What GFS is:

Posted Jun 28, 2004 5:37 UTC (Mon) by daniel (subscriber, #3181) [Link]

GFS is more of a SAN filesystem. The idea is that it can support concurrent access by multiple systems over a SCSI (multi-initiator), fibre channel or potentially even a firewire bus/fabric.

Nice succinct answer. Additionally, GFS is also used with block devices exported over a network. For my own development, I don't have a shared disk at all, instead I export a SCSI drive using Hyperscsi. iSCSI will work as well. The grand GPL release also includes GNBD, a network block device exporter suitable for use with GFS.

Of course one could then export this local/SAN fs over NFS to additional clients.

That is one of several main applications of GFS out there in the field: basically, server amplification. You have a core of GFS nodes, each of which exports as many NFS clients as it can handle, and you can have a bigger NFS network for less money (because the cost of the cluster scales linearly, whereas the cost of a multicpu server of equivalent power scales quadradically).

You can also do html server amplification, particularly for sites generating dynamic content. A filesystem shared _efficiently_ between all the servers in an html farm is a big help both from the point of view of performance and maintainance.

However, it's more interesting for custered applications that need more tightly coupled access to their storage (where network and IP layers impose too much overhead --- latency in particular).

It's only interesting to applications that need tight coupling to their storage. Fortunately, that is most applications :-)

Network latency isn't much these days, particularly when you use something like HyperSCSI that sits right on the ethernet protocol, so doesn't incur the IP overhead. No TCP -> less latency. This is perfectly practical and useful.

Besides, GFS does a very good job of caching in-kernel, much as a local filesystem does, which tends to amortize the cost of network data transfers. And besides again, GigE gives Fiber Channel a pretty good run for its money.