[Linux-cluster] How to push through NFS unmount if NFS server is gone?

[Gentoo Momo]

Hi,
    I'm configuring a two-node cluster with NFS3 shared storage via "netfs"
agent for data repository. I've also configured a virtual ip on the
interface for NFS access. If the server is gone or the link for NFS is
broken ("tested by ifconfig ethX down"), the cluster tries to stop the
service by unmounting the NFS while it always hangs there. Is there any sort
which can solve this problem?
    Any suggestions are appreciated!
    Thanks!

Han.
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