RAID consideration for 24 Disk Array

Question

I have here a 24 Disk Array with 3.7 TB disks in it. What would be, performance wise a good configuration when using a RAID 6: RAID 6 over all 24 disks or should I use 2x12 disk RAID 6 and than a RAID 0?

I'm not so interested in conversation about the RAID level itself (e.g. 5 or 6 or 10) but more about the arrangement of the disks. If it would be better using multiple smaller RAID groups or one big RAID group for example... what's here the best practice?

Best.

You might want to read the answers to this question. It's not identical, but most of the answers are directly applicable to your situation as well. — MadHatter, Jul 15 '14 at 09:04
What type of disks are installed (dictates the failure and design considerations)? What do you plan to use the storage for (dictates performance requirements)? How will the storage array be connected to the server (to determine throughout and backplane/controller limits)? — ewwhite, Jul 15 '14 at 09:18
@Dani_l: Yes I ment TB not GB, just edited. Ok I see the number of disk in a RAID group depends first on the controller (support wise) and second on the rebuild time if a failed disk gets replaced. — mazebuhu, Jul 17 '14 at 12:03
Possible duplicate of: What are the different widely used RAID levels and when should I consider them? — Chris S, Jul 22 '14 at 14:16

ewwhite · Accepted Answer · 2014-07-15T09:48:00.837

Some of this hinges on the hardware involved. I prefer RAID 1+0 for simplicity and rebuild times. It's tough to give a generic answer without more details, though...

Things to consider:

The disks installed in the system: SAS? SATA? Nearline SAS? This impacts the failure rate and failure mode, as well as array rebuild times.
The anticipated use for storage: Your performance requirements may drive the design. Random I/O? Sequential? Read-biased? Write-biased?
Interconnects: How will the storage array be connected to the server? SAS? Will you be using a single connection to an HBA? Two? Multipath? 3Gbps? 6Gbps? There will be a ceiling in storage throughput because of SAS oversubscription. So this factors into the design because of that performance cap.
Controller: I always come from an HP SmartArray perspective, but I suppose the rest of the world uses LSI and PERC controllers. This may be a moot discussion, as LSI controllers can't have more than 16 disks in a single-level virtual drive; e.g. you wouldn't be able to create a 24-disk RAID6 volume. You can do this with HP controllers, though.
Resiliency: Do you plan to have online spares? When you consider a nested RAID level like 60, that becomes important.

So, assuming a controller capable of both. Your options are really 4 x 6-disk RAID6+0, 3 x 8-disk RAID6+0, 2 x 12-disk RAID6+0 and a 24-disk RAID6.

Determine the space needs, as they vary. Then evaluate the sequential performance capabilities of each. I'd suggest 3 x 8-disk as a reasonable if you go nested and aren't interested in RAID 1+0.

IMO the controller plays a greater role in the decision than mere raid-group size. Memory and concurrency features of the controller directly determine how well it can handle multichannel commands in parallel required for efficient nested raids, and see http://arecaraid.cineraid.com/forum/viewtopic.php?f=7&t=5&sid=ee7d848b3da5045d79eb6b9a2f2b93a3 (how do I make links in comments?). The other issue is that on a 8x3 raid 60 you only have 18 working spindles for any operation compared to 22 in raid6. See link examples of greater performance in raid6 over raid60 when controller is understrength. — Dani_l, Jul 15 '14 at 13:08

score 0 · Answer 2 · answered Jul 15 '14 at 09:32

One RAID6:

Advantages:

You only lose 2 Disks for parity (maximizes space)

Disadvantages:

Writes are fairly slow
You only have 2 Disks for parity in a 24-disk array, you can only survive 2 concurrent disk failures

RAID60:

Advantages

Faster writes
You can survive up to 2* <number of sub-raids> disk failures
Lower chance of a sub-raid failing because of lesser number of disks used

Disadvantages

You will lose 2* <number of sub-raids> on parity disks (less space)

I usually prefer safety of data over performance, and safety/performance over disk space.

So I would probably even go with 3x RAID6 and also account for Hot Spare(s)

RAID consideration for 24 Disk Array

2 Answers2