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From Björn Hegerfors (JIRA) <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-8243) DTCS can leave time-overlaps, limiting ability to expire entire SSTables
Date Mon, 03 Nov 2014 00:01:34 GMT
Björn Hegerfors created CASSANDRA-8243:
------------------------------------------

             Summary: DTCS can leave time-overlaps, limiting ability to expire entire SSTables
                 Key: CASSANDRA-8243
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8243
             Project: Cassandra
          Issue Type: Bug
            Reporter: Björn Hegerfors
            Assignee: Björn Hegerfors
            Priority: Minor
             Fix For: 2.0.12, 2.1.2


CASSANDRA-6602 (DTCS) and CASSANDRA-5228 are supposed to be a perfect match for tables where
every value is written with a TTL. DTCS makes sure to keep old data separate from new data.
So shortly after the TTL has passed, Cassandra should be able to throw away the whole SSTable
containing a given data point.

CASSANDRA-5228 deletes the very oldest SSTables, and only if they don't overlap (in terms
of timestamps) with another SSTable which cannot be deleted.

DTCS however, can't guarantee that SSTables won't overlap (again, in terms of timestamps).
In a test that I ran, every single SSTable overlapped with its nearest neighbors by a very
tiny amount. My reasoning for why this could happen is that the dumped memtables were already
overlapping from the start. DTCS will never create an overlap where there is none. I surmised
that this happened in my case because I sent parallel writes which must have come out of order.
This was just locally, and out of order writes should be much more common non-locally.

That means that the SSTable removal optimization may never get a chance to kick in!

I can see two solutions:
1. Make DTCS split SSTables on time window borders. This will essentially only be done on
a newly dumped memtable once every base_time_seconds.
2. Make TTL SSTable expiry more aggressive. Relax the conditions on which an SSTable can be
dropped completely, of course without affecting any semantics.



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