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https://issues.apache.org/jira/browse/HDFS-822?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12800877#action_12800877
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Todd Lipcon commented on HDFS-822:
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Sounds reasonable, so long as we're confident that holding the FSDataset lock
for 10-20 seconds isn't a bug (eg appending to a 250MB block will take 10
seconds at 25M/sec cross-volume copy rate, could very well be slower if either
disk is stressed)
> 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
> Assignee: Hairong Kuang
> Priority: Blocker
> Fix For: 0.21.0, 0.22.0
>
> Attachments: HDFS-822.patch
>
>
> 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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