Matt Corgan created HBASE-6093:
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Summary: Flatten timestamps during flush and compaction
Key: HBASE-6093
URL: https://issues.apache.org/jira/browse/HBASE-6093
Project: HBase
Issue Type: New Feature
Components: io, performance, regionserver
Reporter: Matt Corgan
Priority: Minor
Many applications run with maxVersions=1 and do not care about timestamps, or
they will specify one timestamp per row as a normal KeyValue rather than
per-cell.
Then, DataBlockEncoders like those in HBASE-4218 and HBASE-4676 often encode
timestamps as diffs from the previous or diffs from the minimum timestamp in
the block. If all timestamps in a block are the same, they will all compress
to basically <= 8 bytes total per block. This can be 10% to 25% space savings
for some schemas, and that savings is realized both on disk and in block cache.
We could add a ColumnFamily setting flattenTimestamps=[true/false]. If true,
then all timestamps are modified during a flush/compaction to the
currentTimeMillis() at the start of the flush/compaction. If all timestamps
are made identical in a file, then the encoder will be able to eliminate them.
The simplest use case is probably that where all inserts are type=Put, there
are no overwrites, and there are no deletes. As use cases get more complex,
then so does the implementation.
For example, what happens when there is a Put and a Delete of the same cell in
the same memstore? Maybe for a flush at t=flushStartTime, the Put gets
timestamp=t, and the Delete gets timestamp=t+1. Or maybe HBASE-4241 could take
care of this problem.
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