I work for a large three letter company with a RedHat support contract. RedHat came out to our campus to give a talk about GFS. I learned a couple things.
First, they said there are current deployments of GFS with more than 32 block device nodes. The limiting factor is the lock manager. They now have a load balancing and redundent lock manager servers. So rather than having a single lock manager server, you now have a small cluster of them.
Second, the locks are per-block not as you said per-file. And the locks are revokable if a host with a GFS FS mounted becomes uncommunicative the lock can be recovered from that host.
Third, you do not need a SAN (I'm thinging you meant Fibre Channel SAN) to use GFS. You can cost effectively set up a Gb ethernet LAN and use the GNDB server to serve blocks over TCP. I pressed them for an estimate of the speed up compared to NFS. The response was reluctant, but they said one customer measured speedups around 25 times faster than NFS on the same LAN. The presenter continued that it could be slower or faster than that number depending on usage but that it would never be slower than NFS.
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