Mailing-List: contact issues-help@hbase.apache.org; run by ezmlm
Precedence: bulk
Date: Wed, 26 Sep 2012 21:12:09 +1100 (NCT)
From: "nkeywal (JIRA)" <jira@apache.org>
To: issues@hbase.apache.org
Message-ID: <474432291.127652.1348654329561.JavaMail.jiratomcat@arcas>
In-Reply-To: <442594344.82722.1342804055207.JavaMail.jiratomcat@issues-vm>
Subject: [jira] [Commented] (HBASE-6435) Reading WAL files after a recovery
 leads to time lost in HDFS timeouts when using dead datanodes
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit


    [ https://issues.apache.org/jira/browse/HBASE-6435?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13463678#comment-13463678 ] 

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

As HDFS-3701 (dataloss) is into the branch 1.1 as HDFS-3703 (helps to minimize data reads errors), I think it implies that we should target 1.1 for 0.96 as the recommended minimal version. If it's the case, we can remove this fix, as it contains a dependency on hdfs internals. If we keep it, I need to fix the filename analysis and to add "-splitting" on the directories managed. In both cases, it should be done in a separate jiras, but let's have the discussion here.
                
> 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
>             Fix For: 0.96.0
>
>         Attachments: 6435.unfinished.patch, 6435.v10.patch, 6435.v10.patch, 6435.v12.patch, 6435.v12.patch, 6435.v12.patch, 6435-v12.txt, 6435.v13.patch, 6435.v14.patch, 6435.v2.patch, 6435.v7.patch, 6435.v8.patch, 6435.v9.patch, 6435.v9.patch, 6535.v11.patch
>
>
> HBase writes a Write-Ahead-Log to revover from hardware failure. This log is written 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.

--
This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators
For more information on JIRA, see: http://www.atlassian.com/software/jira