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From "Colin Patrick McCabe (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-6581) Write to single replica in memory
Date Fri, 19 Sep 2014 16:43:37 GMT

    [ https://issues.apache.org/jira/browse/HDFS-6581?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14140845#comment-14140845

Colin Patrick McCabe commented on HDFS-6581:

[~andrew.wang]: I agree that it would be nice to see a short-circuit write benchmark.  I don't
think it's absolutely necessary to see that before merging, though.  But I do think some kind
of benchmark of reading and writing from in-memory replicas is necessary, since as you say,
this is a performance-focused project.  At least __reading__ from these replicas should be
fast, similar to how reading from HDFS-4949 replicas is fast.  Merging without this information
would be like merging the ACL branch without ever trying to create an ACL.  How do you know
if it works?

I also think we absolutely need a better eviction strategy than LRU before merging.  Any kind
of real-world workload will probably show this really clearly.

Sorry for being a party pooper, but I wanted to bring up these concerns before the merge vote.
 If this spends another week or two in development, it will be a small price to pay for something
we can be proud of.

> Write to single replica in memory
> ---------------------------------
>                 Key: HDFS-6581
>                 URL: https://issues.apache.org/jira/browse/HDFS-6581
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: datanode
>            Reporter: Arpit Agarwal
>            Assignee: Arpit Agarwal
>         Attachments: HDFS-6581.merge.01.patch, HDFS-6581.merge.02.patch, HDFS-6581.merge.03.patch,
HDFS-6581.merge.04.patch, HDFS-6581.merge.05.patch, HDFS-6581.merge.06.patch, HDFSWriteableReplicasInMemory.pdf,
> Per discussion with the community on HDFS-5851, we will implement writing to a single
replica in DN memory via DataTransferProtocol.
> This avoids some of the issues with short-circuit writes, which we can revisit at a later

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