[jira] [Created] (CASSANDRA-8885) Cold sstable count increases indefinitely in STCS

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

Anuj created CASSANDRA-8885:
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             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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