GFS does support mmap. You get local mmap semantics over the cluster.
Yes you can. For the initial single node setup, simply setup GFS using the nolock locking method. Make sure you create the file system with enough journals to support the number of nodes you wish to add later. (If you do not add enough, you can add journals later, but you must add additional space to the volume GFS is on to do so.)
Once you want to add more nodes, you need to setup the
cluster infrastructure just as you would in an initial
multi-node configuration. You also need to modify the
gfs superblock with gfs_tool to switch it to a
multi-node locking setup. Use the values you would have
given to gfs_mkfs - instead of the '-p
Once these changes and additions are made, fire up the cluster infrastructure and mount GFS.