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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HADOOP-2657) Enhancements to DFSClient to support flushing data at any point in time
Date Fri, 07 Mar 2008 00:08:58 GMT

     [ https://issues.apache.org/jira/browse/HADOOP-2657?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

dhruba borthakur updated HADOOP-2657:
-------------------------------------

    Attachment: flush5.patch

Incorporated Raghu's review comments.

One question is "what is the semantics of flush?". My opinion is that the client should confirm
that the data has reached the OS buffers on all datanodes in the pipeline before the flush
call returns. This will enable applications like HBase to use this flush API on the HBase
transaction log (which is a HDFS file) and rest easy that it is persisted.

If the DFSOutputStream.flush() does not guarantee that the data has reached the OS buffers
on datanode(s) then this API migth not be useful for HBase.

> Enhancements to DFSClient to support flushing data at any point in time
> -----------------------------------------------------------------------
>
>                 Key: HADOOP-2657
>                 URL: https://issues.apache.org/jira/browse/HADOOP-2657
>             Project: Hadoop Core
>          Issue Type: New Feature
>          Components: dfs
>            Reporter: dhruba borthakur
>            Assignee: dhruba borthakur
>         Attachments: flush.patch, flush2.patch, flush3.patch, flush4.patch, flush5.patch
>
>
> The HDFS Append Design (HADOOP-1700) requires that there be a public API to flush data
written to a HDFS file that can be invoked by an application. This API (popularly referred
to a fflush(OutputStream)) will ensure that data written to the DFSOutputStream is flushed
to datanodes and any required metadata is persisted on Namenode.
> This API has to handle the case when the client decides to flush after writing data that
is not a exact multiple of io.bytes.per.checksum.

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