Eagerly re-write data at read time ("superseding")
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Key: CASSANDRA-2503
URL: https://issues.apache.org/jira/browse/CASSANDRA-2503
Project: Cassandra
Issue Type: New Feature
Components: Core
Reporter: Stu Hood
Once CASSANDRA-2498 is implemented, it will be possible to implement an
optimization to eagerly rewrite ("supersede") data at read time. If a
successful read needed to hit more than a certain threshold of sstables, we can
eagerly rewrite it in a new sstable, and 2498 will allow only that file to be
accessed. This basic approach would improve read performance considerably, but
would cause a lot of duplicate data to be written, and would make compaction's
work more necessary.
Augmenting the basic idea, if when we superseded data in a file we marked it as
superseded somehow, the next compaction that touched that file could remove the
data. Since our file format is immutable, the values that a particular sstable
superseded could be recorded in a component of that sstable. If we always
supersede at the "block" level (as defined by CASSANDRA-674 or CASSANDRA-47),
then the list of superseded blocks could be represented using a generation
number and a bitmap of block numbers. Since 2498 would already allow for
sstables to be eliminated due to timestamps, this information would probably
only be used at compaction time (by loading all superseding information in the
system for the sstables that are being compacted).
Initially described on
[1608|https://issues.apache.org/jira/secure/EditComment!default.jspa?id=12477095&commentId=12920353].
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