Clusters and DECnet

[leichterj at rani.DEC]

	Newsgroups: net.unix-wizards,net.unix
	Path: decwrl!decvax!linus!philabs!cmcl2!seismo!umcp-cs!chris
	Subject: Re: VAXclusters and UN*X
	Posted: Fri May 31 11:14:10 1985

	I believe (why I don't know, since I have no way of knowing) that VMS
	doesn't use dual-porting for clusters either.
False.  All disks in a cluster can be dual-ported, though they can't neces-
sarily be accessed along two paths at the same time.  For MASSBUS disks, both
paths to the disk are usable simultaneously.  For disks on an HSC-50, one path
is active and the other is standby.  It's not practical to use both paths at
once because the "switch port" command takes a long time - seconds - to com-
plete on an HSC (due, I gather, to various error checks and diagnostics).

	I've been told that the way the cluster controller works is that it
	speaks DECnet and does VMS QIO level stuff with some protocol on top
	of DECnet.  This makes quite a bit of sense to me . . . .  Anyway, if
	this is true, then with DECnet support, it shouldn't be more than a
	couple months' effort to hack up kernel stuff that translates UNIX
	file requests to VMS QIOs, if one had all the documentation....  (Just
	use one huge VMS file to store your disk in, right?)
Nope.  DECnet is way too general, hence expensive, a protocol for this appli-
cation.  Communication in a cluster takes place over the CI bus, at 70 mbps
or so.  The communication software provides a datagram service and a virtual
circuit service.  The disks are accessed through this link as devices, not
files - the HSC-50 has no knowledge of file structure or VMS QIO's or anything
like that - it accesses blocks on disks.

The description you've given actually corresponds to transparent remote file
access, which has been in VMS from early on.  When you give RMS a file spec,
you can include a remote node name.  RMS - not, actually, the QIO level -
sets up a DECnet connection to the remote machine and talks DAP - Device
Access Protocol - to a FAL - File Access Listener - at the other end.  While
there are some restrictions, virtually all file accesses - sequential, random,
what have you - work through such a link exactly as they would work to a local
file.  However, the performance is, at best, an order of magnitude slower than
what you'd get in a cluster.

DECnet/Ultrix certainly includes code for both ends of a FAL link, but how
you access it, I don't know.

	In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 4251)

							-- Jerry