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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-395) DFS Scalability: Incremental block reports
Date Thu, 14 Jul 2011 20:59:00 GMT

    [ https://issues.apache.org/jira/browse/HDFS-395?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13065530#comment-13065530
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Todd Lipcon commented on HDFS-395:
----------------------------------

bq. Todd, please correct me if I am wrong, but I don't see that this is happening in FSDataset.invalidate().


Ah, it seems we have three separate but similar AsyncDiskServices in Hadoop :) The MRAsyncDiskService
class from MAPREDUCE-1302 uses a separate "toBeDeleted" directory as described above - I thought
we used the same technique in the HDFS side, but it appears it's just a deferred delete of
the file in the same location as you described.

Can we share some code here? There's already another AsyncDiskService superclass in common.

> DFS Scalability: Incremental block reports
> ------------------------------------------
>
>                 Key: HDFS-395
>                 URL: https://issues.apache.org/jira/browse/HDFS-395
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>            Reporter: dhruba borthakur
>            Assignee: dhruba borthakur
>         Attachments: blockReportPeriod.patch, explicitDeleteAcks.patch
>
>
> I have a cluster that has 1800 datanodes. Each datanode has around 50000 blocks and sends
a block report to the namenode once every hour. This means that the namenode processes a block
report once every 2 seconds. Each block report contains all blocks that the datanode currently
hosts. This makes the namenode compare a huge number of blocks that practically remains the
same between two consecutive reports. This wastes CPU on the namenode.
> The problem becomes worse when the number of datanodes increases.
> One proposal is to make succeeding block reports (after a successful send of a full block
report) be incremental. This will make the namenode process only those blocks that were added/deleted
in the last period.

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