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Re: [Qemu-devel] [PATCH v2 06/45] block: add support for job pause/resum
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH v2 06/45] block: add support for job pause/resume |
Date: |
Thu, 27 Sep 2012 15:51:15 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:13.0) Gecko/20120605 Thunderbird/13.0 |
Am 27.09.2012 14:57, schrieb Paolo Bonzini:
> Il 27/09/2012 14:45, Kevin Wolf ha scritto:
>> Am 27.09.2012 14:27, schrieb Paolo Bonzini:
>>> Il 27/09/2012 14:18, Kevin Wolf ha scritto:
>>>>>>
>>>>>> Signed-off-by: Paolo Bonzini <address@hidden>
>>>> I think there's a problem with terminology at least. What does "paused"
>>>> really mean? Is it that the job has been requested to pause, or that it
>>>> has actually yielded and is inactive?
>>>>
>>>> The commit message seems to use the latter semantics (which I would
>>>> consider the intuitive one),
>>>
>>> You mean this: "Paused jobs cannot be canceled without first resuming
>>> them". I can add a specification, like "(even if the job actually has
>>> not reached the sleeping point and thus is still running)".
>>
>> I actually meant "pause happens at the next sleeping point", which isn't
>> unspecific at all.
>
> Hmm, there are two aspects: 1) when things stop running; 2) when the job
> reports itself to be paused. The commit message describes (1)
> precisely, and doesn't say anything about (2). That's too specific for
> a commit message, but the header file describes it precisely.
Yes, I understood that, I just found it confusing that both were called
"paused" in different contexts.
> However, in the QMP documentation, the good comment for "bool paused;"
> must be replicated in BlockJobInfo's "paused" member.
>
>>> From the QMP client's point of view it doesn't really matter, does it?
>>>
>>> - even after a job that writes to disk X has "really" paused, you cannot
>>> read or write disk X. It's still owned by QEMU, it hasn't been flushed,
>>> it may play games like lazy refcounts.
>>
>> I'm not sure about this one. Consider things like a built-in NBD server.
>> Probably we'll find more cases in the future, where some monitor command
>> might seem to be safe while a job is paused.
>
> Ok, that's a good point. I'll add a "busy" member to BlockJobInfo.
Ok, thanks. Together with the comment from the bool paused field this
should make pretty clear what clients would have to check for.
Kevin
- [Qemu-devel] [PATCH v2 02/45] blockdev: rename block_stream_cb to a generic block_job_cb, (continued)
[Qemu-devel] [PATCH v2 07/45] qmp: add block-job-pause and block-job-resume, Paolo Bonzini, 2012/09/26
[Qemu-devel] [PATCH v2 08/45] qemu-iotests: add test for pausing a streaming operation, Paolo Bonzini, 2012/09/26
[Qemu-devel] [PATCH v2 09/45] block: rename block_job_complete to block_job_completed, Paolo Bonzini, 2012/09/26
[Qemu-devel] [PATCH v2 11/45] iostatus: move BlockdevOnError declaration to QAPI, Paolo Bonzini, 2012/09/26