Re: HDFS HA IO Fencing
I use NFS V4 to test the java FileLock.
The 192.168.1.233 machine is NFS Server, the nfs configuration are
/home/hdfs.ha/share 192.168.1.221(rw,sync,no_root_squash)
/home/hdfs.ha/share 192.168.1.222(rw,sync,no_root_squash)
in /etc/exports file.
I run below commands to start nfs server:
service nfs start
service nfslock start
The 192.168.1.221 and 192.168.1.222 machines are NFS Client, the nfs
configuration is
192.168.1.223:/home/hdfs.ha/share /home/hdfs.ha/share nfs
rsize=8192,wsize=8192,timeo=14,intr in /etc/fstab file.
I run below commands to start nfs client:
service nfs start
service nfslock start
I write one programm to receive file lock:
public class FileLockTest {
FileLock lock;
public void lock(String path,boolean isShare) throws IOException {
this.lock = tryLock(path,isShare);
if (lock == null) {
String msg = Cannot lock storage + path
+ . The directory is already locked.;
System.out.println(msg);
throw new IOException(msg);
}
}
private FileLock tryLock(String path,boolean isShare) throws
IOException {
boolean deletionHookAdded = false;
File lockF = new File(path);
if (!lockF.exists()) {
lockF.deleteOnExit();
deletionHookAdded = true;
}
RandomAccessFile file = new RandomAccessFile(lockF, rws);
FileLock res = null;
try {
res = file.getChannel().tryLock(0,Long.MAX_VALUE,isShare);
} catch (OverlappingFileLockException oe) {
file.close();
return null;
} catch (IOException e) {
e.printStackTrace();
file.close();
throw e;
}
if (res != null !deletionHookAdded) {
// If the file existed prior to our startup, we didn't
// call deleteOnExit above. But since we successfully locked
// the dir, we can take care of cleaning it up.
lockF.deleteOnExit();
}
return res;
}
public static void main(String[] s) {
FileLockTest fileLockTest =new FileLockTest();
try {
fileLockTest.lock(s[0], Boolean.valueOf(s[1]));
Thread.sleep(1000*60*60*1);
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
I do two test cases.
1. The network is OK
I run java -cp ./filelock.jar lock.FileLockTest
/home/hdfs.ha/share/test.lock false command in 192.168.1.221 to hold file
lock, and then I run same command to hold same file lock in 192.168.1.222,
throw below exception:
Cannot lock storage /home/hdfs.ha/share/test.lock. The directory is already
locked.
java.io.IOException: Cannot lock storage /home/hdfs.ha/share/test.lock. The
directory is already locked.
at lock.FileLockTest.lock(FileLockTest.java:18)
at lock.FileLockTest.main(FileLockTest.java:53)
2. machine which hold file lock is diconnected
I run java -cp ./filelock.jar lock.FileLockTest
/home/hdfs.ha/share/test.lock false command on 192.168.1.221, then
192.168.1.221 machine is disconnected from network . After three minutes ,
I run the java -cp ./filelock.jar lock.FileLockTest
/home/hdfs.ha/share/test.lock false command on 192.168.1.222, that can
hold the file lock.
I use mount | grep nfs command to examine the mount nfs directory on
192.168.1.221, the share directory /home/hdfs.ha/share/ is disappear on
192.168.1.221 machine. So I think when the machine is disconnected for
a long time, other machine can receive the same file lock.
Re: HDFS HA IO Fencing
If you use NSFv4 you should be able to use locks and when a machine dies /
fails to renew the lease, the other machine can take over.
On Friday, October 26, 2012, Todd Lipcon wrote:
NFS Locks typically last forever if you disconnect abruptly. So they are
not sufficient -- your standby wouldn't be able to take over without manual
intervention to remove the lock.
If you want to build an unreliable system that might corrupt your data,
you could set up 'shell(/bin/true)' as a second fencing method. But, it's
really a bad idea. There are failure scenarios which could cause split
brain if you do this, and you'd very likely lose data.
-Todd
On Fri, Oct 26, 2012 at 1:59 AM, lei liu
liulei...@gmail.comjavascript:_e({}, 'cvml', 'liulei...@gmail.com');
wrote:
We are using NFS for Shared storage, Can we use linux nfslcok service to
implement IO Fencing ?
2012/10/26 Steve Loughran ste...@hortonworks.com javascript:_e({},
'cvml', 'ste...@hortonworks.com');
On 25 October 2012 14:08, Todd Lipcon t...@cloudera.comjavascript:_e({},
'cvml', 't...@cloudera.com');
wrote:
Hi Liu,
Locks are not sufficient, because there is no way to enforce a lock in
a distributed system without unbounded blocking. What you might be
referring to is a lease, but leases are still problematic unless you can
put bounds on the speed with which clocks progress on different machines,
_and_ have strict guarantees on the way each node's scheduler works. With
Linux and Java, the latter is tough.
on any OS running in any virtual environment, including EC2, time is
entirely unpredictable, just to make things worse.
On a single machine you can use file locking as the OS will know that
the process is dead and closes the file; other programs can attempt to open
the same file with exclusive locking -and, by getting the right failures,
know that something else has the file, hence the other process is live.
Shared NFS storage you need to mount with softlock set precisely to stop
file locks lasting until some lease has expired, because the on-host
liveness probes detect failure faster and want to react to it.
-Steve
--
Todd Lipcon
Software Engineer, Cloudera
--
Thanks
-balaji
--
http://balajin.net/blog/
http://flic.kr/balajijegan
Re: HDFS HA IO Fencing
We are using NFS for Shared storage, Can we use linux nfslcok service to implement IO Fencing ? 2012/10/26 Steve Loughran ste...@hortonworks.com On 25 October 2012 14:08, Todd Lipcon t...@cloudera.com wrote: Hi Liu, Locks are not sufficient, because there is no way to enforce a lock in a distributed system without unbounded blocking. What you might be referring to is a lease, but leases are still problematic unless you can put bounds on the speed with which clocks progress on different machines, _and_ have strict guarantees on the way each node's scheduler works. With Linux and Java, the latter is tough. on any OS running in any virtual environment, including EC2, time is entirely unpredictable, just to make things worse. On a single machine you can use file locking as the OS will know that the process is dead and closes the file; other programs can attempt to open the same file with exclusive locking -and, by getting the right failures, know that something else has the file, hence the other process is live. Shared NFS storage you need to mount with softlock set precisely to stop file locks lasting until some lease has expired, because the on-host liveness probes detect failure faster and want to react to it. -Steve
Re: HDFS HA IO Fencing
NFS Locks typically last forever if you disconnect abruptly. So they are not sufficient -- your standby wouldn't be able to take over without manual intervention to remove the lock. If you want to build an unreliable system that might corrupt your data, you could set up 'shell(/bin/true)' as a second fencing method. But, it's really a bad idea. There are failure scenarios which could cause split brain if you do this, and you'd very likely lose data. -Todd On Fri, Oct 26, 2012 at 1:59 AM, lei liu liulei...@gmail.com wrote: We are using NFS for Shared storage, Can we use linux nfslcok service to implement IO Fencing ? 2012/10/26 Steve Loughran ste...@hortonworks.com On 25 October 2012 14:08, Todd Lipcon t...@cloudera.com wrote: Hi Liu, Locks are not sufficient, because there is no way to enforce a lock in a distributed system without unbounded blocking. What you might be referring to is a lease, but leases are still problematic unless you can put bounds on the speed with which clocks progress on different machines, _and_ have strict guarantees on the way each node's scheduler works. With Linux and Java, the latter is tough. on any OS running in any virtual environment, including EC2, time is entirely unpredictable, just to make things worse. On a single machine you can use file locking as the OS will know that the process is dead and closes the file; other programs can attempt to open the same file with exclusive locking -and, by getting the right failures, know that something else has the file, hence the other process is live. Shared NFS storage you need to mount with softlock set precisely to stop file locks lasting until some lease has expired, because the on-host liveness probes detect failure faster and want to react to it. -Steve -- Todd Lipcon Software Engineer, Cloudera
Re: HDFS HA IO Fencing
On 26 October 2012 15:37, Todd Lipcon t...@cloudera.com wrote: NFS Locks typically last forever if you disconnect abruptly. So they are not sufficient -- your standby wouldn't be able to take over without manual intervention to remove the lock. +1. This is why you are told to mount your shared edit log NFS servers with the soft option -otherwise you can't swap over to the other machines. see: http://www.faqs.org/docs/linux_network/x-087-2-nfs.mountd.html If you want to build an unreliable system that might corrupt your data, you could set up 'shell(/bin/true)' as a second fencing method. But, it's really a bad idea. There are failure scenarios which could cause split brain if you do this, and you'd very likely lose data You could do it on a cluster where you were prepared to lose/corrupt all the data. But in that world: why bother?
Re: HDFS HA IO Fencing
Hi Liu, Locks are not sufficient, because there is no way to enforce a lock in a distributed system without unbounded blocking. What you might be referring to is a lease, but leases are still problematic unless you can put bounds on the speed with which clocks progress on different machines, _and_ have strict guarantees on the way each node's scheduler works. With Linux and Java, the latter is tough. You may want to look into QuorumJournalManager which doesn't require setting up IO fencing. -Todd On Thu, Oct 25, 2012 at 1:27 AM, lei liu liulei...@gmail.com wrote: I want to use HDFS HA function, I find the IO Fencing function is complex in hadoop2.0. I think we can use file lock to implement the IO Fencing function, I think that is simple. Thanks, LiuLei -- Todd Lipcon Software Engineer, Cloudera
Re: HDFS HA IO Fencing
On 25 October 2012 14:08, Todd Lipcon t...@cloudera.com wrote: Hi Liu, Locks are not sufficient, because there is no way to enforce a lock in a distributed system without unbounded blocking. What you might be referring to is a lease, but leases are still problematic unless you can put bounds on the speed with which clocks progress on different machines, _and_ have strict guarantees on the way each node's scheduler works. With Linux and Java, the latter is tough. on any OS running in any virtual environment, including EC2, time is entirely unpredictable, just to make things worse. On a single machine you can use file locking as the OS will know that the process is dead and closes the file; other programs can attempt to open the same file with exclusive locking -and, by getting the right failures, know that something else has the file, hence the other process is live. Shared NFS storage you need to mount with softlock set precisely to stop file locks lasting until some lease has expired, because the on-host liveness probes detect failure faster and want to react to it. -Steve
