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Hadoop, mail # user - Is this a fair summary of HDFS failover?


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Re: Is this a fair summary of HDFS failover?
Allen Wittenauer 2011-02-16, 19:32

I'm more than a little concerned that you missed the whole multiple directories--including a remote one--for the fsimage thing.  That's probably the #1 thing that most of the big grids do to maintain the NN data.  I can only remember one failure where the NFS copy wasn't used to recover a namenode in all the failures I've personally been involved (and that was an especially odd bug, not a NN failure, per se).  The only reason to fall back to the 2ndary NN in 0.20 should be is if you've hit a similarly spectacular bug.  Point blank: anyone who runs the NN without it writing to a remote copy doesn't know what they are doing.

Also, until AvatarNode comes of age (which, from what I understand, FB has only been doing for very long themselves), there is no such thing as HA NN.  We all have high hopes that it works out, but it likely isn't anywhere near ready for primetime yet.

On Feb 14, 2011, at 2:52 PM, Mark Kerzner wrote:

> I completely agree, and I am using yours and the group's posting to define
> the direction and approaches, but I am also trying every solution - and I am
> beginning to do just that, the AvatarNode now.
>
> Thank you,
> Mark
>
> On Mon, Feb 14, 2011 at 4:43 PM, M. C. Srivas <[EMAIL PROTECTED]> wrote:
>
>> I understand you are writing a book "Hadoop in Practice".  If so, its
>> important that what's recommended in the book should be verified in
>> practice. (I mean, beyond simply posting in this newsgroup - for instance,
>> the recommendations on NN fail-over should be tried out first before
>> writing
>> about how to do it). Otherwise you won't know your recommendations really
>> work or not.
>>
>>
>>
>> On Mon, Feb 14, 2011 at 12:31 PM, Mark Kerzner <[EMAIL PROTECTED]
>>> wrote:
>>
>>> Thank you, M. C. Srivas, that was enormously useful. I understand it now,
>>> but just to be complete, I have re-formulated my points according to your
>>> comments:
>>>
>>>  - In 0.20 the Secondary NameNode performs snapshotting. Its data can be
>>>  used to recreate the HDFS if the Primary NameNode fails. The procedure
>> is
>>>  manual and may take hours, and there is also data loss since the last
>>>  snapshot;
>>>  - In 0.21 there is a Backup Node (HADOOP-4539), which aims to help with
>>>  HA and act as a cold spare. The data loss is less than with Secondary
>> NN,
>>>  but it is still manual and potentially error-prone, and it takes hours;
>>>  - There is an AvatarNode patch available for 0.20, and Facebook runs
>> its
>>>  cluster that way, but the patch submitted to Apache requires testing
>> and
>>> the
>>>  developers adopting it must do some custom configurations and also
>>> exercise
>>>  care in their work.
>>>
>>> As a conclusion, when building an HA HDFS cluster, one needs to follow
>> the
>>> best
>>> practices outlined by Tom
>>> White<
>>> http://www.cloudera.com/wp-content/uploads/2010/03/HDFS_Reliability.pdf
>>> ,
>>> and may still need to resort to specialized NSF filers for running the
>>> NameNode.
>>>
>>> Sincerely,
>>> Mark
>>>
>>>
>>>
>>> On Mon, Feb 14, 2011 at 11:50 AM, M. C. Srivas <[EMAIL PROTECTED]>
>> wrote:
>>>
>>>> The summary is quite inaccurate.
>>>>
>>>> On Mon, Feb 14, 2011 at 8:48 AM, Mark Kerzner <[EMAIL PROTECTED]>
>>>> wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>> is it accurate to say that
>>>>>
>>>>>  - In 0.20 the Secondary NameNode acts as a cold spare; it can be
>> used
>>>> to
>>>>>  recreate the HDFS if the Primary NameNode fails, but with the delay
>>> of
>>>>>  minutes if not hours, and there is also some data loss;
>>>>>
>>>>
>>>>
>>>> The Secondary NN is not a spare. It is used to augment the work of the
>>>> Primary, by offloading some of its work to another machine. The work
>>>> offloaded is "log rollup" or "checkpointing". This has been a source of
>>>> constant confusion (some named it incorrectly as a "secondary" and now
>> we
>>>> are stuck with it).
>>>>
>>>> The Secondary NN certainly cannot take over for the Primary. It is not
>>> its
>>>> purpose.