FAQ
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


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().

--
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.

Search Discussions

Related Discussions

Discussion Navigation
viewthread | post
Discussion Overview
groupcommon-dev @
categorieshadoop
postedOct 14, '09 at 5:20p
activeOct 14, '09 at 5:20p
posts1
users1
websitehadoop.apache.org...
irc#hadoop

1 user in discussion

Hairong Kuang (JIRA): 1 post

People

Translate

site design / logo © 2022 Grokbase