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@redhat.com > >> https://www.redhat.com/mailman/listinfo/linux-cluster > > > > > > ________________________________ > > > > > > This email and any files transmitted with it are confidential and are > > intended solely for the use of the individual or entity to whom they are > > addressed. If you are not the original recipient or the person responsible > > for delivering the email to the intended recipient, be advised that you > > have received this email in error, and that any use, dissemination, > > forwarding, printing, or copying of this email is strictly prohibited. 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