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https://issues.apache.org/jira/browse/HADOOP-2186?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12542621
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Raghu Angadi commented on HADOOP-2186:
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I don't have data but some time back for essentially random accesses, 5 x no of
disks was pretty re good. In this case, these are hardlinks but treating it as
random access would be conservative.
I think something like 10-15 threads will cover most common case of disks and
cores.
> DFS Upgrade should process dfs.data.dirs in parallel
> ----------------------------------------------------
>
> Key: HADOOP-2186
> URL: https://issues.apache.org/jira/browse/HADOOP-2186
> Project: Hadoop
> Issue Type: Improvement
> Components: dfs
> Affects Versions: 0.15.0
> Reporter: Stu Hood
> Priority: Minor
>
> I just upgraded from 0.14.2 to 0.15.0, and things went very smoothly, if a
> little slowly.
> The main reason the upgrade took so long was the block upgrades on the
> datanodes. Each of our datanodes has 3 drives listed for the dfs.data.dir
> parameter. From looking at the logs, it is fairly clear that the upgrade
> procedure does not attempt to upgrade all listed dfs.data.dir's in parallel.
> I think even if all of your dfs.data.dir's are on the same physical device,
> there would still be an advantage to performing the upgrade process in
> parallel. The less downtime, the better: especially if it is potentially 20
> minutes versus 60 minutes.
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