Re: [Gluster-devel] What functionality is expected from persistent NFS-client tracking?

[J. Bruce Fields]

On Fri, Jan 25, 2013 at 03:23:28PM +0100, Niels de Vos wrote:
> Hi all,
> 
> the last few days I have been looking into making the tracking of 
> NFS-clients more persistent. As it is today, the NFS-clients are kept in 
> a list in memory on the NFS-server. When the NFS-server restarts, the 
> list is recreated from scratch and does not contain the NFS-clients that 
> still have the export mounted (Bug 904065).
> 
> NFSv3 depends on the MOUNT protocol. When an NFS-client mounts an 
> export, the MOUNT protocol is used to get the initial file-handle. With 
> this handle, the NFS-service can be contacted. The actual services 
> providing the MOUNT and NFSv3 protocol can be separate (Linux kernel 
> NFSd) or implemented closely together (Gluster NFS-server). 
> 
> Now, when the Linux kernel NFS-server is used, the NFS-clients are saved 
> my the rpc.mountd process (which handles the MOUNT protocol) in 
> /var/lib/nfs/rwtab. This file is modified on mounting and unmounting.  
> Implementing a persistent cache similar to this is pretty straight 
> forward and is available for testing and review in [1].
> 
> There are however some use-cases that may require some different 
> handling. When an NFS-server starts to mount an export, the MOUNT 
> protocol is handled on a specific server. After getting the initial 
> file-handle for the export, any Gluster NFS-server can be used to talk 
> NFSv3 and do I/O. When the NFS-clients are kept only on the NFS-server 
> that handled the initial MOUNT request, and due to fail-over (think CTDB 
> and similar here) an other NFS-server is used, the persistent cache of 
> 'connected' NFS-clients is inaccurate.
> 
> The easiest way I can think of to remedy this issue, is to place the 
> persistent NFS-client cache on a GlusterFS volume. When CTDB is used, 
> the locking-file and is placed on a shared storage as well, so the same 
> volume can be used for the NFS-client cache. Providing an option to set 
> the volume/path of the NFS-client cache would be needed for this. 
> I guess that this could result in a chicken-and-egg situation 
> (NFS-server is started, but no volume mounted yet)?
> 
> Any ideas or recommendations are welcome. The patch in [1] is not final 
> yet and I'd like some feedback before I proceed any further.

My only comment is that this doesn't need to be perfect or even all that
good.

The list can already get out of sync in other ways: clients can just
fail to unmount, for example.

I don't think it's used by anything other than showmount.

--b.