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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-8833) Stop opening compaction results early
Date Mon, 09 Mar 2015 11:09:39 GMT


Benedict commented on CASSANDRA-8833:

bq. run benchmarks again, this time on ssds (Ryan McGuire could your team help out with that?)

This is a little tricky, since we've disabled the eviction of page cache entries for the new
files in 2.1, so we'll need to roll a patch to reintroduce it. It's also worth caveating in
advance that, since testing SSDs we likely care about latency more than throughput, Gil Tene's
arguments (my final thoughts about which can be found [here|,d.d24])
about stress in its current state do hold water, and we we will significantly underestimate
any negative effect on latency, although I may be able to calculate a rough adjustment after
the fact. This isn't a problem for provisioning tests, since you should never saturate throughput
for such a test, but for testing these kinds of effects we absolutely should. Fixing stress
to properly simulate all of the messages at a target throughput rate is on my hit list, but
won't be done for a few weeks at least.

> Stop opening compaction results early
> -------------------------------------
>                 Key: CASSANDRA-8833
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Marcus Eriksson
>             Fix For: 2.1.4
> We should simplify the code base by not doing early opening of compaction results. It
makes it very hard to reason about sstable life cycles since they can be in many different
states, "opened early", "starts moved", "shadowed", "final", instead of as before, basically
just one (tmp files are not really 'live' yet so I don't count those). The ref counting of
shared resources between sstables in these different states is also hard to reason about.
This has caused quite a few issues since we released 2.1
> I think it all boils down to a performance vs code complexity issue, is opening compaction
results early really 'worth it' wrt the performance gain? The results in CASSANDRA-6916 sure
look like the benefits are big enough, but the difference should not be as big for people
on SSDs (which most people who care about latencies are)
> WDYT [~benedict] [~jbellis] [~iamaleksey] [~JoshuaMcKenzie]?

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