[jira] [Created] (CASSANDRA-12201) Burst Hour Compaction Strategy

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

Pedro Gordo created CASSANDRA-12201:
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             Summary: Burst Hour Compaction Strategy
                 Key: CASSANDRA-12201
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-12201
             Project: Cassandra
          Issue Type: New Feature
          Components: Compaction
            Reporter: Pedro Gordo


Although it may be subject to changes, for the moment I plan to create a strategy that will revolve around taking advantage of periods of the day where there's less I/O on the cluster. This time of the day will be called “Burst Hour” (BH), and hence the strategy will be named “Burst Hour Compaction Strategy” (BHCS). 
The following process would be fired during BH:

1. Read all the SSTables and detect which partition keys are present in more than a configurable value which I'll call referenced_sstable_limit. This value will be three by default.

2. Group all the repeated keys with a reference to the SSTables containing them.

3. Calculate the total size of the SSTables which will be merged for the first partition key on the list created in step 2. If the size calculated is bigger than property which I'll call max_sstable_size (also configurable), more than one table will be created in step 4.

4. During the merge, the data will be streamed from SSTables up to a point when we have a size close to max_sstable_size. After we reach this point, the stream is paused, and the new SSTable will be closed, becoming immutable. Repeat the streaming process until we've merged all tables for the partition key that we're iterating.

5. Cycle through the rest of the collection created in step 2 and remove any SSTables which don't exist anymore because they were merged in step 5. An alternative course of action here would be to, instead of removing the SSTable from the collection, to change its reference to the SSTable(s) which was created in step 5. 

6. Repeat from step 3 to step 6 until we traversed the entirety of the collection created in step 2.


This strategy addresses three issues of the existing compaction strategies:
- Due to max_sstable_size_limit, there's no need to reserve disc space for a huge compaction, as it can happen on STCS.
- The number of SSTables that we need to read from to reply to a read query will be consistently maintained at a low level and controllable through the referenced_sstable_limit property. This addresses the scenario of STCS when we might have to read from a lot of SSTables.
- It removes the dependency of a continuous high I/O of LCS.



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