Re: To RAID or not to RAID (...or how to RAID)

On 13 Sep 2002 16:18:26 +0100 (BST), Andrew Mobbs <andrewm_at_chiark.greenend.org.uk> wrote:

>Ed Stevens <spamdump_at_nospam.noway.nohow> wrote:
>>
>>RAID 5+0? Striped RAID-5? Never heard of these configurations. Can you point
>>me to a description of this? RAID-5 *is* striped. The difference between 0 and
>>5 is that 5 adds a parity check for drive recoverability. I'm trying to
>>visualize what a doubly striped set would look like.
>
>Essentially, visualise each RAID-5 set as a single device, not worrying
>about the internals any more than you worry about how a HDD distributes
>data between its platters. There's then just a RAID-0 stripe across N
>devices. It's not quite that easy, since you do have to be careful about
>stripe widths though, so you don't end up with suboptimal interactions.
>
>The advantages of this is that you can have many disks in the logical
>volume, without the disadvantages of doing this in straight RAID-5.
>When you lose a disk in RAID-5, you have to read *all* the disks to
>reconstruct the data from the missing one, this is one reason why nobody
>likes very large RAID-5 sets.
>
>The other is that they're not resilient against double failure. RAID
>5+0 will survive a double failure if and only if the failures occur in
>different RAID 5 sets, which isn't as good as RAID 1+0, but isn't too bad.
>Just like every other use of RAID 5, its a compromise, better options exist
>but they use more disks, so cost more.
>
>Hope that helps - Sorry, I've no references to hand and I'm sure you
>can use Google as well as I can. (I do however have a large RAID 5+0
>configuration in the machine room here).
>
>--
>Andrew Mobbs - http://www.chiark.greenend.org.uk/~andrewm/

Well, I've learned something new today. (Can I go home now!?)

Sounds interesting. Sounds like it would require LOTS of drives. Given that each R-5 set would require a minimum of 3 drives, and I presume the R-0 layered over that would require 3 R5 sets (certainly would require at least 2) we're talking about a minimum of 6 or 9 drives. How many drives worth of capacity is 'lost' in this configuration? Certainly one out of each R5 set. Any more when the R0 is layered on?

Not that I believe that this 'lost' capacity is a concern -- you're buying speed and resiliancy, and that seems to me to be a good thing. But I find that a tough (no, impossible) sell to the guys that control the hardware purchasing and configuration. All they can see is the total capacity they are purchasing, and feel that RAID-5 is resiliant enough, even though we just dodged the bullet on that one a couple of months ago. And as devices keep getting larger and faster, it becomes a tougher sell. They about s**t a brick when I asked for a test server to be configured Raid 0+1 and they 'lost' half of the drives.

--
Ed Stevens
(Opinions expressed do not necessarily represent those of my employer.)