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From "stack (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3113) Provide a configurable way for DFSOututStream.flush() to flush data to real block file on DataNode.
Date Tue, 01 Apr 2008 23:19:24 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3113?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12584353#action_12584353

stack commented on HADOOP-3113:

> I can make the configurable to be per file, but maybe it makes more sense to make it
applicable to the entire system. 

When the setting is not per file:

1. HBase often is new-kid on the block and but one of the many users of an HDFS install. 
HBase installers may find it difficult convincing HDFS admins they need to make the change.
2. If per-file, HBase can manage the configuration.  Otherwise, its a two-step process.  HBase
installers may plain forget.
3. Minor-point: HBase needs append only for its Write-Ahead Log; nowhere else.

If its an 'entire system' setting, will it require an HDFS restart to take effect?

Sounds like we should set the blocksize for our Write-Ahead Log to be a good deal smaller
than default.

I took a quick look at the patch.

The hadoop-default.xml entry description is all on one line.  You might want to break it up.
 Also, is there a downside to setting the dfs.datanode.skipTmpFile flag (Reading the description,
in my head I'm thinking there must be or why even bother with this configuration?)

Otherwise, patch looks good to me. 

Do you have a suggestion for a test I might run to exercise this new functionality?

Thanks Dhruba

> Provide a configurable way for DFSOututStream.flush() to flush data to real block file
on DataNode.
> ---------------------------------------------------------------------------------------------------
>                 Key: HADOOP-3113
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3113
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: dfs
>            Reporter: dhruba borthakur
>            Assignee: dhruba borthakur
>         Attachments: noTmpFile.patch
> DFSOutputStream has a method called flush() that persists block locations on the namenode
and sends all outstanding data to all datanodes in the pipeline. However, this data goes to
the tmp file on the datanode(s). When the block is closed, the tmp files is renamed to be
the real block file. If the datanode(s) dies before the block is compete, then entire block
is lost. This behaviour wil be fixed in HADOOP-1700.
> However, in the short term, a configuration paramater can be used to allow datanodes
to write to the real block file directly, thereby avoiding writing to the tmp file. This means
that data that is flushed successfully by a client does not get lost even if the datanode(s)
or client dies.
> The Namenode already has code to pick the largest replica (if multiple datanodes have
different sizes of this block). Also, the namenode has code to not trigger replication request
if the file is still being written to.
> The only caveat that I can think of is that the block report periodicity should be much
much smaller that the lease timeout period. A block report adds the being-written-to blocks
to the blocksMap thereby avoiding any cleanup that a lease expiry processing might have otherwise
> Not all requirements specified by HADOOP-1700 are supported by this approach, but it
could still be helpful (in the short term) for a wide range of applications.

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