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From Allen Wittenauer <awittena...@linkedin.com>
Subject Re: Is this a fair summary of HDFS failover?
Date Wed, 16 Feb 2011 19:32:33 GMT

	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 <mcsrivas@gmail.com> 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 <markkerzner@gmail.com
>>> 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 <mcsrivas@gmail.com>
>> wrote:
>>>> The summary is quite inaccurate.
>>>> On Mon, Feb 14, 2011 at 8:48 AM, Mark Kerzner <markkerzner@gmail.com>
>>>> 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.
>>>> Yes, there is data loss.
>>>>>  - in 0.21 there are streaming edits to a Backup Node (HADOOP-4539),
>>>> which
>>>>>  replaces the Secondary NameNode. The Backup Node can be used as a
>>> warm
>>>>>  spare, with the failover being a matter of seconds. There can be
>>>> multiple
>>>>>  Backup Nodes, for additional insurance against failure, and
>> previous
>>>> best
>>>>>  common practices apply to it;
>>>> There is no "Backup NN" in the manner you are thinking of. It is
>>> completely
>>>> manual, and requires restart of the "whole world", and takes about 2-3
>>>> hours
>>>> to happen. If you are lucky, you may have only a little data loss
>> (people
>>>> have lost entire clusters due to this -- from what I understand, you
>> are
>>>> far
>>>> better off resurrecting the Primary instead of trying to bring up a
>>> Backup
>>>> NN).
>>>> In any case, when you run it like you mention above, you will have to
>>>> (a) make sure that the primary is dead
>>>> (b) edit hdfs-site.xml on *every* datanode to point to the new IP
>> address
>>>> of
>>>> the backup, and restart each datanode.
>>>> (c) wait for 2-3 hours for all the block-reports from every restarted
>> DN
>>> to
>>>> finish
>>>> 2-3 hrs afterwards:
>>>> (d) after that, restart all TT and the JT to connect to the new NN
>>>> (e) finally, restart all the clients (eg, HBase, Oozie, etc)
>>>> Many companies, including Yahoo! and Facebook, use a couple of NetApp
>>>> filers
>>>> to hold the actual data that the NN writes. The two NetApp filers are
>> run
>>>> in
>>>> "HA" mode with NVRAM copying.  But the NN remains a single point of
>>>> failure,
>>>> and there is probably some data loss.
>>>>>  - 0.22 will have further improvements to the HDFS performance, such
>>>>>  as HDFS-1093.
>>>>> Does the paper on HDFS Reliability by Tom
>>>>> White<
>>> http://www.cloudera.com/wp-content/uploads/2010/03/HDFS_Reliability.pdf
>>>>>> still
>>>>> represent the current state of things?
>>>> See Dhruba's blog-post about the Avatar NN + some custom "stackable
>> HDFS"
>>>> code on all the clients + Zookeeper + the dual NetApp filers.
>>>> It helps Facebook do manual, controlled, fail-over during software
>>>> upgrades,
>>>> at the cost of some performance loss on the DataNodes (the DataNodes
>> have
>>>> to
>>>> do 2x block reports, and each block-report is expensive, so it limits
>> the
>>>> DataNode a bit).  The article does not talk about dataloss when the
>>>> fail-over is initiated manually, so I don't know about that.
>> http://hadoopblog.blogspot.com/2010/02/hadoop-namenode-high-availability.html
>>>>> Thank you. Sincerely,
>>>>> Mark

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