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From "nkeywal (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-6435) Reading WAL files after a recovery leads to time lost in HDFS timeouts when using dead datanodes
Date Fri, 20 Jul 2012 21:21:35 GMT

    [ https://issues.apache.org/jira/browse/HBASE-6435?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13419581#comment-13419581
] 

nkeywal commented on HBASE-6435:
--------------------------------

My thinking was it could make it on a hdfs release that accepts changing public interfaces.
I fully agree with you Todd, we need to do our homeworks and push hdfs to ensure that what
we need is understood and makes it to a release. On the other hand, if I look at how it worked
for much simpler stuff like JUnit and surefire, our changes are in theie trunk for a few months
and we're still waiting. These things take time. But I will do my homeworks on hdfs, I promise
(I may need your help actually). The Jira will be created next week and if I have enough feedback
I will propose a patch.

I was also wondering if proposing natively to have interceptors would not be interesting for
hdfs. It was available a long time in an orb called orbix and was great to use. But they would
need to be per conf, so cannot be available with static stuff.

bq. Do we have to do this in both master and regionserver? Can't do it in HFileSystem constructor
assuming it takes a Conf (or that'd be too late?)
It can be put pretty late, basically before we start a recovery process. But we don't want
it client side, so I will check this.

bq. Rather than have it called a reorderProxy, call it an HBaseDFSClient? Might want to add
more customizations while waiting on HDFS fix to arrive.
I've intercepted a lower level call: I'm between the DFSClient and the namenode. This because
the DFSClient does more than just transferring calls: it contains some logic. Hence going
in front of the namenode. But yes, I could make it more generic.

                
> Reading WAL files after a recovery leads to time lost in HDFS timeouts when using dead
datanodes
> ------------------------------------------------------------------------------------------------
>
>                 Key: HBASE-6435
>                 URL: https://issues.apache.org/jira/browse/HBASE-6435
>             Project: HBase
>          Issue Type: Improvement
>          Components: master, regionserver
>    Affects Versions: 0.96.0
>            Reporter: nkeywal
>            Assignee: nkeywal
>         Attachments: 6435.unfinished.patch
>
>
> HBase writes a Write-Ahead-Log to revover from hardware failure.
> This log is written with 'append' on hdfs.
> Through ZooKeeper, HBase gets informed usually in 30s that it should start the recovery
process. 
> This means reading the Write-Ahead-Log to replay the edits on the other servers.
> In standards deployments, HBase process (regionserver) are deployed on the same box as
the datanodes.
> It means that when the box stops, we've actually lost one of the edits, as we lost both
the regionserver and the datanode.
> As HDFS marks a node as dead after ~10 minutes, it appears as available when we try to
read the blocks to recover. As such, we are delaying the recovery process by 60 seconds as
the read will usually fail with a socket timeout. If the file is still opened for writing,
it adds an extra 20s + a risk of losing edits if we connect with ipc to the dead DN.
> Possible solutions are:
> - shorter dead datanodes detection by the NN. Requires a NN code change.
> - better dead datanodes management in DFSClient. Requires a DFS code change.
> - NN customisation to write the WAL files on another DN instead of the local one.
> - reordering the blocks returned by the NN on the client side to put the blocks on the
same DN as the dead RS at the end of the priority queue. Requires a DFS code change or a kind
of workaround.
> The solution retained is the last one. Compared to what was discussed on the mailing
list, the proposed patch will not modify HDFS source code but adds a proxy. This for two reasons:
> - Some HDFS functions managing block orders are static (MD5MD5CRC32FileChecksum). Implementing
the hook in the DFSClient would require to implement partially the fix, change the DFS interface
to make this function non static, or put the hook static. None of these solution is very clean.

> - Adding a proxy allows to put all the code in HBase, simplifying dependency management.
> Nevertheless, it would be better to have this in HDFS. But this solution allows to target
the last version only, and this could allow minimal interface changes such as non static methods.
> Moreover, writing the blocks to the non local DN would be an even better solution long
term.

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