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https://issues.apache.org/jira/browse/CASSANDRA-1902?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13030504#comment-13030504
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Peter Schuller commented on CASSANDRA-1902:
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Chris,
Looks like I missed at least two things when I looked at the patch last time.
Firstly I thought it *was* checking mincore() prior to reading (this is why I
said it was not an implementation detail of isRangeInCache() that it does not
reflect current reality, since it is called after reading). Secondly, as per
the discussion in CASSANDRA-1470 you have fsync/fdatasync prior to DONTNEED in
order for the DONTNEED to have any effect. I had a quick look again at the
patch and I don't see anything doing this.
Is it possible the effect you're seeing is entirely because of this? IIRC from
last time I looked at the source it really looked like it would loop over all
pages and invalidate them *AS LONG AS* they aren't dirty.
Note: To be clear, once again just like with direct I/O, there will be a
performance penalty associated with the fsync() (approximately the same as the
one you see on each write with direct I/O) so benchmarking has to take that
into account. The fsync/DONTNEED frequency needs to be balanced with the
latency potentially incurred by fsync.
> 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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