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https://issues.apache.org/jira/browse/CASSANDRA-6357?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13978917#comment-13978917
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Benedict commented on CASSANDRA-6357:
-------------------------------------
Well strictly speaking it's not _necessary_ with the latest version of 6916, as
all sstablereaders of the same table now equals() each other, but I felt a
little uncomfortable getting rid of it. The idea is(/was) that anybody using
the set would see the most recent version of the readers that were passed in.
Both of the rewriters mutate the set (never simultaneously) whenever they
perform a reopen, and replace the old readers in the DataTracker so both
rewriters are always working with the actually extant copies.
I'm pretty sure it's safe to get rid of it, though, so perhaps we should as it
is clearly not intuitive.
> Flush memtables to separate directory
> -------------------------------------
>
> Key: CASSANDRA-6357
> URL: https://issues.apache.org/jira/browse/CASSANDRA-6357
> Project: Cassandra
> Issue Type: New Feature
> Components: Core
> Reporter: Patrick McFadin
> Assignee: Jonathan Ellis
> Priority: Minor
> Labels: performance
> Fix For: 2.1 beta1
>
> Attachments: 6357-revert.txt, 6357-v2.txt, 6357.txt,
> c6357-2.1-stress-write-adj-ops-sec.png,
> c6357-2.1-stress-write-latency-99th.png,
> c6357-2.1-stress-write-latency-median.png,
> c6357-stress-write-latency-99th-1.png
>
>
> Flush writers are a critical element for keeping a node healthy. When several
> compactions run on systems with low performing data directories, IO becomes a
> premium. Once the disk subsystem is saturated, write IO is blocked which will
> cause flush writer threads to backup. Since memtables are large blocks of
> memory in the JVM, too much blocking can cause excessive GC over time
> degrading performance. In the worst case causing an OOM.
> Since compaction is running on the data directories. My proposal is to create
> a separate directory for flushing memtables. Potentially we can use the same
> methodology of keeping the commit log separate and minimize disk contention
> against the critical function of the flushwriter.
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