Re: RHEL/CentOS-6 HA NFS Configuration Question

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Thanks for all the useful information on this.

I realise the bz is not for this issue, I just included it as it has the
suggestion that nfsd should actually live in user space (which seems
sensible).

Out of interest is there a bz # for this issue?

Colin


On Thu, 2012-05-17 at 10:26 +0200, Fabio M. Di Nitto wrote:
> On 05/16/2012 08:19 PM, Colin Simpson wrote:
> > This is interesting.
> >
> > We very often see the filesystems fail to umount on busy clustered NFS
> > servers.
>
> Yes, I am aware the issue since I have been investigating it in details
> for the past couple of weeks.
>
> >
> > What is the nature of the "real fix"?
>
> First, the bz you mention below is unrelated to the unmount problem we
> are discussing. clustered nfsd locks are a slightly different story.
>
> There are two issues here:
>
> 1) cluster users expectations
> 2) nfsd internal design
>
> (and note I am not blaming either cluster or nfsd here)
>
> Generally cluster users expect to be able to do things like (fake meta
> config):
>
> <service1..
>  <fs1..
>   <nfsexport1..
>    <nfsclient1..
>     <ip1..
> ....
> <service2
>  <fs2..
>   <nfsexport2..
>    <nfsclient2..
>     <ip2..
>
> and be able to move services around cluster nodes without problem. Note
> that it is irrelevant of the fs used. It can be clustered or not.
>
> This setup does unfortunately clash with nfsd design.
>
> When shutdown of a service happens (due to stop or relocation is
> indifferent):
>
> ip is removed
> exportfs -u .....
> (and that's where we hit the nfsd design limitation)
> umount fs..
>
> By design (tho I can't say exactly why it is done this way without
> speculating), nfsd will continue to serve open sessions via rpc.
> exportfs -u will only stop new incoming requests.
>
> If nfsd is serving a client, it will continue to hold a lock on the
> filesystem (in kernel) that would prevent the fs to be unmounted.
>
> The only way to effectively close the sessions are:
>
> - drop the VIP and wait for connections timeout (nfsd would effectively
>   also drop the lock on the fs) but it is slow and not always consistent
>   on how long it would take
>
> - restart nfsd.
>
>
> The "real fix" here would be to wait for nfsd containers that do support
> exactly this scenario. Allowing unexport of single fs and lock drops
> etc. etc. This work is still in very early stages upstream, that doesn't
> make it suitable yet for production.
>
> The patch I am working on, is basically a way to handle the clash in the
> best way as possible.
>
> A new nfsrestart="" option will be added to both fs and clusterfs, that,
> if the filesystem cannot be unmounted, if force_unmount is set, it will
> perform an extremely fast restart of nfslock and nfsd.
>
> We can argue that it is not the final solution, i think we can agree
> that it is more of a workaround, but:
>
> 1) it will allow service migration instead of service failure
> 2) it will match cluster users expectations (allowing different exports
> and live peacefully together).
>
> The only negative impact that we have been able to evaluate so far (the
> patch is still under heavy testing phase), beside having to add a config
> option to enable it, is that there will be a small window in which all
> clients connect to a certain node for all nfs services, will not be
> served because nfsd is restarting.
>
> So if you are migrating export1 and there are clients using export2,
> export2 will also be affected for those few ms required to restart nfsd.
> (assuming export1 and 2 are running on the same node of course).
>
> Placing things in perspective for a cluster, I think that it is a lot
> better to be able to unmount a fs and relocate services as necessary vs
> a service failing completely and maybe node being fenced.
>
>
>
>
> >
> > I like the idea of NFSD fully being in user space, so killing it would
> > definitely free the fs.
> >
> > Alan Brown (who's on this list) recently posted to a RH BZ that he was
> > one of the people who moved it into kernel space for performance reasons
> > in the past (that are no longer relevant):
> >
> > https://bugzilla.redhat.com/show_bug.cgi?id=580863#c9
> >
> > , but I doubt this is the fix you have in mind.
>
> No that's a totally different issue.
>
> >
> > Colin
> >
> > On Tue, 2012-05-15 at 20:21 +0200, Fabio M. Di Nitto wrote:
> >> This solves different issues at startup, relocation and recovery
> >>
> >> Also note that there is known limitation in nfsd (both rhel5/6) that
> >> could cause some problems in some conditions in your current
> >> configuration. A permanent fix is being worked on atm.
> >>
> >> Without extreme details, you might have 2 of those services running on
> >> the same node and attempting to relocate one of them can fail because
> >> the fs cannot be unmounted. This is due to nfsd holding a lock (at
> >> kernel level) to the FS. Changing config to the suggested one, mask the
> >> problem pretty well, but more testing for a real fix is in progress.
> >>
> >> Fabio
> >>
> >> --
> >> Linux-cluster mailing list
> >> Linux-cluster@xxxxxxxxxx
> >> https://www.redhat.com/mailman/listinfo/linux-cluster
> >
> >
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