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https://issues.apache.org/jira/browse/CASSANDRA-1658?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12924258#action_12924258
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Peter Schuller commented on CASSANDRA-1658:
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Clarification: The reason why sstable choice should be deterministic on row key
(or something similar) is that it means that whenever some particular data is
picked from the new sstable, they are never again read from the old. This is
important both because it means the transition can happen faster, and because
it means you do not increase the working set size as significantly as otherwise.
(In an ideal case the algorithm to determine sstable should take into account
actual on-disk placement in the new sstable, given that the OS cache will often
be pulling in other things than what is being asked for due to locality. But
even lacking that, being deterministic is a significant improvement.)
> support incremental sstable switching
> -------------------------------------
>
> Key: CASSANDRA-1658
> URL: https://issues.apache.org/jira/browse/CASSANDRA-1658
> Project: Cassandra
> Issue Type: Improvement
> Reporter: Peter Schuller
> Priority: Minor
>
> I have been thinking about how to minimize the impact of compaction further
> beyond CASSANDRA-1470. 1470 deals with the impact of the compaction process
> itself in that it avoids going through the buffer cache; however, once
> compaction is complete you are still switching to new sstables which will
> imply cold reads.
> Instead of switching all at once, one could keep both the old and new
> sstables around for a bit and incrementally switch over traffic to the new
> sstables.
> A given request would go to the new or old sstable depending on e.g. the hash
> of the row key couple with the point in time relative to compaction
> completion and relative to the intended target sstable switch-over.
> In terms of end-user configuration/mnemonics, one would specify, for a given
> column family, something like "sstable transition period per gb of data" or
> similar. The "per gb of data" would refer to the size of the newly written
> sstable after a compaction. So; for a major compaction you would wait for a
> very significant period of time since the entire database just went cold. For
> a minor compaction, you would only wait for a short period of time.
> The result should be a reasonable negative impact on e.g. disk space usage,
> but hopefully a very significant impact in terms of making the sstable
> transition as smooth as possible for the node.
> I like this because it feels pretty simple, is not relying on OS specific
> features or otherwise rely on specific support from the OS other than a "well
> functioning cache mechanism", and does not imply something hugely significant
> like writing our own page cache layer. The performance w.r.t. CPU should be
> very small, but the improvement in terms of disk I/O should be very
> significant for workloads where it matters.
> The feature would be optional and per-sstable (or possibly global for the
> node).
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