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From Jonathan Disher <jdis...@parad.net>
Subject Re: DataNode internal balancing, performance recommendations
Date Tue, 04 Jan 2011 06:29:00 GMT
That's what we've been doing.  Again, the problem is, we still have to pull the datanode out
of rotation and change config, replace disk, put it back... even if I have spares on hand
and finish this in a few minutes, I still have one empty disk and many tens of not-empty disks.
 Monitoring and identifying the failure isn't the problem, we have that down pat.  I'm hoping
for a better way to re-balance the disks in the node after a failure.  I suspect the sad answer
is that what I'm doing now is the best thing for it.


On Jan 3, 2011, at 10:21 PM, Esteban Gutierrez Moguel wrote:

> Jonathan,
> Hadoop will throw an exception according to the kind of error: AccessControlException
if its permission related or IOException for any other disk related task.
> A safer approach to handle physical failures would be monitoring syslog messages (Syslog4j,
nagios, ganglia, etc.) and if you are lucky enough and the node doesn't hangs after the disk
failure, you could shutdown it gracefully.
> esteban.
> On Mon, Jan 3, 2011 at 13:55, Jonathan Disher <jdisher@parad.net> wrote:
> The problem is, what do you define as a failure?  If the disk is failing, writes will
fail to the filesystem - how does Hadoop differentiate between permissions and physical disk
failure?  They both return error.
> And yeah, the idea of stopping the datanode, removing the affected mount from hdfs-site.xml,
and restarting has been discussed.  The problem is, when that disk gets replaced, and readded,
then I have horrible internal balance issues.  Thus causing the problem I have now :(
> -j
> On Jan 3, 2011, at 9:07 AM, Eli Collins wrote:
> > Hey Jonathan,
> >
> > There's an option (dfs.datanode.failed.volumes.tolerated, introduced
> > in HDFS-1161) that allows you to specify the number of volumes that
> > are allowed to fail before a datanode stops offering service.
> >
> > There's an operational issue that still needs to be addressed
> > (HDFS-1158) that you should be aware of - the DN will still not start
> > if any of the volumes have failed, so to restart the DN you'll need
> > you'll need to either unconfigure the failed volumes or fix them. I'd
> > like to make DN startup respect the config value so it tolerates
> > failed volumes on startup as well.
> >
> > Thanks,
> > Eli
> >
> > On Sun, Jan 2, 2011 at 7:20 PM, Jonathan Disher <jdisher@parad.net> wrote:
> >> I see that there was a thread on this in December, but I can't retrieve it to
reply properly, oh well.
> >>
> >> So, I have a 30 node cluster (plus separate namenode, jobtracker, etc).  Each
is a 12 disk machine - two mirrored 250GB OS disks, ten 1TB data disks in JBOD.  Original
system config was six 1TB data disks - we added the last four disks months later.  I'm sure
you can all guess, we have some interesting internal usage balancing issues on most of the
nodes.  To date, when individual disks get critically low on space (earlier this week I had
a node with six disks around 97% full, four around 70%), we've been pulling them from the
cluster, formatting the data disks, and sticking them back in (with a rebalance running to
keep the cluster in some semblance of order).
> >>
> >> Obviously if there was a better way to do this, I'd love to see it.  I see that
there are recommendations of killing the DataNode process and manually moving files, but my
concern is that the DataNode process will spend an enormous amount of time tracking down these
moves (currently around 820,000 blocks/node).  And it's not necessarily easy to automate,
so there's the danger of nuking blocks, and making the problems worse.  Are there alternatives
to manual moves (or more automated ways that exist)?  Or has my brute-force rebalance got
the best chance of success, albeit slowly?
> >>
> >> We are also building a new cluster - starting around 1.2PB raw, eventually growing
to around 5PB, for near-line storage of data.  Our storage nodes will probably be 4U systems
with 72 data disks each (yeah, good times).  The problem with this becomes obvious - with
the way Hadoop works today, if a disk fails, the datanode process chokes and dies when it
tries to write to it.  We've been told repeatedly that Hadoop doesn't perform well when it
operates on RAID arrays, but, to scale efffectively, we're going to have to do just that -
three 24 disk controllers in RAID-6 mode.  How bad is this going to be?  JBOD just doesn't
scale beyond a couple disks per machine, the failure rate will knock machines out of the cluster
too often (and at 60TB per node, rebalancing will take forever, even if I let it saturate
> >>
> >> I appreciate opinions and suggestions.  Thanks!
> >>
> >> -j

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