Losing the number of disks with raid and hot spare

[alihahaie]

hi all,
My QNAP have 16 disk.
it have 2 strorage pool with raid 6 and in each pool 8 disks.
also set disk 8 and 16 to hot spare .
Therefore, we don't have 3 disks per pool!
The question is this:
When raid 6 is configured and two disks are reserved for it for possible problems(in each pool), then what is the purpose of allocating another disk for hot spare?
thanks

[FSC830]

Unclear description!

...
it have 2 strorage pool with raid 6 and in each pool 8 disks.
...
Therefore, we don't have 3 disks per pool!
...

So how can a pool contains 8 disk if disk 8 and 16 set to hotspare?
Each pool contains 7 disks according to this description!
Raid6 in a 7-disk pool means 5 data disks and 2 parity disks, so again the "we dont have 3 disks per pool" is very unclear?

If you decide to use a hot spare (even if raid1/5/6/10/50 is used), you minimize the time a raid is degraded.
The rebuild starts immediately, so the raid is fully functional earlier than waiting until a new disk is delivered and the broken disk replaced.
You need to judge if an additional hot spare is worth the loss of capacity.
Anyhow, a hot spare do not protect in the case a second/third (depends to used raid level)fails during a rebuild!

And keep in mind: a raid is not a replacement for a backup!

Regards