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https://issues.apache.org/jira/browse/HADOOP-6313?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12771205#action_12771205
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Sanjay Radia commented on HADOOP-6313:
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+1
> Expose flush APIs to application users
> --------------------------------------
>
> Key: HADOOP-6313
> URL: https://issues.apache.org/jira/browse/HADOOP-6313
> Project: Hadoop Common
> Issue Type: New Feature
> Components: fs
> Reporter: Hairong Kuang
> Assignee: Hairong Kuang
> Fix For: 0.21.0
>
> Attachments: hflushCommon.patch, hflushCommon1.patch
>
>
> Earlier this year, Yahoo, Facebook, and Hbase developers had a roundtable
> discussion where we agreed to support three types of flush in HDFS (API1, 2,
> and 3) and the append project aims to implement API2. Here is a proposal to
> expose these APIs to application users.
> 1. Three flush APIs
> * API1: flushes out from the address space of client into the socket to the
> data nodes. On the return of the call there is no guarantee that that data
> is out of the underlying node and no guarantee of having reached a DN. New
> readers will eventually see this data if there are no failures.
> * API2: flushes out to all replicas of the block. The data is in the buffers
> of the DNs but not on the DN's OS buffers. New readers will see the data
> after the call has returned.
> * API3: 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).
> 2. Support flush APIs in FS
> * FSDataOutputStream#flush supports API1
> * FSDataOutputStream implements Syncable interface defined below. If its
> wrapped output stream (i.e. each file system's stream) is Syncable,
> FSDataOutputStream#hflush() and hsync() call its wrapped output stream's
> hflush & hsync.
> {noformat}
> public interface Syncable {
> public void hflush() throws IOException; // support API2
> public void hsync() throws IOException; // support API3
> }
> {noformat}
> * In each file system, if only hflush() is implemented, hsync() by default
> calls hflush(). If only hsync() is implemented, hflush() by default calls
> flush().
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