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https://issues.apache.org/jira/browse/HDFS-1312?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13462960#comment-13462960
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Steve Hoffman commented on HDFS-1312:
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{quote}
The other thing is that as you grow a grid, you care less and less about the
balance on individual nodes. This issue is of primary important to smaller
installations who likely are under-provisioned hardware-wise anyway.
{quote}
Our installation is about 1PB so I think we can say we are past "small". We
typically run at 70-80% full as we are not made of money. And at 90% the disk
alarms start waking people out of bed.
I would say we very much care about the balance of a single node. When that
node fills, it'll take out the region server, the M/R jobs running on it and
generally anger people who's jobs have to be restarted.
I wouldn't be so quick to discount this. And when you have enough machines,
you are replacing disks more and more frequently. So ANY manual process is $
wasted in people time. Time to re-run jobs, times to take down datanode and
move blocks. Time = $. To turn Hadoop into a more mature product, shouldn't
we be striving for "it just works"?
> Re-balance disks within a Datanode
> ----------------------------------
>
> Key: HDFS-1312
> URL: https://issues.apache.org/jira/browse/HDFS-1312
> Project: Hadoop HDFS
> Issue Type: New Feature
> Components: data-node
> Reporter: Travis Crawford
>
> Filing this issue in response to ``full disk woes`` on hdfs-user.
> Datanodes fill their storage directories unevenly, leading to situations
> where certain disks are full while others are significantly less used. Users
> at many different sites have experienced this issue, and HDFS administrators
> are taking steps like:
> - Manually rebalancing blocks in storage directories
> - Decomissioning nodes & later readding them
> There's a tradeoff between making use of all available spindles, and filling
> disks at the sameish rate. Possible solutions include:
> - Weighting less-used disks heavier when placing new blocks on the datanode.
> In write-heavy environments this will still make use of all spindles,
> equalizing disk use over time.
> - Rebalancing blocks locally. This would help equalize disk use as disks are
> added/replaced in older cluster nodes.
> Datanodes should actively manage their local disk so operator intervention is
> not needed.
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