[PLUG] striping in LVM or md?

[Eli Stair]

On 6/6/07, Eric Wilhelm <scratchcomputing at gmail.com> wrote:
> # from Eli Stair
> # on Wednesday 06 June 2007 02:00 pm:
>
> >On 6/6/07, Eric Wilhelm <scratchcomputing at gmail.com> wrote:
> >> I've got four SATA disks and want to setup a raid 10 with md.
> >>
> >> I'll be using md to create two mirror pairs.
>
> >In short, I'd suggest using the md-raid10 module.  It handles in one
> >module/management point the striping/mirroring.
>
> I forgot to mention that I have a pair of 120GB disks and one pair of
> 160GB disks.  Does that change the answer.
>
> On further reading, it sounds like md wants similar-sized devices for
> raid0, though I guess I could do "linear" on top of raid1?

Any method other than JBOD will lose space in excess of the
lower-common-denominator disk, so you'll end up with a raid of 4x120G
(240G partitioned).  JBOD will give you a linear increase in volume
size based on #*size of disks, with a corresponding increase in
likelihood of full-filesystem-failure due to any one disk error
trashing the whole filesystem/volume.

> >Feel free to layer LVM on top.
>
> Wouldn't that cost me just as much (or more) performance than doing the
> striping in LVM?  That's mainly what I was wondering with the original
> question.  i.e. if I'm doing LVM anyway, it couldn't possibly be
> without cpu cost, so shouldn't I just mirror in md and then make a VG
> out of md0 and md1?

Assuming you use md-raid10, you wouldn't use any striping/allocation
features of LVM, just saying you could still use it for the ability to
resize later (if you add disks/resize the array underneath).  That
type of usage adds no processing overhead for any operations.

>
> Anybody got any references to benchmarks on that?  All google gives me
> is questions.
>
> Thanks,
> Eric
> --
> "You can't win. You can't break even. You can't quit."
> --Ginsberg's Restatement of the Three Laws of Thermodynamics
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