Hi all,
We're in the process of adding some extra disks to our FAS2050A and I'm having a tough time figuring out the "best" way to allocate these disks (and hand-in-hand with this, choose the "best" RAID group sizes).
Current system is a 2050A with 20 x 144GB disks - 16 data/parity + 1 spare presently allocated to controller #1 (currently out "active" controller), 2 data/parity + 1 spare to controller #2 (currently acting only as a "passive" controller).
Additional disks are 28 x 144GB in two DS14MK4 shelves.
As noted above, I'm trying to plan how we spread these disks across the two controllers, taking into account both our current setup and also possible future expansion.
A few possible possible configs I've come up with - all RGs would use RAID-DP:
Option 1:
Controller #1:
Data: 16 disks (internal shelf) in one 16-disk RG
Spare: 2 disks (internal shelf)
Controller #2:
Data: 28 disks (DS14MK4 shelves) in two 14-disk RGs or one 28-disk RG
Spare: 2 disks (internal shelf)
Option 2:
Controller #1:
Data: 18 disks (internal shelf) in one 18-disk RG
Spare: 2 disks (internal shelf)
Controller #2:
Data: 26 disks (DS14MK4 shelves) in two 13-disk RGs or one 26-disk RG
Spare: 2 disks (DS14MK4 shelves)
Some questions arising out of this:
1) Any other configurations I should consider?
2) Thoughts on RG size for controller #2 - 13/14 disks in below the default, 26/28 is approaching maximum. Obviously the choice is a trade-off of performance & efficiency vs reliability, but can anyone offer some insight into the "best" choice here?
3) Can someone clarify something for me to do with RG sizes and performance please? If we were comparing performance of 2 x 14 disk RGs vs 1 x 28 disk RG, would the former (ignoring parity disks) only give us the performance of a 14-disk stripe (the size of each RG), or would it give us the performance of a 28-disk stripe (the size of the two RGs combined)?
4) The reason for considering option 1, where controller #1 only uses 16 data/parity disks and controller #2 has its spares in the internal shelf, is to do with future expansion.
Assuming we were to add another 28 disks in the future (2 more DS14MK4's) and wanted to add these disks to existing aggregates then I believe option #1 would be better. By adding a whole shelf to controller #1 and reclaiming the two internal-shelf spares assigned to controller #2 we could create a 2 x 16 disk RG aggregate with 2 internal-shelf spares on controller #1, giving us balanced RG sizes and following the best practice recommendation of assinging whole shelves to a single controller. Likewise, the other additional shelf could be added to controller #2 and would provide for 2 replacement spares + expanding controller #2's RGs to 2 x 20 or 1 x 28 and 1 x 12. We would be mixing SAS and FC in one of the controller #1 RGs though... I know we can do this, but not sure if it has performance implications?
On the flip-side, I think option 2 is a better design for us now - it maintains the best-practice recommendation of assigning whole shelves to a single controller with the present setup, and does a slightly better job of balancing the number of disks per controller. It does make further expansion - e.g. adding another 28 disks - tricker though, as we'd need to either go for unbalanced RGs on controller #1 (18 + 14) or break the assign-whole-shelves-to-single-controllers best practice recommendation (2x18 disk RG on controller #1, with the second RG taking a whole shelf + part of another).
I'm guessing the "right" choice here probably comes down to our specific environment + timeframe for additional expansion, but does anyone have any other thoughts on the matter? Or am I over-thinking this, with there being very little real-world difference between the two options?
Thanks all, really appreciate any advice that is forthcoming.
Cheers,
Matt
