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

["Benoît Canet" <benoit.canet@irqsave.net>]

> 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

Regards,

Benoît

> Regards,
> 
> Anthony Liguori
>