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T Jake Luciani commented on CASSANDRA-1902:
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Right, *not* DONTNEEDing the pages needed should prob be fine just though will
need might ensure these pages are kept over the presumably active older
sstables.
So you are recommending a binary flag like "compaction.hotness.migration.thresh
= .5"? I was thinking the same for simplicity sake.
> Migrate cached pages during compaction
> ---------------------------------------
>
> Key: CASSANDRA-1902
> URL: https://issues.apache.org/jira/browse/CASSANDRA-1902
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Affects Versions: 0.7.1
> Reporter: T Jake Luciani
> Assignee: T Jake Luciani
> Fix For: 0.7.1
>
> Original Estimate: 32h
> Remaining Estimate: 32h
>
> Post CASSANDRA-1470 there is an opportunity to migrate cached pages from a
> pre-compacted CF during the compaction process.
> First, add a method to MmappedSegmentFile: long[] pagesInPageCache() that
> uses the posix mincore() function to detect the offsets of pages for this
> file currently in page cache.
> Then add getActiveKeys() which uses underlying pagesInPageCache() to get the
> keys actually in the page cache.
> use getActiveKeys() to detect which SSTables being compacted are in the os
> cache and make sure the subsequent pages in the new compacted SSTable are
> kept in the page cache for these keys. This will minimize the impact of
> compacting a "hot" SSTable.
> A simpler yet similar approach is described here:
> http://insights.oetiker.ch/linux/fadvise/
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