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From "Allen Wittenauer (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-664) Add a way to efficiently replace a disk in a live datanode
Date Wed, 30 Sep 2009 17:13:23 GMT

    [ https://issues.apache.org/jira/browse/HDFS-664?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12760959#action_12760959
] 

Allen Wittenauer commented on HDFS-664:
---------------------------------------

TT needs the same functionality if you consider that chances are high there is mapred space
on the same drive.

More and more it is evident that Hadoop needs just a general file system management daemon
that can inform all processes on the box that a drive and its associated file systems are
dead.

> Add a way to efficiently replace a disk in a live datanode
> ----------------------------------------------------------
>
>                 Key: HDFS-664
>                 URL: https://issues.apache.org/jira/browse/HDFS-664
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>          Components: data-node
>    Affects Versions: 0.22.0
>            Reporter: Steve Loughran
>
> In clusters where the datanode disks are hot swappable, you need to be able to swap out
a disk on a live datanode without taking down the datanode. You don't want to decommission
the whole node as that is overkill. on a system with 4 1TB HDDs, giving 3 TB of datanode storage,
a decommissioning and restart will consume up to 6 TB of bandwidth. If a single disk were
swapped in then there would only be 1TB of data to recover over the network. More importantly,
if that data could be moved to free space on the same machine, the recommissioning could take
place at disk rates, not network speeds. 
> # Maybe have a way of decommissioning a single disk on the DN; the files could be moved
to space on the other disks or the other machines in the rack.
> # There may not be time to use that option, in which case pulling out the disk would
be done with no warning, a new disk inserted.
> # The DN needs to see that a disk has been replaced (or react to some ops request telling
it this), and start using the new disk again -pushing back data, rebuilding the balance. 
> To complicate the process, assume there is a live TT on the system, running jobs against
the data. The TT would probably need to be paused while the work takes place, any ongoing
work handled somehow. Halting the TT and then restarting it after the replacement disk went
in is probably simplest. 
> The more disks you add to a node, the more this scenario becomes a need.

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