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Re: How to configure 36 disks ?

To: Raz <raziebe@xxxxxxxxx>
Subject: Re: How to configure 36 disks ?
From: Bill Davidsen <davidsen@xxxxxxx>
Date: Mon, 23 Mar 2009 11:35:08 -0400
Cc: linux-fsdevel@xxxxxxxxxxxxxxx, Linux RAID Mailing List <linux-raid@xxxxxxxxxxxxxxx>, linux-xfs@xxxxxxxxxxx, linux-aio@xxxxxxxxx, "linux-ide@xxxxxxxxxxxxxxx" <linux-ide@xxxxxxxxxxxxxxx>
In-reply-to: <5d96567b0903230659t734677a3pb4fd77cccb54008b@xxxxxxxxxxxxxx>
Organization: TMR Associates Inc, Schenectady NY
References: <5d96567b0903230659t734677a3pb4fd77cccb54008b@xxxxxxxxxxxxxx>
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.19) Gecko/20081217 Fedora/1.1.14-1.fc9 SeaMonkey/1.1.14
Raz wrote:
Hello
I need to configure 3xDAS'es, each with 12 disks.
All three DAS'es are connected to a single machine.
I have the following requirements (in this order of importance)
from the storage:

1. redundancy.
   having two disks failing in one raid5 breaks the entire raid. when
you have 30TB storage
   it is a disaster.

2. performance.
   My code eliminates Linux raid5/6 write penalty. I managed to do by
   manipulating xfs and patching linux raid5 a bit.

3. modularity ( a "grow" and it will be nice to have "shrink" )
   file system and volume must be able to grow. shrinking is possible
by unifying multiple file systems
   under unionfs or aufs.

4. Utilize storage size.

I assume each disk is 1TB.

___ snip ___

Any other ideas ?

Yes, you have the whole solution rotated 90 degrees. Consider your original solution #2 below... You have no redundancy if one whole DAS box fails, which is certainly a possible failure mode. If you put the RAID0 horizontally, two arrays size six in each DAS, then RAID6 vertically, if one DAS fails completely you still have a functioning system, and the failure results for individual drives remains about the same, while the rebuild time will be longer.

Solution #2
                             raid0
DAS1: raid6: D,D,D,D,D,D     |
      raid6: D,D,D,D,D,D     |
                              |
DAS2: raid6: D,D,D,D,D,D     | xfs.
      raid6: D,D,D,D,D,D     |
                             |
DAS3: raid6: D,D,D,D,D,D     |
      raid6: D,D,D,D,D,D     |


In addition, you can expand this configuration by adding more DAS units. This addresses several of your goals.

In practice, just to get faster rebuild as the array gets larger, I suspect you would find it was worth making the horizontal arrays RAID5 instead of RAID0, just to minimize time to full performance.

--
bill davidsen <davidsen@xxxxxxx>
 CTO TMR Associates, Inc

"You are disgraced professional losers. And by the way, give us our money back."
   - Representative Earl Pomeroy,  Democrat of North Dakota
on the A.I.G. executives who were paid bonuses  after a federal bailout.


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