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From "Patrick McFadin (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-6357) Flush memtables to separate directory
Date Mon, 24 Mar 2014 22:31:45 GMT


Patrick McFadin commented on CASSANDRA-6357:

I don't think we proved out the initial point for this patch. How can we minimize (or remove)
flush writer blocks when disks are under stress?

Under load, the performance difference would be nice, but my intent was to prevent heap pressure
and eventual node problems. That's what I would like to show if possible. 

> Flush memtables to separate directory
> -------------------------------------
>                 Key: CASSANDRA-6357
>                 URL:
>             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-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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