On Wed, May 31, 2000 at 09:10:30AM -0400, Andy Poling wrote:
> That's the error you will get any time that you try to layer raid levels
> that md does not support layering. It's a safety belt mechanism of sorts.
Arguably, any combination should be allowed, but 0+1 and 1+0 at minimum.
> Either way, specific pairs of disks can fail without consequence. With 0+1,
> sda & sdb can fail or sdc & sdd can fail.
>
> 0+1 and 1+0 are equally safe as far as that's concerned... i.e. not very. :-)
There are two major benefits of striped mirrors over mirrored stripes (I
forget which is 0+1 and which is 1+0, so I'll just be verbose):
1) reconstruction is much faster (resync the failed disk, not the
failed array).
2) the chance that the whole array will fail is less, even though certain
2 disk failures will take either array down. Taking the 2x2 example:
With either striped mirrors or mirrored stripes, the chance that the
first disk will take down the array is 0% (the mirror will cover it).
However, with striped mirrors, there's only a 1 in 3 chance that the
next disk failure will take the array down (both disks in the same
mirror have to die).
Mirrored stripes, though, gives you a 2 in 3 change that the second
disk failure takes the array down. (the first disk knocked out the
first stripe, so any failure in the second stripe brings the whole
thing down.)
These values get more disjointed as the arrays grow:
3x2: 1/5 vs 3/5
4x2: 1/7 vs 4/7
...
Nx2: 1/(2N-1) vs N/(2N-1)
NxP: 1/(PN-1) vs N/(PN-1)
Striped mirrors is costly for administration (mirrored stripes is always 3
arrays (mir,mir,stripe), whereas striped mirrors depend on the number of
disks (Nx2 is N+1 arrays, n mir + stripe)), but the costs are worth it IMHO.
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