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From "Anuj (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-8885) Cold sstable count increases indefinitely in STCS
Date Mon, 02 Mar 2015 11:04:04 GMT
Anuj created CASSANDRA-8885:
-------------------------------

             Summary: 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


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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