Björn Hegerfors created CASSANDRA-8361:
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Summary: Make DTCS split SSTables to perfectly fit time windows
Key: CASSANDRA-8361
URL: https://issues.apache.org/jira/browse/CASSANDRA-8361
Project: Cassandra
Issue Type: Improvement
Reporter: Björn Hegerfors
Priority: Minor
The time windows that DTCS uses are what the strategy tries to align SSTables
to, in order to get the right structure, for best performance. I added the
ticket CASSANDRA-8360, taking SSTables one step closer to aligning with these
windows in a 1:1 manner.
The idea in this ticket is to perfectly align SSTables with the DTCS time
windows, by splitting SSTables that cross window borders. This can lead to
certain benefits, perhaps mostly in consistency and predictability terms, where
it will be very well defined where every value is stored that is old enough to
have stabilized.
Read queries can be aligned with windows in order to guarantee a single disk
seek (although then the client needs to know the right window placements).
Basically, SSTables can be made to align perfectly on day borders, for example.
Right now, there would be an SSTable that almost represents a day, but not
perfectly. So some data is still in another SSTable.
It could also be a useful property for tombstone expiration and repairs.
Practically all splits would happen only in the latest time windows with the
newest and smallest SSTables. After those are split, DTCS would never compact
SSTables across window borders. I have a hard time seeing when this could cause
an expensive operation except for when switching from another compaction
strategy (or even from current DTCS), and after a major compaction. In fact
major compaction for DTCS should put data perfectly in windows rather than
everything in one SSTable.
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