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From "nkeywal (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HBASE-6435) Reading WAL files after a recovery leads to time lost in HDFS timeouts when using dead datanodes
Date Sun, 19 Aug 2012 08:56:38 GMT

     [ https://issues.apache.org/jira/browse/HBASE-6435?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

nkeywal updated HBASE-6435:

    Release Note: 
This JIRA adds a hook in the HDFS client to reorder the replica locations for HLog files.
The default ordering in HDFS is rack aware + random. When reading a HLog file, we prefer not
to use the replica on the same server as the region server that wrote the HLog: this server
is likely to be not available, and this will delay the HBase recovery by one minute. This
occurs because the recovery starts sooner in HBase than in HDFS: 3 minutes by default in HBase
vs. 10:30 minutes in HDFS. This will be changed in HDFS-3703. Moreover, when a HDFS file is
already opened for writing, a read triggers another call to get the file size, leading to
another timeout (see HDFS-3704), but as well a wrong file size value (see HDFS-3701 and HBASE-6401).
- his hook won't be useful anymore when HDFS-3702 or HDFS-3705 or HDFS-3706 is available and
used in HBase.
- the hook intercepts the calls to the nanemode and reorder the locations it returned, extracting
the region server name from the HLog file. This server is put at the end of the list, ensuring
it will be tried only if all the others fail.
- It has been tested with HDFS 1.0.3. of HDFS 2.0 apha.
- It can be deactivated (at master & region server start-up) by setting "hbase.filesystem.reorder.blocks"
to false in the HBase configuration.

> Reading WAL files after a recovery leads to time lost in HDFS timeouts when using dead
> ------------------------------------------------------------------------------------------------
>                 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.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 with 'append' on hdfs.
> Through ZooKeeper, HBase gets informed usually in 30s that it should start the recovery
> 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

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