[jira] [Created] (IOTDB-981) Give higher priority to unseq data in chunk cache

["Tian Jiang (Jira)" <ji...@apache.org>]

Tian Jiang created IOTDB-981:
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             Summary: Give higher priority to unseq data in chunk cache
                 Key: IOTDB-981
                 URL: https://issues.apache.org/jira/browse/IOTDB-981
             Project: Apache IoTDB
          Issue Type: Improvement
          Components: Core/Engine
            Reporter: Tian Jiang


Cache unseq data chunk may provide large benefits for queries, consider the following situation:
We have 3 seq chunks: s0, s1, and s2; and 1 unseq chunk: u0, which overlaps with all 3 seq chunks. Assuming the 3 chunks are stored more closely (having lowing seek costs when accessed sequentially).
If we can only cache one chunk in chunk cache, and if we choose to cache any of the seq chunks, we then must seek between seq chunks and unseq chunks. If we cache the unseq chunk, then only seeks between seq chunks are required, which is clearly more efficient.

The simple example shows that giving a higher priority to unseq chunks is beneficial, but it remains to be seen whether this conclusion can be generalized. 

Any further discussion is welcomed here.



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