What GFS is:
Posted Jun 28, 2004 5:37 UTC (Mon) by
daniel (subscriber, #3181)
In reply to:
What GFS is: by AnswerGuy
Parent article:
GFS released under the GPL
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.
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