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From "Carl Yeksigian (JIRA)" <j...@apache.org>
Subject [jira] [Resolved] (CASSANDRA-8885) Cold sstable count increases indefinitely in STCS
Date Mon, 02 Mar 2015 21:23:04 GMT

     [ https://issues.apache.org/jira/browse/CASSANDRA-8885?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Carl Yeksigian resolved CASSANDRA-8885.
---------------------------------------
       Resolution: Duplicate
    Fix Version/s: 2.0.4

As part of [~thobbs]'s fix for CASSANDRA-6483, he addressed an issue where even if cold_reads_to_omit
was 0.0, it would still filter out sstables which hadn't been read from. Upgrading to 2.0.4+
should address this issue.

> Cold sstable count increases indefinitely in STCS
> -------------------------------------------------
>
>                 Key: CASSANDRA-8885
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8885
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>         Environment: Cassandra 2.0.3
>            Reporter: Anuj
>            Priority: Minor
>             Fix For: 2.0.4
>
>
> This issue is regarding compaction of cold sstables.
> Compaction Strategy for CF: Size Tiered (Min Threshold=4 & Max Threshold=8)
> Scenario: Run heavy write load (with NO reads) on all 3 nodes of Cassandra cluster. After
some time, compactions stop and sstable count increases continuously to large numbers (90+)
. As writes continue, small sstable of similar size are added after every few minutes. When
writes stop and Cassandra is idle, compaction doesn't happen automatically and sstable count
is still very high. When we start reads on sstables, compaction is automatically triggered
and sstable count keeps on decreasing to reasonable levels.
> We think that behaviour is unexpected because of following reasons:
> 	1. As per documentation (https://issues.apache.org/jira/browse/CASSANDRA-6109 and  
http://www.datastax.com/dev/blog/optimizations-around-cold-sstables), cold_reads_to_omit is
disabled by default in 2.0.3. We are using 2.0.3 and not enabled cold_reads_to_omit explicitly.
Still, cold sstables are not getting compacted. Coldness should not come in picture by default
in 2.0.3.
> 	Note:compactionstats show 0 pending tasks while compaction process is not happening.
> 	
> 	2. In our scenario, we have heavy writes (say for loading data) followed by reads. Till
the time read starts, you have huge no. of sstables to compact.Significant compactions need
to be done before read performance is restored. 
> 	
> 	3. Even when Cassandra is kept idle after writes, compaction doesn't happen. Dont you
think compaction should not just look at coldness? It should also consider load on server
and utilize time for compaction when possible. Cold sstables can be compacted together. This
way we will have much less compactions to do if cold sstables turn hot.



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