RAID 5
RAID 5 is a way of storing data across at least three disks so the set can still be read after any one disk fails. RAID means redundant array of independent disks.
The data is split into stripes and spread across the disks. For each stripe the array also stores parity, a calculated value that can rebuild one missing piece. That parity is rotated, so no single disk holds all of it. The array can lose one disk and still return the data by recalculating what was on the missing disk. It cannot do that once two disks are missing.
How does RAID 5 store data?
Each write is broken into blocks. Those blocks, plus the parity for that stripe, are written to different disks. With equal-sized disks, usable space is roughly the number of disks minus one, times the smallest disk, before formatting overhead. Three disks of the same size provide about as much usable space as two of them.
Why does a second disk failure matter?
A second failure means the missing pieces can no longer be calculated. With one disk already missing, every read of the missing blocks depends on every disk that remains, and there is no second copy of the parity, so if another disk then has a physical failure, or cannot be read, nothing is left to calculate them from.
What should you do if a RAID 5 array reports a failed disk?
Don't start a rebuild until the remaining disks have been copied. An array that is already missing one disk has no redundancy left, so a rebuild at that point can turn a second fault into data loss.
Recovery of a failed RAID 5 array is the subject of the RAID 5 data recovery page.
What might you notice?
- The array reports itself as degraded, or shows one disk offline
- A rebuild has started, or the controller is offering one
- A second disk has stopped responding
- The volume was available yesterday and is missing today
Related terms
Common misconceptions
Is RAID 5 a backup?
No. RAID 5 tolerates one failed disk while the array is still in use. It does not keep an earlier copy of a file you deleted, and it does not protect a file system that has been formatted. A backup is a separate copy, kept somewhere else.
Is the array safe while it is still online?
Not fully. An array that is already missing one disk is degraded. It can still serve files, and it has nothing in reserve if a second disk fails.