Re: NFSv4 high availability setups

[Michael Schwartzkopff <misch@schwartzkopff.org>]

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> On Tue, 10 Apr 2012 14:55:52 +0200
> 
(...)
> > > > Can we put this state dir on a shared volume so that this state dir
> > > > is common for all the front-ends serving the same content? Is is
> > > > supposed to work and NFSv4 can merge its state with existing state
> > > > on a shared disk?
> > > 
> > > Not properly, no. nfsd expects to have complete control over that
> > > directory. There's no locking or merging of the data there. A node will
> > > also clean that directory out in some cases, and that will throw your
> > > state tracking off.
> > 
> > Thank you for information.
> > 
> > Is there any (preferably simple) way to demonstrate that this does not
> > work properly? E.g., if I share the same export through two or more
> > NFSv4 front-ends that share the v4recovery directory, do I trigger
> > problems with this tool
> > http://nfsv4.bullopensource.org/tools/tests/locktest.php?
> 
> Nope. It'll all work just great...until it doesn't. I don't have any
> specific failure scenarios, but most of the problems will be issues
> with state recovery when a server node is restarted.
> 
> That may manifest in different ways -- problems reclaiming locks for
> instance, or even silent data corruption depending on the application.
> 
> For instance, a node might hand out a lock and the client release it,
> after a server node reboots but before a client that really "owns" it
> reclaims it. Depending on the application, that may cause serious
> problems.

Hi,

I don't think a active/active NFS server is possible with 
/var/lib/nfs/v4recovery on a shared media. I think you will get into major 
trouble if two or more nodes access that directory at the same time.

On the other hand an active/passive setup is quite easy. There are some HOWTOs 
on the internet. I like the one of linbit most:
http://www.linbit.com/de/training/tech-guides/highly-available-nfs-with-drbd-
and-pacemaker/

The guide provides a basic path to follow. You have to tune it according to 
which distribution you use. Not all distributions have the necessary features.

See: leasetime, grace time, ...

Greetings,
-- 
Dr. Michael Schwartzkopff
Guardinistr. 63
81375 München

Tel: (0163) 172 50 98
Fax: (089) 620 304 13

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