Re: Creating a 3-disk RAID6 array

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On 5/16/2012 8:24 PM, Oliver Martin wrote:
> Hi,
> is there any specific reason why md refuses to create a RAID6 array with
> 3 disks? My (probably naive) understanding suggests it should be the
> same as a 3-disk RAID1, similar to a 2-disk RAID5.
> The reason I'm asking is that I currently have space on three disks for
> a new array, and would like to expand it when I add a fourth. I tried
> this scenario with a few loopback devices, but the only way to go from a
> 3-disk RAID1 to a 4-disk RAID6 seems to be via an intermediate 3-disk
> RAID5, requiring two reshapes. I'd like to avoid one of them, if at all
> possible.

Make a 3 partition (seems you're using partitions) md RAID 10 (RAID 1E)
array with what you have now.  When you add a disk down the road, backup
your filesystem, create your 4 disk md RAID 6 array, format, restore.

Now for my $0.02:  You're better served all around with a 4 partition
RAID 10 or 1+0, than a 4 partition RAID 6.  Random write performance is
greatly superior, rebuild time is significantly lower, etc.  Sure it
would be nice if mdadm could reshape a 3 disk RAID10 into a 4 disk
RAID10, but it can't.  But since you should be doing a backup/restore
anyway, it doesn't matter.

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