[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
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 .
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  is not final
yet and I'd like some feedback before I proceed any further.
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