Re: [Qemu-devel] [PATCH 0/9] major rework of drive-mirror

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Jun 14, 2016 at 06:25:07PM +0300, Denis V. Lunev wrote:
> Block commit of the active image to the backing store on a slow disk
> could never end. For example with the guest with the following loop
> inside
>     while true; do
>         dd bs=1k count=1 if=/dev/zero of=x
>     done
> running above slow storage could not complete the operation with a
> resonable amount of time:
>     virsh blockcommit rhel7 sda --active --shallow
>     virsh qemu-monitor-event
>     virsh qemu-monitor-command rhel7 \
>         '{"execute":"block-job-complete",\
>           "arguments":{"device":"drive-scsi0-0-0-0"} }'
>     virsh qemu-monitor-event
> Completion event is never received.
> 
> This problem could not be fixed easily with the current architecture. We
> should either prohibit guest writes (making dirty bitmap dirty) or switch
> to the sycnchronous scheme.
> 
> This series switches driver mirror to synch scheme. Actually we can have
> something more intelligent and switch to sync mirroring just after
> the first pass over the bitmap. Though this could be done relatively
> easily during discussion. The most difficult things are here.
> 
> The set also adds some performance improvements dealing with
> known-to-be-zero areas.

Patches 1-5 look pretty good to me.

I think the synchronous I/O mirroring approach will need more discussion
and it could be split into a separate series so the earlier patches can
be merged already.

Stefan

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