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From Nathaniel Cook <nathani...@qualtrics.com>
Subject Re: How to rebuild the shared edits directory
Date Tue, 08 May 2012 18:07:13 GMT
On Tue, May 8, 2012 at 11:44 AM, Todd Lipcon <todd@cloudera.com> wrote:
> On Tue, May 8, 2012 at 10:33 AM, Nathaniel Cook
> <nathanielc@qualtrics.com> wrote:
>> We ran the initializeSharedEdits command and it didn't have any
>> effect, but that my be because of the weird state we got it in.
>> So help me understand: I was under the assumption that if shared edits
>> went away you would lose the ability to failover and that is it. The
>> active namenode would still function but would not failover and all
>> standy namenodes would not try to become active. Is this correct?
> Unfortunately that's not the case. If you lose shared edits, your
> cluster should shut down. We currently require the NFS direcory to be
> highly available itself. This is achievable with even pretty
> inexpensive NAS devices from your vendor of choice.
> The reason for this behavior is as follows: if the active node loses
> access to the mount, it's unable to distinguish whether the mount
> itself died or if the node just had a local issue which broke the
> mount. Imagine for example that the NFS client had a bug which caused
> the mount to go away. Then, you'd continue running for quite some time
> without writing to shared edits. If your NN then crashed, a failover
> would cause you to revert to an old version of the namespace, and
> you'd have a case of permanent data loss due to divergence of the
> image before and after failover.

Ok this makes sense but I still have a question. What if when the
active namenode looses its connection to the shared edits instead of
shutting down the cluster degrades to a "non-failover" cluster. So in
the case of the active node losing its access to the mount, it could
tell the standby nodes to either shutdown or to not ever attempt to
become active using the already in place failover communication
systems. Then you could fix the NFS mount while in "non-failover" mode
and once its back up sync the missing edits. Then tell the standby
nodes to come back online.  This way both the active namenode and
shared edits have to fail at the same time for the cluster to go down.
The worst that can happen when the shared edits goes down is that you
loose the ability to failover. It this plausible or are there parts of
the system that would prevent this type of behavior?

> There's work under way to remove this restriction which should be
> available for general use some time this summer or early fall, if I
> had to take a guess on timeline.
>> If
>> it is the case that namenodes quit when they lose connection to the
>> shared edits dir than doesn't the shared edits becomes the new single
>> point of failure?
> Yes, but it's an easy one to resolve. Most of our customers already
> have a NAS device in their datacenter, which has dual heads, dual
> PDUs, etc, and at least 5 9s of uptime. This HA setup is basically the
> same as you see in most enterprise HA systems which rely on shared
> storage.

What about cloud environments where you dont have the ability to
create a NAS? AWS for example.

>> Unfortunately we have cleared the logs from this test but we could try
>> to reproduce it.
> That would be great, thanks!
> -Todd
>> On Tue, May 8, 2012 at 10:28 AM, Todd Lipcon <todd@cloudera.com> wrote:
>>> On Tue, May 8, 2012 at 7:46 AM, Nathaniel Cook <nathanielc@qualtrics.com>
>>>> We have be working with an HA hdfs cluster, testing several failover
>>>> scenarios.  We have a small cluster of 4 machines spun up for testing.
>>>> We run a namenode on two of the machines and hosted an nfs share on
>>>> the third for the shared edits directory. The fourth machine is just a
>>>> datanode. We configured the cluster for automatic failover using ZKFC.
>>>> We can start and stop the namenodes with no problems, failover happens
>>>> as expected. Then we tested breaking the shared edits directory. We
>>>> stopped the nfs share and then reenabled it. This caused the loss of a
>>>> few edits.
>>> Really? What mount options are you using on your NFS mount?
>>> The active NN should abort immediately if the shared edits dir
>>> disappears. Do you have logs available from your NNs during this time?
>>>> This had no effect, as expected, on the namenodes, and the
>>>> cluster functioned normally.
>>> On the contrary, I'd expect the NN to bail out on the next edit (since
>>> it has no place to reliably fsync it)
>>>> We stopped the standby namenode and tried
>>>> to start it again, it would not start because of the missing edits. No
>>>> matter what we tried we could not rebuild the shared edits directory
>>>> and thus get the second namenode back online. In this state the hdfs
>>>> cluster continued to function but it was no longer an HA cluster. To
>>>> get the cluster back in HA mode we had to reformat the namenode data
>>>> with the shared edits. In this case how do you rebuild the shared
>>>> edits data so you can get the cluster back to an HA mode?
>>> It sounds like something went wrong with the facility that's supposed
>>> to make the active NN crash if shared edits go away. The logs will
>>> help.
>>> To answer your question, though, you can run the
>>> "initializeSharedEdits" process again to re-initialize that edits dir.
>>> Thanks
>>> -Todd
>>> --
>>> Todd Lipcon
>>> Software Engineer, Cloudera
>> --
>> -Nathaniel Cook
> --
> Todd Lipcon
> Software Engineer, Cloudera

-Nathaniel Cook

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