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From Jim Cistaro <jcist...@netflix.com>
Subject Re: Cassandra pending compaction tasks keeps increasing
Date Sat, 19 Jan 2013 18:49:18 GMT
1) In addition to iostat, dstat is a good tool to see wht kind of disck throuput your are getting.
 That would be one thing to monitor.
2) For LCS, we also see pending compactions skyrocket.  During load, LCS will create a lot
of small sstables which will queue up for compaction.
3) For us the biggest concern is not how high the pending count gets, but how often it gets
back down near zero.  If your load is something you can do in segments or pause, then you
can see how fast the cluster recovers on the compactions.
4) One thing which we tune per cluster is the size of the files.  Increasing this from 5MB
can sometimes improve things.  But I forget if we have ever changed this after starting data
load.

Is your cluster receiving read traffic during this data migration? If so, I would say that
read latency is your best measure.  If the high number of SSTables waiting to compact is not
hurting your reads, then you are probably ok.  Since you are on SSD, there is a good chance
the compactions are not hurting you.  As for compactionthroughput, we set ours high for SSD.
 You usually wont use it all because the compactions are usually single threaded.  Dstat will
help you measure this.

I hope this helps,
jc

From: Wei Zhu <wz1975@yahoo.com<mailto:wz1975@yahoo.com>>
Reply-To: "user@cassandra.apache.org<mailto:user@cassandra.apache.org>" <user@cassandra.apache.org<mailto:user@cassandra.apache.org>>,
Wei Zhu <wz1975@yahoo.com<mailto:wz1975@yahoo.com>>
Date: Friday, January 18, 2013 12:10 PM
To: Cassandr usergroup <user@cassandra.apache.org<mailto:user@cassandra.apache.org>>
Subject: Cassandra pending compaction tasks keeps increasing

Hi,
When I run nodetool compactionstats

I see the number of pending tasks keep going up steadily.

I tried to increase the  compactionthroughput, by using

nodetool setcompactionthroughput

I even tried the extreme to set it to 0 to disable the throttling.

I checked iostats and we have SSD for data, the disk util is less than 5% which means it's
not I/O bound, CPU is also less than 10%

We are using levelcompaction and in the process of migrating data. We have 4500 writes per
second and very few reads. We have about 70G data now and will grow to 150G when the migration
finishes. We only have one CF and right now the number of  SSTable is around 15000, write
latency is still under 0.1ms.

Anything needs to be concerned? Or anything I can do to reduce the number of pending compaction?

Thanks.
-Wei



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