Forcibly Unmounting NFS Filesystems

forcibly unmounting nfs filesystems

preventative steps

use the “soft” NFS

mount option

By default, NFS filesystems are mounted with the “hard” option, which instructs the

client’s kernel to indefinitely retransmit any NFS request that is not responded to by the

NFS server. In other words, if a process on an NFS client attempts to access a file on an

NFS server that is down or otherwise unresponsive, the client will continue to resend NFS

requests to the server until that system (or a different system masquerading as the server)

responds. This behavior is intentional and designed to guarantee data integrity.

While there are definite benefits to this “hard” mount behavior, there are also drawbacks

– especially in environments where NFS servers are frequently unavailable. In these

environments it may be preferable to allow NFS clients to eventually “give up” when

trying to communicate with a down server and return control to the requesting

applications. This can be done by mounting the NFS filesystems using the “soft” option.

In the previous example on pages 7 through 10, the NFS filesystem was mounted with

the default “hard” option, forcing the client to continually resend NFS requests to the

server until it received a response. If a “surrogate” server had not been configured, or

the original NFS server not been restored to service, any client applications accessing this

filesystem would hang indefinitely.

In Figure 4 below the same exercise is repeated, except in this case the NFS filesystem is

mounted with the “soft” option. When the NFS server crashes (in step 3) the client’s

kernel will attempt to communicate with the server by retransmitting the NFS requests that

have not been responded to. However, the “soft” option instructs the client to eventually

give up and return an I/O error to the dd(1) process. Once all of the processes

accessing this hung NFS filesystem exit the filesystem can be successfully unmounted.

Figure 4 – Writing to a down NFS server via a “soft” mounted filesystem

Refer to the example shown on pages 7 through 10.