Forcibly Unmounting NFS Filesystems

forcibly unmounting nfs filesystems

other vendors’ solution & hp’s future direction

other vendors’

solution

Several NFS vendors, including Sun Microsystems, have added support for a new

forcible unmount feature to their operating systems. Sun introduced a new “-f” option to

the umount(1M) command in Solaris 8, which instructs the client to forcibly unmount the

filesystem regardless of whether any processes are accessing the filesystem or not.

This forcible unmount feature has the following characteristics and implications:

• The filesystem being forcibly unmounted simply disappears from the namespace

• Any existing processes using the filesystem are returned an I/O error (EIO)

• The system is prevented from consuming any new resources for the unmounted

filesystem and any resources in use by the filesystem prior to the unmount request

are cleaned up as much as possible

• Any locks held by the NFS client for files residing in the forcibly unmounted

filesystem are released

• When a file I/O operation is attempted on a memory-mapped file in the

unmounted filesystem the application will receive either a segmentation violation

(SIGSEGV) or a bus error (SIGBUS)

• Any data being written to the unmounted filesystem that has not been committed

would be lost

• The processes using this unmounted filesystem may not receive any indication

that the filesystem has been forcibly unmounted

As the above list implies, the use of this option is not without potentially adverse effects to

client-side applications. The potential exists for data loss, and any application using the

filesystem being unmounted faces the possibility of aborting due to a SIGBUS or

SIGSEGV error being returned from a pending I/O operation.

Of course, the same potential for data corruption exists if your only course of action is to

reboot your NFS client systems, and most customers would prefer to avoid rebooting their

systems whenever possible.

hp’s future

direction

Hewlett-Packard recognizes the potential benefits offered by this forcible unmount feature

and we are considering adding this feature to a future OS release. As of the time of this

writing, the target release for this feature is 11.31 (11i v3), but these plans could change

at any time.