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Re: [Qemu-block] Making QMP 'block-job-cancel' transactionable

From: Eric Blake
Subject: Re: [Qemu-block] Making QMP 'block-job-cancel' transactionable
Date: Tue, 28 Mar 2017 09:49:40 -0500
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On 03/24/2017 07:34 AM, Kashyap Chamarthy wrote:
> While debugging some other issue, I happened to stumble across an old
> libvirt commit[*] that adds support for pivot (whether QEMU should
> switch to a target copy or not) operation as a result of issuing QMP
> 'block-job-cancel' to a 'drive-mirror' (in libvirt parlance, "block
> copy").
> In the libvirt commit message[*] Eric Blake writes:
>     "[...] There may be potential improvements to the snapshot code to
>     exploit block copy over multiple disks all at one point in time.
>     And, if 'block-job-cancel' were made part of 'transaction', you
>     could copy multiple disks at the same point in time without pausing
>     the domain. [...]"
> I realize that 'block-job-cancel' is currently not part of the
> @TransactionAction.  Is it worthwhile to do so?

Yes, it is still worthwhile to make that improvement, although it is now
2.10 material.

> Given the current behavior of QMP 'drive-mirror':
>   - Upon 'block-job-complete', synchronization will end, and live QEMU
>     pivots to the target (i.e. the copy)
>   - Upon 'block-job-cancel', a point-in-time (at the time of cancel)
>     copy gets created, and live QEMU will _not_ pivot.
> I realize that it is not possible to perform a "block copy" of multiple
> disks at the same point in time without pausing QEMU.  From a brief chat
> with Stefan Hajnoczi on IRC, he does confirm that there's no current API
> for doing it atomically across multiple disks.
> Since Stefan asked if a bug exists for adding 'transaction' support to
> 'block-job-cancel', thought I might bring it up here first.
> [*] https://libvirt.org/git/?p=libvirt.git;a=commit;h=eaba79d --
>     blockjob: support pivot operation on cancel

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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