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From "Robert Coli (JIRA)" <>
Subject [jira] Created: (CASSANDRA-1074) check bloom filters to make minor compaction able to delete (some) tombstones
Date Mon, 10 May 2010 23:19:30 GMT
check bloom filters to make minor compaction able to delete (some) tombstones

                 Key: CASSANDRA-1074
             Project: Cassandra
          Issue Type: Improvement
          Components: Core
    Affects Versions: 0.6.1
            Reporter: Robert Coli
            Priority: Minor
             Fix For: 0.6.2

Given a tombstoned key which is older than GCGraceSeconds, current (0.6.1) compaction implementation
still requires a major compaction for the key to actually be deleted. The major compaction
is required is because we must know whether there is a version of the key inside all SSTables
associated with the columnfamily, including ones not involved in minor compactions. However,
as we have bloom filters into each one of these SSTables, minor compaction can relatively
inexpensively check for existence of this key in SSTable files not involved in the current
minor compaction, and thereby delete the key, assuming all bloom filters return negative.
If the filter returns positive, a major compaction would of course still be required.

For use cases like CASSANDRA-1041 where users are strongly motivated to not do a major compaction,
this seems to answer the jbellis objection :

"You don't want to skip large files in major compactions, since the definition of major is
"compact everything so it is safe to remove tombstones." "

The above described improvement appears to provide "safe to remove (some) tombstones" without
requiring "compact everything", and so may be a useful optimization.


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