RAID 1
RAID 1 is a way of storing the same data on two or more disks at once, so the array can still be read if one of them fails. It is also called mirroring.
Every block written to a RAID 1 array is written to each disk in the set, so each disk holds a full copy. When one disk fails, the array keeps reading from the copy that remains. The cost is capacity: two mirrored disks hold only one disk’s worth of data.
RAID 1 vs RAID 5: what’s the difference?
RAID 1 keeps complete copies of the data, while RAID 5 spreads data across at least three disks and stores calculated parity instead of a second copy. Both survive one failed disk. On a RAID 1 the surviving disk can be read on its own as an ordinary copy. On a RAID 5 the missing data has to be calculated from all the disks that remain.
What happens when one disk in a RAID 1 fails?
The array keeps working from the other disk, but it is now a degraded array with no copy to spare. If the remaining disk then fails or has unreadable areas, those blocks have no second copy to come from. See RAID 1 data recovery.
What might you notice?
- The controller or NAS reports the array as degraded
- One disk is marked failed, missing or out of sync
- The array is slow while one disk copies to the other
Related terms
Common misconceptions
Does RAID 1 mean I don’t need a backup?
No. A mirror copies every change to both disks, including a deletion, an overwrite or a damaged file system. It protects against one disk failing, not against the data itself going wrong.