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https://issues.apache.org/jira/browse/CASSANDRA-1902?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13030013#comment-13030013
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Chris Goffinet commented on CASSANDRA-1902:
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We've been testing this patch out for awhile with some modifications. It
actually makes things worse. First off, it does not check if a page is hot/cold
before reading. In our tests we went ahead and added that support so we can
make sure pages stay in a consistent state. What we are seeing though is that
the kernel is ignoring a small percent of hints on a set of pages during the
compaction process. Because of this, this patch ends up migrating pages that it
thinks are hot into the new SSTables over time. You can start to really see the
effects when you are running for days and weeks. Unfortunately since the kernel
is ignoring hints on some of the pages and we can't really have full control I
am -1 on this ticket all together after spending weeks on this. We found just
letting the OS deal with page management to be better. We originally thought
this was going to be better, and in many synthetic benchmarks it has promise.
But after running it in a real production system, the latency is much higher.
> 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-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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