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Re: SATA controllers

Subject: Re: SATA controllers
From: Liam Proven
Date: Sat, 13 Mar 2010 14:11:11 +0000
On Wed, Mar 10, 2010 at 3:19 PM, Dave Howorth
<dhoworth@xxxxxxxxxxxxxxxxx> wrote:
> I'm speccing a new machine and as part of it I'd like to have a
> linux-controlled 4-disk RAID 6 array using SATA 3 Gbps disks (aka STA 2).

You want more than 4 disks for RAID6. Seriously.

RAID6 uses 2 parity drives; this means you get the capacity of (N-2)
drives, where N is the number of drives. Ergo, use 4 drives, you only
get the capacity of 2. This is pointless, because if you lose half the
capacity, you would get /much/ better performance from RAID10 (a
mirror of stripes) or RAID 0+1 (a stripe set of mirror pairs).

(I may have got the definitions of 0+1 and 10 transposed, but it's not
really important at this point!)

RAID levels 0+1 and 10 use simple mirroring and striping, requiring
little CPU, therefore making them very fast, whereas with RAID6 you
impose 2 sets of parity calculations on the system, making writes
slow.

This means the *minimum* number of drives for a RAID6 is 5 drives,
which will give you the capacity of 3× a single drive.

I don't know how smart the Linux software RAID system is, but if it
has good rules built in, it won't let you create a RAID 6 out of <5
drives and should outright block 4. It's an invalid config.

-- 
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