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Benedict edited comment on CASSANDRA-7031 at 4/23/14 9:29 AM:
--------------------------------------------------------------
-[~enigmacurry] was this with just the segment size changed, not the
total_commit_log_space? Or the whole patch? Because if the latter I'm very
surprised, but if only the segment size it's not too surprising at all.-
Hmm. That said, I suppose that's probably the explanation of worse 99.9th
percentiles: if the CL disk is slow then large segments could take longer to
sync() than the configured interval, causing writes to wait.
Okay, let's just bump the total space. That's the main improvement anyway -
want to avoid memtable flushes being forced regularly by CL, which now we have
a more efficient memtable is pretty darn likely with a 1Gb default size. Sound
reasonable [~jbellis]?
was (Author: benedict):
[~enigmacurry] was this with just the segment size changed, not the
total_commit_log_space? Or the whole patch? Because if the latter I'm very
surprised, but if only the segment size it's not too surprising at all.
> Increase default commit log total space + segment size
> ------------------------------------------------------
>
> Key: CASSANDRA-7031
> URL: https://issues.apache.org/jira/browse/CASSANDRA-7031
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Reporter: Benedict
> Assignee: Benedict
> Priority: Trivial
> Fix For: 2.1 beta2
>
> Attachments: 7031.txt
>
>
> I would like to increase the default commit log total space and segment size
> options for 64-bit JVMs:
> The current default of 1Gb and 32Mb is quite constrained and can have some
> (very minor) negative performance implications, for no major benefit:
> # 32Mb files are actually quite small, and if during the 10s interval we have
> completely filled multiple of them (quite easy) it would be more efficient to
> write fewer larger files, as we can issue fewer fsyncs and permit the OS to
> schedule the writes more efficiently. On my box this has a small but
> noticeable impact. Although I would expect on decent server hardware this
> would be smaller still, since we immediately drop the pages from cache on
> writing there isn't a great deal of advantage to keeping the files so small.
> The only advantage I can see is that during a drop KS/CF or other event that
> forces log rollover we're wasting less space until log recycling. 128-256Mb
> are modest increases that seem more appropriate to me.
> # 1Gb is too small for the default total log space. We can find that we force
> memtable flushes as a result of log utilisation instead of memtable occupancy
> quite often (esp. as a result of increased effective memtable space from
> recent improvements), especially on machines with more addressable memory. I
> suggest 8Gb as a minimum. The only disadvantage of having more log data is
> that replay on restart may be slightly slower, but since most of the events
> will be ignored it should be relatively benign, and I would rather take the
> penalty on startup instead of during running, no matter how small the running
> penalty.
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