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From "Pavel Yaskevich (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-1902) Migrate cached pages during compaction
Date Sat, 16 Apr 2011 19:40:06 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-1902?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13020651#comment-13020651
] 

Pavel Yaskevich commented on CASSANDRA-1902:
--------------------------------------------

Thanks for you comment Chris, let me clarify one thing - compaction process uses BRAF and
reads use SSTableReader.dfile which is by default memory mapped, compaction with DONTNEED
has a minimal impact on the hot pages even right now it doesn't take away hot pages from the
memory mappings (only can re-assign them to the BRAF when kernel experiences lack of the pages),
you can check that by yourself using slightly modified mincore program from http://insights.oetiker.ch/linux/fadvise/.
I think that we should reject using BRAF for compaction and move everything to use SSTableReader.dfile
(SegmentedFile) plus make sure that we don't make "hot" what we don't need, it also has other
benefits that I have listed in CASSANDRA-2459.

> 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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