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issuing [block-]job-complete to jobs in STANDBY state

From: John Snow
Subject: issuing [block-]job-complete to jobs in STANDBY state
Date: Thu, 1 Apr 2021 15:02:35 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1

Hi; downstream we've run into an issue where VMs under heavy load with many simultaneously concurrent block jobs running might occasionally flicker into the STANDBY state, during which time they will be unable to receive JOB COMPLETE commands. I assume this flicker is due to child_job_drained_begin().

BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1945635

It's safe to just retry this operation again, but it may be difficult to understand WHY the job is paused at the application level, since the flush event may be asynchronous and unpredictable.

We could define a transition to allow COMPLETE to be applied to STANDBY jobs, but is there any risk or drawback to doing so? On QMP's side, we do know the difference between a temporary pause and a user pause/error pause (Both use the user_pause flag.)

I imagine it's safe to continue rejecting COMPLETE commands if user_paused is set ("No, go fix this first!") and we could define a pathway for implicitly STANDBY jobs only. However, in this case, we don't really know how long STANDBY will last. Do we have the ability to easily accept an async "intent" to complete a job without tying up the monitor?

ATM I think only mirror uses .complete, but it looks like it tries to actually set up the pivot a good deal before delegating to the bottom half, so I worry it's not safe to try to run this when we are in the middle of a drain.

Any thoughts?


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