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https://issues.apache.org/jira/browse/HDFS-7928?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14380682#comment-14380682
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Rushabh S Shah commented on HDFS-7928:
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Thanks [~kihwal] for reviewing and committing.
> Scanning blocks from disk during rolling upgrade startup takes a lot of time
> if disks are busy
> ----------------------------------------------------------------------------------------------
>
> Key: HDFS-7928
> URL: https://issues.apache.org/jira/browse/HDFS-7928
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: datanode
> Affects Versions: 2.6.0
> Reporter: Rushabh S Shah
> Assignee: Rushabh S Shah
> Fix For: 2.8.0
>
> Attachments: HDFS-7928-v1.patch, HDFS-7928-v2.patch, HDFS-7928.patch
>
>
> We observed this issue in rolling upgrade to 2.6.x on one of our cluster.
> One of the disks was very busy and it took long time to scan that disk
> compared to other disks.
> Seeing the sar (System Activity Reporter) data we saw that the particular
> disk was very busy performing IO operations.
> Requesting for an improvement during datanode rolling upgrade.
> During shutdown, we can persist the whole volume map on the disk and let the
> datanode read that file and create the volume map during startup after
> rolling upgrade.
> This will not require the datanode process to scan all the disk and read the
> block.
> This will significantly improve the datanode startup time.
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