Hi Colin, On 5/17/2012 11:47 AM, Colin Simpson wrote: > 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). Understood. I can´t really say if userland or kernel would make any difference in this specific unmount issue, but for "safety reasons" I need to assume their design is the same and behave the same way. when/if there will be a switch, we will need to look more deeply into it. With current kernel implementation we (cluster guys) need to use this approach. > > Out of interest is there a bz # for this issue? Yes one for rhel5 and one for rhel6, but they are both private at the moment because they have customer data in it. I expect that the workaround/fix (whatever you want to label it) will be available via RHN in 2/3 weeks. Fabio > > 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 >>> >>> >>> ________________________________ >>> >>> >>> 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. If you received this email in error, please immediately notify the sender and delete the original. >>> >>> >>> -- >>> Linux-cluster mailing list >>> Linux-cluster@xxxxxxxxxx >>> 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. If you received this email in error, please immediately notify the sender and delete the original. > -- Linux-cluster mailing list Linux-cluster@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/linux-cluster
