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

Niels de Vos ndevos at redhat.com
Fri Jan 25 14:23:28 UTC 2013

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.


[1] http://review.gluster.org/4430

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