[Linux-cluster] self_fence in <fs> resource tag?

["André Fernandes" <andreezer gmail com>]

Hi.

I was configuring a HA cluster with a shared ext3 filesystem and came across this option in the <fs> resource tag.
What does the self_fence option do? I could not find it detailed anywhere.

Here's the sample from my 
cluster.conf:
...
<resources>
            ...
            <fs device="/dev/scalixdata/scalixmail" force_fsck="1" force_unmount="1" fsid="8848" fstype="ext3" mountpoint="/var/opt/scalix" name="scalix_mail_data" options="" 
self_fence="1"/>
            ...

thanks in advance.