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Date: Fri, 11 Sep 2015 17:28:48 +0000 (UTC)
From: "Benedict (JIRA)" <jira@apache.org>
To: commits@cassandra.apache.org
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Subject: [jira] [Commented] (CASSANDRA-10249) Make buffered read size
 configurable
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    [ https://issues.apache.org/jira/browse/CASSANDRA-10249?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14741191#comment-14741191 ] 

Benedict commented on CASSANDRA-10249:
--------------------------------------

In order to make this forwards compatible with CASSANDRA-8894, it may be best to make any configurable property a maximum (or a max and min)

> Make buffered read size configurable
> ------------------------------------
>
>                 Key: CASSANDRA-10249
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-10249
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Albert P Tobey
>             Fix For: 2.1.x
>
>         Attachments: patched-2.1.9-dstat-lvn10.png, stock-2.1.9-dstat-lvn10.png, yourkit-screenshot.png
>
>
> On read workloads, Cassandra 2.1 reads drastically more data than it emits over the network. This causes problems throughput the system by wasting disk IO and causing unnecessary GC.
> I have reproduce the issue on clusters and locally with a single instance. The only requirement to reproduce the issue is enough data to blow through the page cache. The default schema and data size with cassandra-stress is sufficient for exposing the issue.
> With stock 2.1.9 I regularly observed anywhere from 300:1  to 500 disk:network ratio. That is to say, for 1MB/s of network IO, Cassandra was doing 300-500MB/s of disk reads, saturating the drive.
> After applying this patch for standard IO mode https://gist.github.com/tobert/10c307cf3709a585a7cf the ratio fell to around 100:1 on my local test rig. Latency improved considerably and GC became a lot less frequent.
> I tested with 512 byte reads as well, but got the same performance, which makes sense since all HDD and SSD made in the last few years have a 4K block size (many of them lie and say 512).
> I'm re-running the numbers now and will post them tomorrow.


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