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From "Jeff Jirsa (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-13241) Lower default chunk_length_in_kb from 64kb to 4kb
Date Mon, 27 Feb 2017 21:04:45 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-13241?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15886529#comment-15886529
] 

Jeff Jirsa commented on CASSANDRA-13241:
----------------------------------------

4k chunks will will give much better IO for sstables not in page cache, but come at the cost
of significant offheap memory requirements, and compression ratios will suffer. There might
be a better default, but I'm not sure going all the way to 4k is the right answer. 

{quote}
Thanks for your vote, but ... maybe this is a stupid question: Who will finally decide if
that change is accepted?
{quote}

Generally, a committer can push it as long as they have a +1 vote. However, for something
like this, most committers will (should) look for consensus among the other committers. Ultimately,
the final say will come from consensus among the PMC when it goes to voting for a release
- if a member of the PMC ultimately decides it doesn't like the change, that member can/will/should
vote -1 on the release until the commit is removed. 


> Lower default chunk_length_in_kb from 64kb to 4kb
> -------------------------------------------------
>
>                 Key: CASSANDRA-13241
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-13241
>             Project: Cassandra
>          Issue Type: Wish
>          Components: Core
>            Reporter: Benjamin Roth
>
> Having a too low chunk size may result in some wasted disk space. A too high chunk size
may lead to massive overreads and may have a critical impact on overall system performance.
> In my case, the default chunk size lead to peak read IOs of up to 1GB/s and avg reads
of 200MB/s. After lowering chunksize (of course aligned with read ahead), the avg read IO
went below 20 MB/s, rather 10-15MB/s.
> The risk of (physical) overreads is increasing with lower (page cache size) / (total
data size) ratio.
> High chunk sizes are mostly appropriate for bigger payloads pre request but if the model
consists rather of small rows or small resultsets, the read overhead with 64kb chunk size
is insanely high. This applies for example for (small) skinny rows.
> Please also see here:
> https://groups.google.com/forum/#!topic/scylladb-dev/j_qXSP-6-gY
> To give you some insights what a difference it can make (460GB data, 128GB RAM):
> - Latency of a quite large CF: https://cl.ly/1r3e0W0S393L
> - Disk throughput: https://cl.ly/2a0Z250S1M3c
> - This shows, that the request distribution remained the same, so no "dynamic snitch
magic": https://cl.ly/3E0t1T1z2c0J



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