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about mirror cancel
From: |
Vladimir Sementsov-Ogievskiy |
Subject: |
about mirror cancel |
Date: |
Thu, 15 Apr 2021 21:46:05 +0300 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.9.1 |
Hi all!
Recently I've implemented fast-cancelling of mirror job: do
bdrv_cancel_in_flight() in mirror_cancel().
Now I'm in doubt: is it a correct thing? I heard, that mirror-cancel is a kind
of valid mirror completion..
Looking at documentation:
# Note that if you issue 'block-job-cancel' after 'drive-mirror' has indicated
# (via the event BLOCK_JOB_READY) that the source and destination are
# synchronized, then the event triggered by this command changes to
# BLOCK_JOB_COMPLETED, to indicate that the mirroring has ended and the
# destination now has a point-in-time copy tied to the time of the cancellation.
So, in other words, do we guarantee something to the user, if it calls
mirror-cancel in ready state? Does this abuse exist in libvirt?
====
Note, that if cancelling all in-flight requests on target is wrong on mirror
cancel, we still don't have real bug, as the only implementation of
.bdrv_cancel_in_flight is stopping reconnect waiting in nbd driver. So, we'll
cancel requests only if connection is already lost anyway.
But that probably means, that correct name of the handler would be
.bdrv_cancel_in_fligth_requests_that_will_most_probably_fail_anyway()..
And it also means, that abuse of mirror-cancel as valid completion is a bad
idea, as we can't distinguish the cases when user calls job-cancel to have a
kind of point-in-time copy, or just to cancel job (and being not interested in
the final state of target).
So, probably we need an option boolean argument for blockjob-cancel, like
"hard", that will cancel in-flight writes on target node..
--
Best regards,
Vladimir
- about mirror cancel,
Vladimir Sementsov-Ogievskiy <=