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https://issues.apache.org/jira/browse/CASSANDRA-1902?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13030505#comment-13030505
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Peter Schuller commented on CASSANDRA-1902:
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(Without the fsync() I expect the results of DONTNEED to vary on a per-call
basis depending on how the kernel happened to flush data to disk. This should
vary a lot depending on the CPU vs. disk boundness of the compaction, the
DONTNEED frequency in terms of bytes, and the overall I/O load of the system.
As such it's difficult to make generalizations about the results, but it's
clear that if the intent of page cache hinting is to work as expected in ways
you can reason with, the hint needs to be done such that they consistently work
or at least work/don't work in ways that are understood)
> 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: Pavel Yaskevich
> Fix For: 1.0
>
> Attachments:
> 0001-CASSANDRA-1902-cache-migration-impl-with-config-option.txt,
> 1902-BufferedSegmentedFile-logandsleep.txt, 1902-formatted.txt,
> 1902-per-column-migration-rebase2.txt, 1902-per-column-migration.txt,
> CASSANDRA-1902-v10-trunk-rebased.patch, CASSANDRA-1902-v3.patch,
> CASSANDRA-1902-v4.patch, CASSANDRA-1902-v5.patch, CASSANDRA-1902-v6.patch,
> CASSANDRA-1902-v7.patch, CASSANDRA-1902-v8.patch,
> CASSANDRA-1902-v9-trunk-rebased.patch,
> CASSANDRA-1902-v9-trunk-with-jmx.patch, CASSANDRA-1902-v9-trunk.patch,
> CASSANDRA-1902-v9.patch
>
> Original Estimate: 32h
> Time Spent: 56h
> Remaining Estimate: 0h
>
> Post CASSANDRA-1470 there is an opportunity to migrate cached pages from a
> pre-compacted CF during the compaction process. This is now important since
> CASSANDRA-1470 caches effectively nothing.
> For example an active CF being compacted hurts reads since nothing is cached
> in the new SSTable.
> The purpose of this ticket then is to make sure SOME data is cached from
> active CFs. This can be done my monitoring which Old SSTables are in the page
> cache and caching active rows in the New SStable.
> A simpler yet similar approach is described here:
> http://insights.oetiker.ch/linux/fadvise/
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