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Re: [Qemu-block] [PATCH v2 resend] block/mirror: change the semantic of

From: John Snow
Subject: Re: [Qemu-block] [PATCH v2 resend] block/mirror: change the semantic of 'force' of block-job-cancel
Date: Fri, 2 Mar 2018 11:20:55 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

On 03/02/2018 10:39 AM, Eric Blake wrote:
> On 02/26/2018 08:05 PM, Liang Li wrote:
>> When doing drive mirror to a low speed shared storage, if there was heavy
>> BLK IO write workload in VM after the 'ready' event, drive mirror
>> block job
>> can't be canceled immediately, it would keep running until the heavy
>> workload stopped in the VM.
>> Libvirt depends on the current block-job-cancel semantics, which is that
>> when used without a flag after the 'ready' event, the command blocks
>> until data is in sync.  However, these semantics are awkward in other
>> situations, for example, people may use drive mirror for realtime
>> backups while still wanting to use block live migration.  Libvirt cannot
>> start a block live migration while another drive mirror is in progress,
>> but the user would rather abandon the backup attempt as broken and
>> proceed with the live migration than be stuck waiting for the current
>> drive mirror backup to finish.
>> The drive-mirror command already includes a 'force' flag, which libvirt
>> does not use, although it documented the flag as only being useful to
>> quit a job which is paused.  However, since quitting a paused job has
>> the same effect as abandoning a backup in a non-paused job (namely, the
>> destination file is not in sync, and the command completes immediately),
>> we can just improve the documentation to make the force flag obviously
>> useful.
> How does this interact with John's ongoing work to redo block job
> semantics:
> https://lists.gnu.org/archive/html/qemu-devel/2018-02/msg06167.html
>> Cc: Paolo Bonzini <address@hidden>
>> Cc: Jeff Cody <address@hidden>
>> Cc: Kevin Wolf <address@hidden>
>> Cc: Max Reitz <address@hidden>
>> Cc: Eric Blake <address@hidden>
>> Cc: John Snow <address@hidden>
>> Reported-by: Huaitong Han <address@hidden>
>> Signed-off-by: Huaitong Han <address@hidden>
>> Signed-off-by: Liang Li <address@hidden>
>> ---
> But in isolation, the patch looks reasonable to me.
> Reviewed-by: Eric Blake <address@hidden>

In fairness, this patch hit the list before mine did, so...

I think it'll be OK to accommodate -- I think. It just changes the
nature of how fast the cancellation happens in mirror, which shouldn't
muck around with the general flow of job management in general. I think.

Famous last words.


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