On 17/05/12 03:38, NeilBrown wrote:
On Thu, 17 May 2012 03:24:26 +0200 Oliver Martin<oliver@xxxxxxxxxxxxxxxx>
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.
I'll have to leave for for hpa to answer. I've occasionally thought that
maybe it should be fixed, but it never seemed worth the effort.
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.
Yes, not possible at present.
It might be as simple and finding the places that impose the limit and delete
them...
NeilBrown
Theoretically, a 3-disk RAID6 is like a 2-disk RAID5 or a 1-disk RAID1 -
it is possible, but not much use except as a building block for
expansion later. mdadm supports 1-disk RAID1 (you need a "--force" to
persuade mdadm that you know what you are doing, which is nice), and I
think it supports a 2-disk RAID5, though I haven't tried it. I can't
think of any good reason for it /not/ to support 3-disk RAID6, as there
is nothing in the algorithms to hinder it.
mvh.,
David
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