On 02/11/2011 08:02, keld@keldix.com wrote:

On Wed, Nov 02, 2011 at 12:48:16PM +1100, NeilBrown wrote:

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On Wed, 2 Nov 2011 02:37:56 +0100 keld@keldix.com wrote:

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On Wed, Nov 02, 2011 at 09:25:26AM +1100, NeilBrown wrote:

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On Tue, 1 Nov 2011 23:15:39 +0100 keld@keldix.com wrote:

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On Tue, Nov 01, 2011 at 04:13:26PM -0400, Miles Fidelman wrote:

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David Brown wrote:

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No, md RAID10 does /not/ offer more redundancy than RAID1.  You are
right that md RAID10 offers more than RAID1 (or traditional RAID0 over
RAID1 sets) - but it is a convenience and performance benefit, not a
redundancy benefit.  In particular, it lets you build RAID10 from any
number of disks, not just two.  And it lets you stripe over all disks,
improving performance for some loads (though not /all/ loads - if you
have lots of concurrent small reads, you may be faster using plain
RAID1).

In fact raid10 mas a bit less redundancy than raid1+0.
It is as far as I know built as raid0+1 with a disk layout
where you can only loose eg 1 out of 4 disks, while raid1+0
in some cases can loose 2 disks out of 4.

With md/raid10 you can in some case lose 2 out of 4 disks and survive, just
like raid1+0.

OK, in which cases, and when is this not the case?

best regards
keld

"just like raid1+0"

No, that is not the case AFAIK. Eg the layout of raid10,f2 with 4 disks is
"just like raid0+1".

And raid0+1 can also survive two disk failures in some cases.

It boils down to this - if you have a two-way mirror (RAID1, RAID10, 
RAID1+0, RAID0+1), then you can keep losing disks unless you lose both 
copies of part of your data.

Look at the layout diagrams on 
<http://en.wikipedia.org/wiki/Non-standard_RAID_levels#Linux_MD_RAID_10> 
for the four drive cases.  You can lose disk 1 and 3, or disk 2 and 4, 
in either the "near 2" or the "far 2" cases.  But if you lose disks 1 
and 2, or disks 3 and 4, your data is gone:

RAID10,n2, or RAID1+0 (stripe of mirrors) :

Good array        Lost 1+3 (OK)    Lost 1+2 (Dead)
A1 A1 A2 A2       x A1 x A2        x x A2 A2
A3 A3 A4 A4       x A3 x A4        x x A4 A4
A5 A5 A6 A6       x A5 x A6        x x A6 A6
A7 A7 A8 A8       x A7 x A8        x x A8 A8


RAID10,f2 :

Good array        Lost 1+3 (OK)    Lost 1+2 (Dead)
A1 A2 A3 A4       x A2 x A4        x x A3 A4
A5 A6 A7 A8       x A6 x A8        x x A7 A8
....
A4 A1 A2 A3       x A1 x A3        x x A2 A3
A8 A5 A6 A7       x A5 x A7        x x A6 A7


RAID10,02 :

Good array        Lost 1+3 (OK)    Lost 1+2 (Dead)
A1 A2 A3 A4       x A2 x A4        x x A3 A4
A4 A1 A2 A3       x A1 x A3        x x A2 A3
A5 A6 A7 A8       x A6 x A8        x x A7 A8
A8 A5 A6 A7       x A5 x A7        x x A6 A7

RAID0+1 (mirror of stripes) :

Good array        Lost 1+3 (Dead)  Lost 1+2 (OK)
A1 A2 A1 A2       x A2 x A2        x x A1 A2
A3 A4 A3 A4       x A4 x A4        x x A3 A4
A5 A6 A5 A6       x A6 x A6        x x A5 A6
A7 A8 A7 A8       x A8 x A8        x x A7 A8