hadoop-hdfs-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Todd Lipcon <t...@cloudera.com>
Subject Re: HDFS HA IO Fencing
Date Fri, 26 Oct 2012 14:37:14 GMT
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.


On Fri, Oct 26, 2012 at 1:59 AM, lei liu <liulei412@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 <stevel@hortonworks.com>
>> On 25 October 2012 14:08, Todd Lipcon <todd@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

View raw message