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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-9533) Make batch commitlog mode easier to tune
Date Tue, 02 Jun 2015 18:25:49 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-9533?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14569535#comment-14569535
] 

Benedict commented on CASSANDRA-9533:
-------------------------------------

bq. matches the comment *less* well 

You're being a bit selective in which parts you bold :)

 "it will wait up to" implies _it will wait_ - which it would not, at all. The reference to
Postgres' behaviour also indicates it will actually wait that period (although with separate
sibling requirements, and on a microsecond time horizon which is much more sensible). Further,
our docs say "To avoid syncing after every write, Cassandra groups the mutations into batches
and *syncs every* {{commitlog_batch_window_in_ms.}}" Not at least that often, but - implcitly
- exactly that often, as we do now.

bq.  I thought we did it that way because we don't have a queue of operations to peek into
anymore, so it's difficult to provide the old behavior of "stop sleeping when the queue is
empty."

No, unfortunately the best I can find of the etymology of this change is some offline discussion
between Brandon, Jeremiah and myself, which occurred 2-3 months after commit:

{quote}
Benedict Elliott Smith	 so, just figured out why that CL unit test @yukim found went from
hero to zero in 2.1
Benedict Elliott Smith	 SHANP on IRC has found the old Batch CL code doesn't behave in the
same way
Benedict Elliott Smith	 the window only serves as a maximum for buffering
Benedict Elliott Smith	 so if you get one record arrive, it will immediately sync
Benedict Elliott Smith	 in 2.1 this changed. i'm not sure if I got the wrong end of the stick
(and it seems everyone else who's been discussing it since, maybe), or if this is a mistake
that's been present all along
Benedict Elliott Smith	 but we should probably decide which behaviour we want to go with in
2.1
dr driftx	 I don't think it was a mistake, exactly, I've explained it lots of times in training
that way (pre-2.1 behavior)
Benedict Elliott Smith	 right
Benedict Elliott Smith	 so question is: did I just misunderstand, or did somebody tell me
to implement it this way? and ignoring the answer, do we want to restore the old behaviour?
Benedict Elliott Smith	 "To avoid syncing after every write, Cassandra groups the mutations
into batches and syncs every {{commitlog_batch_window_in_ms. }}"
Benedict Elliott Smith	 anyway, i'm easy, and heading to bed. if nobody says anything i'll
leave it how it is :-)
ZJeremiah D Jordan	 As long as the window time is the max time between syncs and writes block
until they sync. I think your code is fine. As that is how we have documented it working all
along
Benedict Elliott Smith	@jd my version has it as the the exact time between syncs
{quote}

So it is not at all clear why it happened. My recollection was that we discussed it and decided
to normalise on the doc behaviour, but I cannot find a reference to that, so it is possible
I simply implemented it how the docs described it, and you reviewed it with the same lens.
Either way, it was decided to let the change stand due to it matching the doc descriptions
and the lack of further feedback.

We can certainly wait for the "queue" to empty, by issuing Barriers on the OpOrder; we already
issue one and wait for any active at that moment to complete, which is probably good enough.
If we want to wait until _none_ can be active, we can just (on completion of the barrier),
check if there are any now running and issue another if there are.

> Make batch commitlog mode easier to tune
> ----------------------------------------
>
>                 Key: CASSANDRA-9533
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-9533
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Jonathan Ellis
>            Assignee: Benedict
>             Fix For: 3.x
>
>
> As discussed in CASSANDRA-9504, 2.1 changed commitlog_sync_batch_window_in_ms from a
maximum time to wait between fsync to the minimum time, so one must be very careful to keep
it small enough that most writers aren't kept waiting.



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