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Re: [Qemu-devel] [RFC 00/12] Qorum disk image corruption resiliency


From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [RFC 00/12] Qorum disk image corruption resiliency
Date: Fri, 3 Aug 2012 10:21:24 +0100

On Thu, Aug 2, 2012 at 8:22 PM, Benoît Canet <address@hidden> wrote:
>> I'm not entirely sure I understand the use-case all that well.
>>
>> Wouldn't the more typical approach be RAID-5 and the use of parity
>> instead of relying on voting?
>>
>> Quorum doesn't work well with an odd number of disks whereas RAID-5
>> does.  You also get significantly more usable disk space with RAID-5
>> then with voting.
>>
>
> Hello,
>
> Use case:
>
> A customer using NFS want to setup redudancy across multiple separate
> rooms of the same datacenter.
> In this case only the network is common.
>
> Testing prove that synchronisation between high end storage applicances
> fail in this case.
> Something else is required.
>
> With raid5 a small network glitch between the hypervisor and one
> of the filer can bring down a while md raid-5 disk.
> This involve a rebuild of this disk using heavy parity computation.
> (imagine the load with many disk images)
> Properly done qorum will correct the error on the fly.
>
> Quorum can correct bitflips induced by the network raid5 cannot.
> (bad case ethernet cable sitting around power cord)
>
> Quorum require only two read out of three to reach majority in the
> best case.
>
> Some well known cloud provider already use quorum in their setup

There is discussion about adding end-to-end data integrity checks to NFSv4:

http://www.ietf.org/proceedings/83/slides/slides-83-nfsv4-2.pdf

This doesn't seem to exist yet but I wanted to share the slides.

Stefan



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