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From "Aleksey Yeschenko (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-6357) Flush memtables to separate directory
Date Sat, 15 Feb 2014 02:57:20 GMT


Aleksey Yeschenko commented on CASSANDRA-6357:

Mostly LGTM.

Pushed a slightly tweaked version to,
- simplifies and fixes cassandra.yaml/Config/DD memtable_flush_writers inconsistencies
- fixes the typo in Directories.getKSChildDirectories()
- makes AbstractCompactionTask.getWriteableLocation() consistent with FlushRunnable.getWriteableLocation()

I have an issue with the naming in Directories with getWriteableLocation() vs. getFlushLocation().
I think we should rename getWriteableLocation() to something like getCompactionLocation(),
to make the distinction between the two clear.

Other than that +1.

> 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
>             Fix For: 2.1
>         Attachments: 6357-v2.txt, 6357.txt, 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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