I think the simple litmus test of SAN vs. NAS is "who's allowed to touch the disks." In a NAS setting, as we have with our filers, all disk access is proxied by the filer.. Even if there were a way to open up the disk directly to the client, we probably wouldn't because they're generally considerd untrusted (in terms of making sure they don't hork things up).
From my perspective of a SAN, is that you trust all the attached hosts not to do 'bad' things to the disk; we can be safe to presume they'll behave in a prescribed manner and therfore have a 'network' of sorts that allows native disk access. All of a sudden, we have something that isn't bound by bottlenecks in the filer at all, can be built up to be reliable in whatever fashion suits us, etc.
One problem. It doesn't work.
Somewhere in there, we still need a means to handle locks, as well as regular filesystem maintenance (managing inode allocation and free blocks comes to mind offhand). There's a couple ways that I can think of doing this, one of which is to have your netapp san device acting as storage management. It takes care of these things, and hosts are responsible for negotiating locks and such through it, but when it comes down to getting data, hosts can act entirely independently.
there's a million other issues to be worked out in the process, namely this bizzare hybrid filesystem driver. Would it be worth it? I'm not really sure, but atleast intuitively, it would seem to move the storage bottleneck back down to the disk.
enough for now.
..kg..
On Tue, 30 Mar 1999, Sam Schorr wrote:
Yes, I realize there are tremendous hurdles. It will also take a vendor who builds their own solution ala Network Appliance, or even Sun. They would have to marry it to specific OS/hardware (Sun could "easily" do this, while Netapp would have to sell the internal fibre or SCSI card to go in the server as well as the disk arrays). All that aside, there would be no point if the current network technologies make it unnecessary to separate the two very different traffic requirements. I believe, though, that such an offering would hold tremendous value to large sites, particularly as virtual sites on the internet increase (many servers/services appearing as one).