "Last Erred" state, I/O is still attempted to the slice marked "Last Erred." This is because a "Last Erred" slice contains the last good copy of data from DiskSuite's point of view. With a slice in the "Last Erred" state, the metadevice behaves like a normal device (disk) and returns I/O errors to an application. Usually, at this point some data has been lost.

Always replace slices in the "Maintenance" state first, followed by those in the "Last Erred" state. After a slice is replaced and resynced, use the metastat(1M) command to verify its state, then validate the data to make sure it is good.

Mirrors: If slices are in the "Maintenance" state, no data has been lost. You can safely replace or enable the slices in any order. If a slice is in the "Last Erred" state, you cannot replace it until you first replace all the other mirrored slices in the "Maintenance" state. Replacing or enabling a slice in the "Last Erred" state usually means that some data has been lost. Be sure to validate the data on the mirror after repairing it.

RAID5 Metadevices: A RAID5 metadevice can tolerate a single slice failure. You can safely replace a single slice in the "Maintenance" state without losing data. If an error on another slice occurs, it is put into the "Last Erred" state. At this point, the RAID5 metadevice is a read-only device; you need to perform some type of error recovery so that the state of the RAID5 metadevice is non-errored and the possibility of data loss is reduced. If a RAID5 metadevice reaches a "Last Erred" state, there is a good chance it has lost data. Be sure to validate the data on the RAID5 metadevice after repairing it.