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Clarification on logical array and logical drives.


Clarification on logical array and logical drives.

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.
Víctor Cespón
Honored Contributor

Re: Clarification on logical array and logical drives.

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.