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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-822) Appends to already-finalized blocks can rename across volumes
Date Thu, 10 Dec 2009 00:58:18 GMT

    [ https://issues.apache.org/jira/browse/HDFS-822?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12788428#action_12788428
] 

Todd Lipcon commented on HDFS-822:
----------------------------------

I haven't tried to actually produce this bug yet, but it may be reasonably important, since
this is also a synchronized method, and a full cross-volume block copy could take several
seconds at least, during which the DN can't do a whole lot.

> Appends to already-finalized blocks can rename across volumes
> -------------------------------------------------------------
>
>                 Key: HDFS-822
>                 URL: https://issues.apache.org/jira/browse/HDFS-822
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: data-node
>    Affects Versions: 0.21.0, 0.22.0
>            Reporter: Todd Lipcon
>
> This is a performance thing. As I understand the code in FSDataset.append, if the block
is already finalized, it needs to move it into the RBW directory so it can go back into a
"being written" state. This is done using volumes.getNextVolume without preference to the
volume that the block currently exists on. It seems to me that this could cause a lot of slow
cross-volume copies on applications that periodically append/close/append/close a file. Instead,
getNextVolume could provide an alternate form that gives preference to a particular volume,
so the rename stays on the same disk.

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