[jira] [Created] (HBASE-8709) consider a scheme to allow compating files in any combination

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

Sergey Shelukhin created HBASE-8709:
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             Summary: consider a scheme to allow compating files in any combination
                 Key: HBASE-8709
                 URL: https://issues.apache.org/jira/browse/HBASE-8709
             Project: HBase
          Issue Type: Brainstorming
          Components: Compaction, HFile
            Reporter: Sergey Shelukhin
            Assignee: Sergey Shelukhin


We were discussing something and I came up with the following scheme. Consider this. The main problem for choosing out-of-order files for compactions is full key collisions (k, cf:c, ts are the same). We rely on file seqnum to resolve these. There'd be no problem if we stored seqnum for each KV, but that is an overkill. What can we do is this.
1) Store min seqnum for a file together with the max. Assume file seqnum ranges don't overlap.
2) Store seqnum for each KV in memory of the memstore.
3) On flush, don't write out seqnums unless there's a full conflict inside this memstore. We will have to change file format unfortunately to tuck on a bit somewhere to indicate there's varint seqnum.
4) On compaction, when dropping versions we can drop these seqnums.
5) On compaction, if we see a full conflict with no seqnums (i.e. KVs coming from different files), write out seqnums for the KVs involved as median of the respective file ranges (or something like that). We only ever use these KVs to resolve full conflicts so we don't care about relations between keys. 

Both of the places where we write seqnums we will need to see the next KV before writing previous KV, so there's some complexity, however the "buffering" is never more than one KV long - if we see different k-c-t we know we don't need a seqnum unless we are in conflict with previous KVs which we have already written, if we see the same one we know we need a seqnum. 




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