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Re: compaction falling behind


If your compaction strategy is Leveled the number of sstables in each level is a good indication that compactions  are keeping up.


From: Ben Bromhead <ben@xxxxxxxxxxxxxxx>
Sent: Monday, February 13, 2017 1:49:05 PM
To: user
Subject: Re: compaction falling behind
 

You can do so in two ways:

1) direct observation:
You can keep an eye on the number of pending compactions. This will fluctuate with load, compaction strategy, ongoing repairs and nodes bootstrapping but generally the pattern is it should trend towards 0. 

There have been a number of bugs in past versions of Cassandra whereby the number of pending compactions is not reported correctly, so depending on what version of Cassandra you run this could impact you.

2) Indirect observation
You can keep an eye on metrics that healthy compaction will directly contribute to. These include the number of sstables per read histogram, estimated droppable tombstones, tombstones per read etc. You should keep an eye on these things anyway as they can often show you areas where you can fine tune compaction or your data model. 

Everything exposed by nodetool is consumable via JMX which is great to plug into your metrics/monitoring/observability system :) 

On Mon, 13 Feb 2017 at 13:23 John Sanda <john.sanda@xxxxxxxxx> wrote:
What is a good way to determine whether or not compaction is falling behind? I read a couple things earlier that suggest nodetool compactionstats might not be the most reliable thing to use.



- John
--
Ben Bromhead
CTO | Instaclustr
+1 650 284 9692
Managed Cassandra / Spark on AWS, Azure and Softlayer