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https://issues.apache.org/jira/browse/HDFS-1312?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13462916#comment-13462916
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Steve Hoffman commented on HDFS-1312:
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Wow, I can't believe this is still lingering out there as a new feature
request. I'd argue this is a bug -- and a big one. Here's why:
* You have Nx 12x3TB machines in your cluster.
* 1 disk fails on 12 drive machine. Let's say they each were 80% full.
* You install the replacement drive (0% full), but by the time you do this the
under-replicated blocks have been fixed (on this and other nodes)
* The 0% full drive will fill at the same rate as the blocks on the other
disks. That machine's other 11 disks will fill to 100% as the block placement
is at a node level and the node seems to use a round-robin algorithm even
though there is more space.
The only way we have found to move blocks internally (without taking the
cluster down completely) is to decommission the node and have it empty and then
re-add it to the cluster so the balancer can take over and move block back onto
it.
Hard drives fail. This isn't news to anybody. The larger (12 disk) nodes only
make the problem worse in time to empty and fill again. Even if you had a 1U 4
disk machine it is still bad 'cause you lose 25% of your capacity on 1 disk
failure where the impact of the 12 disk machine is less than 9%.
The remove/add of a complete node seems like a pretty poor option.
Or am I alone in this? Can we please revive this JIRA?
> 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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