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From "Tony Valderrama (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HDFS-664) Add a way to efficiently replace a disk in a live datanode
Date Thu, 05 May 2011 15:39:03 GMT

     [ https://issues.apache.org/jira/browse/HDFS-664?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Tony Valderrama updated HDFS-664:

    Attachment: HDFS-664.0-20-3-rc2.patch.1

Steve, thanks for the review!

One general note, before answering your questions - this patch was developed simply by extracting
the relevant sections from the Datanode startup procedure.  I chose 0.20.3-rc2 as the base
because it's closest to what we use at Tuenti.  If there is actual interest in the patch and
no technical barriers (like the threading issues), I'll gladly forward-port it to trunk.

1. Not to my knowledge - a review from someone experienced would be much appreciated.

2. As mentioned above, much of this patch is identical to code elsewhere in 0.20.3-rc2, so
I'll refrain from updating this particular section.  For a trunk forward-port, these sections
would be replaced with the more recent versions.

3. Thanks for the suggestion, it's included in the new patch.

4. Sorry, it's now included.

5. Unfortunately, we use Mercurial internally, and the export process is terrible.  I've rebased
this patch from our internal branch onto 0.20.3-rc2, so it applies cleanly there.

6. I did some basic testing to simulate swapping a volume by moving the data directory out
of the way, touching a file in place of the directory (which causes the Datanode to register
volume failure when it tries to write or read), and then using the shell to add the out-of-place
volume.  You can also add a second data directory on a disk that's already being used to test
swapping in a clean disk.

7. Hmm... that's troubling.  If you find the JIRA, please link it here.  For now, we've tested
this under low load, and when we have time to revisit this, we'll try it out under heavy load.

> 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
>         Attachments: HDFS-664.0-20-3-rc2.patch.1, HDFS-664.patch
> 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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