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From Todd Lipcon <t...@cloudera.com>
Subject Re: Secondary NameNodes or NFS exports?
Date Thu, 24 Dec 2009 18:34:33 GMT
How long does the checkpoint take? It seems possible to me that if the 2NN
checkpoint takes longer than the interval, it's possible that multiple
checkpoints will overlap and might trigger this. (this is conjecture, so
definitely worth testing)

-Todd

On Wed, Dec 23, 2009 at 6:38 PM, Jason Venner <jason.hadoop@gmail.com>wrote:

> I agree, it seems very wrong, that is why I need a block of time to really
> verify the behavior.
>
> My test case is the following, and the same test fails in 18.3 and 19.0 and
> 19.1
>
> set up a single node cluster, 1 namenode, 1 datanode, 1 secondary, all on
> the same machine.
> set the checkpoint interval to 2 minutes (120 sec)
>
> make a few files, wait, and verify that a checkpoint can happen.
>
> recursively start coping a deep tree into hdfs, what the checkpoint fail
> with a timestamp error.
>
> The code explicitly uses the edits.new for the checkpoint verification
> timestamp.
>
> The window is the time from the take of the edit log to the return of the
> fsimage.
>
> On Wed, Dec 23, 2009 at 5:52 PM, Brian Bockelman <bbockelm@cse.unl.edu
> >wrote:
>
> > Hey Jason,
> >
> > This analysis seems fairly unlikely - are you claiming that no edits can
> be
> > merged if files are being created?  Isn't this what edits.new is for?
> >
> > We roll the edits log successfully during periods of high transfer, when
> a
> > new file is being created every 1 second or so.
> >
> > We have had issues with unmergeable edits before - there might be some
> race
> > conditions in this area.
> >
> > Brian
> >
> > On Dec 23, 2009, at 7:07 PM, Jason Venner wrote:
> >
> > > I have no current solution.
> > > When I can block a few days, I am going to instrument the code a bit
> more
> > to
> > > verify my understanding.
> > >
> > > I believe the issue is that the time stamp is being checked against the
> > > active edit log (the new one created then the checkpoint started)
> rather
> > > than the time stamp of the rolled (old) edit log.
> > > As long as no transactions have hit, the time stamps are the same.
> > >
> > >
> > > On Wed, Dec 23, 2009 at 11:23 AM, Stas Oskin <stas.oskin@gmail.com>
> > wrote:
> > >
> > >> Hi.
> > >>
> > >> What was your solution to this then?
> > >>
> > >> Regards.
> > >>
> > >> On Sat, Dec 5, 2009 at 7:43 AM, Jason Venner <jason.hadoop@gmail.com>
> > >> wrote:
> > >>
> > >>> I have dug into this more, it turns out the problem is unrelated to
> nfs
> > >> or
> > >>> solaris.
> > >>> The issue is that if there is a meta data change, while the secondary
> > is
> > >>> rebuilding the fsimage, the rebuilt image is rejected.
> > >>> On our production cluster, there is almost never a moment where there
> > is
> > >>> not
> > >>> a file being created or altered, and as such the secondary is never
> > make
> > >> a
> > >>> fresh fsimage for the cluster.
> > >>>
> > >>> I have checked this with several hadoop variants and with vanilla
> > >>> distributions with the namenode, secondary and a datanode all running
> > on
> > >>> the
> > >>> same machine.
> > >>>
> > >>> On Tue, Oct 27, 2009 at 8:03 PM, Jason Venner <
> jason.hadoop@gmail.com
> > >>>> wrote:
> > >>>
> > >>>> The namenode would never accept the rebuild fsimage from the
> > secondary,
> > >>> so
> > >>>> the edit logs grew with outbounds.
> > >>>>
> > >>>>
> > >>>> On Tue, Oct 27, 2009 at 10:51 AM, Stas Oskin <stas.oskin@gmail.com>
> > >>> wrote:
> > >>>>
> > >>>>> Hi.
> > >>>>>
> > >>>>> You mean, you couldn't recover the NameNode from checkpoints
> because
> > >> of
> > >>>>> timestamps?
> > >>>>>
> > >>>>> Regards.
> > >>>>>
> > >>>>> On Tue, Oct 27, 2009 at 4:49 PM, Jason Venner <
> > jason.hadoop@gmail.com
> > >>>>>> wrote:
> > >>>>>
> > >>>>>> We have been having some trouble with the secondary on
a cluster
> > >> that
> > >>>>> has
> > >>>>>> one edit log partition on an nfs server, with the namenode
> rejecting
> > >>> the
> > >>>>>> merged images due to timestamp missmatches.
> > >>>>>>
> > >>>>>>
> > >>>>>> On Mon, Oct 26, 2009 at 10:14 AM, Stas Oskin <
> stas.oskin@gmail.com>
> > >>>>> wrote:
> > >>>>>>
> > >>>>>>> Hi.
> > >>>>>>>
> > >>>>>>> Thanks for the advice, it seems that the initial approach
of
> > >> having
> > >>>>>> single
> > >>>>>>> SecNameNode writing to exports is the way to go.
> > >>>>>>>
> > >>>>>>> By the way, I asked this already, but wanted to clarify:
> > >>>>>>>
> > >>>>>>> * It's possible to set how often SecNameNode checkpoints
the data
> > >>>>> (what
> > >>>>>> is
> > >>>>>>> the setting by the way)?
> > >>>>>>>
> > >>>>>>> * It's possible to let NameNode write to exports as
well together
> > >>> with
> > >>>>>>> local
> > >>>>>>> disk, which ensures the latest possible meta-data in
case of disk
> > >>>>> crash
> > >>>>>>> (compared to pereodic check-pointing), but it's going
to slow
> down
> > >>> the
> > >>>>>>> operations due to network read/writes.
> > >>>>>>>
> > >>>>>>> Thanks again.
> > >>>>>>>
> > >>>>>>> On Thu, Oct 22, 2009 at 10:03 PM, Patrick Angeles
> > >>>>>>> <patrickangeles@gmail.com>wrote:
> > >>>>>>>
> > >>>>>>>> From what I understand, it's rather tricky to set
up multiple
> > >>>>> secondary
> > >>>>>>>> namenodes. In either case, running multiple 2ndary
NNs doesn't
> > >> get
> > >>>>> you
> > >>>>>>>> much.
> > >>>>>>>> See this thread:
> > >>>>>>>>
> > >>>>>
> > http://www.mail-archive.com/core-user@hadoop.apache.org/msg06280.html
> > >>>>>>>>
> > >>>>>>>> On Wed, Oct 21, 2009 at 10:44 AM, Stas Oskin <
> > >>> stas.oskin@gmail.com>
> > >>>>>>> wrote:
> > >>>>>>>>
> > >>>>>>>>> To clarify, it's either let single SecNameNode
to write to
> > >>>>> multiple
> > >>>>>> NFS
> > >>>>>>>>> exports, or actually have multiple SecNameNodes.
> > >>>>>>>>>
> > >>>>>>>>> Thanks again.
> > >>>>>>>>>
> > >>>>>>>>> On Wed, Oct 21, 2009 at 4:43 PM, Stas Oskin
<
> > >>> stas.oskin@gmail.com
> > >>>>>>
> > >>>>>>>> wrote:
> > >>>>>>>>>
> > >>>>>>>>>> Hi.
> > >>>>>>>>>>
> > >>>>>>>>>> I'm want to keep a checkpoint data on several
separate
> > >>> machines
> > >>>>> for
> > >>>>>>>>> backup,
> > >>>>>>>>>> and deliberating between exporting these
machines disks via
> > >>> NFS,
> > >>>>> or
> > >>>>>>>>> actually
> > >>>>>>>>>> running Secondary Name Nodes there.
> > >>>>>>>>>>
> > >>>>>>>>>> Can anyone advice what would be better
in my case?
> > >>>>>>>>>>
> > >>>>>>>>>> Regards.
> > >>>>>>>>>>
> > >>>>>>>>>
> > >>>>>>>>
> > >>>>>>>
> > >>>>>>
> > >>>>>>
> > >>>>>>
> > >>>>>> --
> > >>>>>> Pro Hadoop, a book to guide you from beginner to hadoop
mastery,
> > >>>>>> http://www.amazon.com/dp/1430219424?tag=jewlerymall
> > >>>>>> www.prohadoopbook.com a community for Hadoop Professionals
> > >>>>>>
> > >>>>>
> > >>>>
> > >>>>
> > >>>>
> > >>>> --
> > >>>> Pro Hadoop, a book to guide you from beginner to hadoop mastery,
> > >>>> http://www.amazon.com/dp/1430219424?tag=jewlerymall
> > >>>> www.prohadoopbook.com a community for Hadoop Professionals
> > >>>>
> > >>>
> > >>>
> > >>>
> > >>> --
> > >>> Pro Hadoop, a book to guide you from beginner to hadoop mastery,
> > >>> http://www.amazon.com/dp/1430219424?tag=jewlerymall
> > >>> www.prohadoopbook.com a community for Hadoop Professionals
> > >>>
> > >>
> > >
> > >
> > >
> > > --
> > > Pro Hadoop, a book to guide you from beginner to hadoop mastery,
> > > http://www.amazon.com/dp/1430219424?tag=jewlerymall
> > > www.prohadoopbook.com a community for Hadoop Professionals
> >
> >
>
>
> --
> Pro Hadoop, a book to guide you from beginner to hadoop mastery,
> http://www.amazon.com/dp/1430219424?tag=jewlerymall
> www.prohadoopbook.com a community for Hadoop Professionals
>

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