Re: Recovering from the loss of a NFS Server

[Ian Kent <raven@themaw.net>]

On Sat, 2011-03-12 at 23:27 -0500, Breitman, Jason wrote:
> OS
> 	Linux hostname 2.6.18-238.el5 #1 SMP Sun Dec 19 14:22:44 EST 2010 x86_64 x86_64 x86_64 GNU/Linux
> 
> autofs package
> 	autofs-5.0.1-0.rc2.148.bz579312.1.el5
> 
> Mount options
> 	$ cat /etc/auto.master
> 	# Master map for automounter
> 	#
> 	/home             auto_home               -hard,intr,retry=10
> 
> 	$ cat /etc/sysconfig/autofs
> 	TIMEOUT=86400 - we have a long TIMEOUT to avoid mount storms.
> 
> What am I trying to do?
> 	Prior to a disaster recovery test, my home directory will be mounted from my-nfs-server.domainname:/home/jbreitma.
> 	At this point my-nfs-server.domainname points to 1.1.1.1.
> 	There are active reads and writes to my home directory.
> 	Lets say I have a subdirectory called htdocs and am running apache.
> 
> 	Now we are cutoff from 1.1.1.1 because the Data Center where 1.1.1.1 lives is no longer accessible.
> 	We simulate this with an ACL.
> 	We now repoint my-nfs-server.domainname to 2.2.2.2.
> 
> 	The NFS Clients where /home/jbreitma is mounted are now confused.
> 
> 	What is my best coarse of action?
> 		umount -l /home/jbreitma
> 		/etc/init.d/autofs restart
> 		fuser -k /home/jbreitma
> 		kill -USR1 `pgrep automount`
> 		etc ...

That's about all you can do.

The "umount -l" has it's own set of problems.
In particular any process that has an active mount must do a "cd ." (I
believe that will work) to recover from the changed mount otherwise
getcwd(3) will fail and /proc/<pid>/cwd will point to "/" instead of a
valid working directory.

Also, there is pretty much no way to get the RPC layer to give up on
those outstanding IOs which will cause ongoing problems.

> 
> 	How do I recover from this situation?	

There's not much you can do for read/write mounts and even read only
fail over hasn't been implemented within the Linux kernel NFS client.

> 	I am open to a new approach if that is required.

The only way I think high availability NFS can work today is when the
backend deals with the change such as in Clustered environments.

> 
> 
> I have had some success with umount -l /home/jbreitma followed by
> a /etc/init.d/autofs restart, but this does not always work.
> I specifically fail when active writes and or reads are occurring
> to /home/jbreitma.
> 		
> 
> Jason Breitman
> A&T-Tech-GTI
> Jason.Breitman@blackrock.com
> BlackRock
> 
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