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From "Ryan McGuire (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-7567) when the commit_log disk for a single node is overwhelmed the entire cluster slows down
Date Wed, 30 Jul 2014 02:08:41 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-7567?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14078786#comment-14078786
] 

Ryan McGuire commented on CASSANDRA-7567:
-----------------------------------------

Sorry for the confusion, I don't think I tested this on 2.0 after all, I was unable to see
the same latency spikes on 2.0.8 nor 2.0 HEAD. I do seem them on 2.1 HEAD and I'm preparing
logs and traces for that run.

> when the commit_log disk for a single node is overwhelmed the entire cluster slows down
> ---------------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-7567
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7567
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>         Environment: debian 7.5, bare metal, 14 nodes, 64CPUs, 64GB RAM, commit_log disk
sata, data disk SSD, vnodes, leveled compaction strategy
>            Reporter: David O'Dell
>            Assignee: Brandon Williams
>         Attachments: 7567.logs.bz2, write_request_latency.png
>
>
> We've run into a situation where a single node out of 14 is experiencing high disk io.
This can happen when a node is being decommissioned or after it joins the ring and runs into
the bug cassandra-6621.
> When this occurs the write latency for the entire cluster spikes.
> From 0.3ms to 170ms.
> To simulate this simply run dd on the commit_log disk (dd if=/dev/zero of=/tmp/foo bs=1024)
and you will see that instantly all nodes in the cluster have slowed down.
> BTW overwhelming the data disk does not have this same effect.
> Also I've tried this where the overwhelmed node isn't being connected directly from the
client and it still has the same effect.



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