Expose flush APIs to application users
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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().
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