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

[Anthony Liguori <anthony@codemonkey.ws>]

Benoît Canet <benoit.canet@gmail.com> writes:

> This patchset create a block driver implementing a qorum using three qemu disk
> images. Writes are mirrored on the three files.
> For the reading part the three files are read at the same time and a vote is
> done to determine which is the majoritary qiov version. It then return this
> majoritary version to the upper layers.
> When three differents versions of the data are returned by the lower layer the
> qorum is broken and the read return -EIO.
>
> The goal of this patchset is to be turned in a QEMU block filter living just
> above raw-*.c and below qcow2/qed when the required infrastructure will be done.
>
> Main use of this feature will be people using NFS appliances which can be
> subjected to bitflip errors.

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.

Regards,

Anthony Liguori