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Jonathan Ellis commented on CASSANDRA-2503:
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Good point on skipping the commitlog.
The reason to limit to STC is that I don't think of this as a band-aid for
compaction-is-behind (although I suppose it accomplishes that as well) so much
as a limit on the worst-case behavior; even when STC is "fully" compacted (i.e.
not major compacted but there is nothing left for the bucketing to do) you can
have an arbitrary number of sstables contain columns for a given row. Thus, I
think using min_compaction_threshold as the cutoff here makes a lot of sense.
> Eagerly re-write data at read time ("superseding")
> --------------------------------------------------
>
> Key: CASSANDRA-2503
> URL: https://issues.apache.org/jira/browse/CASSANDRA-2503
> Project: Cassandra
> Issue Type: New Feature
> Components: Core
> Reporter: Stu Hood
> Assignee: Jonathan Ellis
> Labels: compaction, performance
> Fix For: 1.0.1
>
> Attachments: 2503.txt
>
>
> 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 Augmenting the basic idea, if when we superseded data in a file we marked it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 as superseded somehow, the next compaction that touched that file could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 particular sstable superseded could be recorded in a component of that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 CASSANDRA-674 or CASSANDRA-47), then the list of superseded blocks could be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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