Clarification on logical array and logical drives.

prxhi — Thu, 13 Nov 2008 04:16:14 GMT

1 x MSA1500 Controller
3 x MSA20 Enclosures
36 x 250GB SATA HDs

I want to maximize the amount of disk space but it seems that there's a 2TB logical drive limitation. When I create an 11 disk array + 1 spare (one full enclosure) and try to create a RAID 5 logical drive, I'm left with a decent amount of unused space.

So, now with the 2TB limitation I got to thinking of how to chop things up in order to use the full amount of disk space with the minimum amount of logical drives.

Here's what I came up with.

Option #1
Create 1 large 34 drive array with 2 spares.
Array A - Disks 1-34
- Logical RAID5 drive 1 = 2TB
- Logical RAID5 drive 2 = 2TB
- Logical RAID5 drive 3 = 2TB
- Logical RAID5 drive 4 = 1.5TB

Option #2
Create 4 arrays. Each array will consist of one RAID5 logical drive.
Array A - Disks 1-9
- 1 2TB RAID5 logical drive
Array B - Disks 10-18
- 1 2TB RAID5 logical drive
Array C - Disks 19-27
- 1 2TB RAID5 logical drive
Array D - Disks 28-36
- 1 2TB RAID5 logical drive

I'm leaning towards option #2 as I've read that I could suffer a performance impact with large arrays as in option #1.

What I'm having trouble with is understanding the physical to logical drive mappings that would occur if I chose option #1.

Here's what I mean. In option #2, there's one logical drive for each array. If I say Array A is drives 1-9 then I know my logical drive resides physically on drives 1-9.

With option #1, I have one big array and 4 logical RAID 5 drives that I have no idea how to locate physically. Also, if two random physical drives were to fail in option #1 do I lose everything?

All advise and criticisms are welcome, I'm just trying to understand this better! Thank you.

Re: Clarification on logical array and logical drives.

Víctor Cespón — Sat, 15 Nov 2008 15:55:03 GMT

In option #1 you would have a single RAID with all the disks (although I think that the limit on HP array controllers is 32 drives per array). Then the total space is divided on several logical drivers, with each logical drive taking a part of each physical disk.
If the logical drives are configured in RAID 5, then a single drive failure will not lead to data loss, but a double failure will cause the loss of all data in all logical drives.
The option #2 is more secure, and the traffic on each logical drive is insolated, so if one is very bussy, it does not affect the others.

topic Clarification on logical array and logical drives. in Disk Enclosures

Clarification on logical array and logical drives.

Re: Clarification on logical array and logical drives.