3.5.1 Matrix Server Administration Guide

Chapter 8: Configure MxFS-Linux 167

Using the NLM Protocol

NLM is the locking protocol used by NFS. By default, it is disabled when

MxFS-Linux is installed. If necessary, NLM can be enabled; however, you

should be aware of the following caveat:

• File locks granted by the NFS server are cluster-coherent. When a

failover occurs, the locks are released by the original server and the

client automatically reclaims them on the new server (the backup

node). However, during the period after the lock is released, another

client or application may compete for and win the lock. Some NFS

clients will return an error to the client applications if the lock cannot

be reclaimed. Other clients (for example, the Linux 2.6 NFS client) will

not return any error. If no error is returned by the client, the

application may proceed under the false assumption that the lock has

been granted. Data corruption may be the result.

To prevent this situation, locking should be enabled only if your

clients are partitioned so that all clients needing a particular lock are

using the same Virtual NFS Service IP address. If a failover occurs, all

of the clients will lose their locks. They can then reclaim those same

locks on the new node without conflicts from outside clients.

The mxnlmconfig command is used to enable or disable NLM locking in

the cluster. The change takes place immediately and may affect clients.

The mxnlmconfig command has this syntax:

/opt/polyserve/bin/mxnlmconfig -q|-e|-d|-?

The options are as follows:

-q

Show the current status of NLM locking in the cluster (either enabled or

disabled).

-e

Enable NLM locking in the cluster. No reboot is necessary; the change is

effective almost immediately.

-d

Disable NLM locking in the cluster. No reboot is necessary; the change is

effective almost immediately.

Display a syntax message.