[jira] [Commented] (CASSANDRA-12591) Re-evaluate the default 160MB sstable_size_in_mb choice in LCS

["Wei Deng (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-12591?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15456278#comment-15456278 ] 

Wei Deng commented on CASSANDRA-12591:
--------------------------------------

So I've done some quick initial tests using latest trunk (i.e. C* 3.10) code just to prove the point whether this is a worthwhile effort. The hardware I'm using is still not a typical/adequate-enough configuration I'd use for a production Cassandra deployment (GCE n1-standard-4, with 4 vCPUs, 15GB RAM and a single 1TB persistent disk that's spindle-based), but I'm already seeing a positive sign that shows bigger max_sstable_size can be helpful for compaction throughput.

Based on the initial results (at each max_sstable_size, I did three runs from scratch; for all runs I set compaction threads to 4, and since there will be no throttling enforced by compaction-stress the setting would be equivalent to setting compaction_throughput_mb_per_sec to 0, the initial SSTable files generated by `compaction-stress write` are using the default 128MB size, which is inline with the typical flush size I saw on this kind of hardware using default cassandra.yaml configuration parameters), using 10GB of stress data generated by the blogpost data model [here|https://gist.githubusercontent.com/tjake/8995058fed11d9921e31/raw/a9334d1090017bf546d003e271747351a40692ea/blogpost.yaml], the overall compaction times with 1280MB max_sstable_size are: 7m16.456s, 7m7.225s, 7m9.102s; the overall compaction times with 160MB max_sstable_size are: 9m16.715s, 9m28.146s, 9m7.192s.

Given these numbers, the average seconds to finish compaction with 1280MB max_sstable_size is 430.66, and the average seconds to finish compaction with 160MB max_sstable_size is 557.33, which is already a 23% improvement.

I realize 10GB data is barely enough to test 1280MB sstable size as the data will only go from L0->L1, so the next run I'm going to use 100GB data size on this hardware (keeping everything else the same) and see how the numbers compare.

> Re-evaluate the default 160MB sstable_size_in_mb choice in LCS
> --------------------------------------------------------------
>
>                 Key: CASSANDRA-12591
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-12591
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Compaction
>            Reporter: Wei Deng
>              Labels: lcs
>
> There has been some effort from CASSANDRA-5727 in benchmarking and evaluating the best max_sstable_size used by LeveledCompactionStrategy, and the conclusion derived from that effort was to use 160MB as the most optimal size for both throughput (i.e. the time spent on compaction, the smaller the better) and the amount of bytes compacted (to avoid write amplification, the less the better).
> However, when I read more into that test report (the short [comment|https://issues.apache.org/jira/browse/CASSANDRA-5727?focusedCommentId=13722571&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13722571] describing the tests), I realized it was conducted on a hardware with the following configuration: "a single rackspace node with 2GB of ram." I'm not sure if this was an ok hardware configuration for production Cassandra deployment at that time (mid-2013), but it is definitely far lower from today's hardware standard now.
> Given that we now have compaction-stress which is able to generate SSTables based on user defined stress profile with user defined table schema and compaction parameters (compatible to cassandra-stress), it would be a useful effort to relook at this number using a more realistic hardware configuration and see if 160MB is still the optimal choice. It might also impact our perceived "practical" node density with LCS nodes if it turns out bigger max_sstable_size actually works better as it will allow less number of SSTables (and hence less level and less write amplification) per node with bigger density.



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