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From "Tsz Wo (Nicholas), SZE (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-744) Support hsync in HDFS
Date Fri, 18 May 2012 00:25:04 GMT

    [ https://issues.apache.org/jira/browse/HDFS-744?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13278444#comment-13278444
] 

Tsz Wo (Nicholas), SZE commented on HDFS-744:
---------------------------------------------

Thanks a lot, Lars!  The patch looks good.  Some comments:

- Let's name the new CreateFlag as SYNC_BLOCK instead of FORCE.  POSIX uses SYNC as you mentioned
but POSIX SYNC means syncing every write.

- DFSOutputStream.hsync(),
-* It should call flush(true).  It is better to sync the current block then not syncing at
all.
-* Need to update the javadoc to say that it only sync the current block.

- Rename flush(force) to flushOrSync(isSync) in BlockReceiver and DFSOutputStream.  Please
also update the javadoc.

- We do not use tabs in Hadoop.  Indentation should use two spaces.

- Please add some new tests.  It is not easy to test whether sync actually works but at least
add some new test to call hsync().  See TestFileAppend and TestFileAppend[234] to get some
ideas.
                
> Support hsync in HDFS
> ---------------------
>
>                 Key: HDFS-744
>                 URL: https://issues.apache.org/jira/browse/HDFS-744
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>          Components: data-node, hdfs client
>            Reporter: Hairong Kuang
>            Assignee: Lars Hofhansl
>         Attachments: HDFS-744-trunk-v2.patch, HDFS-744-trunk-v3.patch, HDFS-744-trunk-v4.patch,
HDFS-744-trunk.patch, hdfs-744-v2.txt, hdfs-744-v3.txt, hdfs-744.txt
>
>
> HDFS-731 implements hsync by default as hflush. As descriibed in HADOOP-6313, the real
expected semantics should be "flushes out to all replicas and all replicas have done posix
fsync equivalent - ie the OS has flushed it to the disk device (but the disk may have it in
its cache)." This jira aims to implement the expected behaviour.

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