[Linux-cluster] redundancy in redhat clusters

[Raz Ben-Jehuda(caro)]

got it. good point.
why do you think raid5 would give poor performance ?
as long as it is not in degredation mode the performance scales
to n-1 disks.


thanks
raz.

On 11/30/05, Jonathan E Brassow <jbrassow at redhat.com> wrote:
>
>
> On Nov 29, 2005, at 9:19 AM, Raz Ben-Jehuda(caro) wrote:
>
> > Question:
> > I need to add to a clsutered environment redundancy.
> >
> > Since the native linux raid 5 is not clustered awared,
> > what would make it aware to the cluster ?
> > What does it lack ?
> >
>
> Clustered file systems and applications will ensure that they are not
> doing simultaneous writes to the same [meta-]data.  However, they have
> no way to tell that a write to one area will conflict with the write to
> another because of the stripe width and parity calculation of the RAID
> device.  This will lead to parity block corruption.
>
> To solve this problem, the RAID 5 implementation must be cluster aware
> and take out single-writer/multiple-reader locks on the stripes -
> ensuring that multiple machines are not writing to the same stripe at
> the same time.
>
> The performance of a cluster-aware software RAID 5 is likely to be
> abysmal, and will probably not rank very high on anyone's priority
> list.
>
> A mirroring solution is in the works, and later, dd-raid may become a
> reality.
>
>   brassow
>
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--
Raz
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