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From "Peter Schuller (JIRA)" <>
Subject [jira] Commented: (CASSANDRA-1658) support incremental sstable switching
Date Tue, 16 Nov 2010 08:16:14 GMT


Peter Schuller commented on CASSANDRA-1658:

I didn't think of that; that we can actually make the transition sequential in terms of sstable
coverage. That's great.

The only case I can think of where it might not be advantageous would be use of the OPP on
a terribly un-even workload where a gradual transition from 0-100% in terms of the node's
ring segment might imply very uneven jumps in the number of requests going to the new cold
sstable at any given moment. (Not that I'm using this as an argument against the idea, just
pointing it out.)

> support incremental sstable switching
> -------------------------------------
>                 Key: CASSANDRA-1658
>                 URL:
>             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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