What is "Raid 5 + Spare" (excerpt from User Manual, Sect 4.17.2, P.54):
RAID5+Spare: RAID 5+Spare is a RAID 5 array in which one disk is
used as spare to rebuild the system as soon as a disk fails (Fig. 79).
At least four disks are required. If one physical disk fails, the data
remains available because it is read from the parity blocks. Data
from a failed disk is rebuilt onto the hot spare disk. When a failed
disk is replaced, the replacement becomes the new hot spare. No
data is lost in the case of a single disk failure, but if a second disk
fails before the system can rebuild data to the hot spare, all data in
the array will be lost.
What is "Raid 6" (excerpt from User Manual, Sect 4.17.2, P.54):
RAID6: In RAID 6, data is striped across all disks (minimum of four)
and a two parity blocks for each data block (p and q in Fig. 80) is
written on the same stripe. If one physical disk fails, the data from
the failed disk can be rebuilt onto a replacement disk. This Raid
mode can support up to two disk failures with no data loss. RAID 6
provides for faster rebuilding of data from a failed disk.
Both "Raid 5 + spare" and "Raid 6" are SO similar … I can't tell the difference.
When would "Raid 5 + Spare" be optimal?
And when would "Raid 6" be optimal"?
The manual dumbs down the different raid with 5 star ratings. "Raid 5 + Spare" only gets 4 stars but "Raid 6" gets 5 stars. If I were to blindly trust the manual I would conclude that "Raid 6" is always better. Is "Raid 6" always better?
Best Answer
In short:
If safety is your main concern then go with RAID6 as it can survive any two drives failing at the same time. If a drive fails in an R5+spare arrangement you are not safe from another failure until the spare has been brought up to speed which could take quite some time with large drives (and it is not unheard of for a drive that has been powered down for ages, such as your spare, to fail to spin up when finally called upon).
If performance is king, go with 5+spare as the write performance will be better when the array is not in a degraded state - though the performance difference between R5 and R6 is significantly smaller than the difference between R5 and other solutions if you have a good controller (i.e. once that makes a partial block write operation "two/three concurrent reads then parity calc then two/three concurrent writes" most of the time rather than "read-then-read(-then-read)-then-parity-calc-then-write-then-write(-then-write)" which is what some very cheap controllers and software RAID may do.
Edit: I missed a potentially important point first time around: