With SSDs, the typical recommendation is up to 0.8-1 compactor per core (depending on other load). How many CPU cores do you have?
From: Kai Wang Reply-To: "user@cassandra.apache.org" Date: Friday, January 15, 2016 at 12:53 PM To: "user@cassandra.apache.org" Subject: compaction throughput Hi, I am trying to figure out the bottleneck of compaction on my node. The node is CentOS 7 and has SSDs installed. The table is configured to use LCS. Here is my compaction related configs in cassandra.yaml: compaction_throughput_mb_per_sec: 160 concurrent_compactors: 4 I insert about 10G of data and start observing compaction. nodetool compaction shows most of time there is one compaction. Sometimes there are 3-4 (I suppose this is controlled by concurrent_compactors). During the compaction, I see one CPU core is 100%. At that point, disk IO is about 20-25 M/s write which is much lower than the disk is capable of. Even when there are 4 compactions running, I see CPU go to +400% but disk IO is still at 20-25M/s write. I use nodetool setcompactionthroughput 0 to disable the compaction throttle but don't see any difference. Does this mean compaction is CPU bound? If so 20M/s is kinda low. Is there anyway to improve the throughput? Thanks.
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