Slow RA 90 disks on DECsystem 5XXX

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In article <28662 at mimsy.umd.edu>, chris at mimsy.umd.edu (Chris Torek) writes:
> In article <1990Dec12.194341.18284 at bernina.ethz.ch>
> karrer at bernina.ethz.ch (Andreas Karrer) writes:
> >We have 6 DEC RA 90 disks ...
> >We are not impressed by their performance.
> 
> I will go *way* out on a limb and make the following claim:
> 
> 	DEC have never produced a fast RA disk, with any setup.
> 
	Define fast.

	All RA disks connect to disk controllers using an SDI
	cable.  The SDI stands for Standard Disk Interconnect and 
	is the protocol used between the disk and controller.  I 
	think the fastest you can move bits down this wire is 
	around 2.2 MB/sec.  That's as fast as it goes.

	I have seen the results of VMS based tests where they can
	read this fast from an RA90.  It's actually pretty simple.
	Issue many asyncronous reads for a track each.  As the early
	reads finish start another one in it's place.

	The problem of course is that in the real ULTRIX world
	the largest I/O is typically 8 KB.  It's probably similar
	for most others that use the Fast File System.  This is
	the ideal I/O size for an RA90, RA70 or RA82.

	That's if you define fast as how many bits you move in a
	second.  How many seeks you can get is a different problem
	and at least partly depends on the distance you're trying
	to traverse.

> It would be nice to see some counter-argument.  The newer RA series
> drives *should* not be so slow; their physical characteristics are OK
> (unlike the RA81).  Perhaps the problem is MSCP.

	I suspect it more how MSCP is (mis)used.  As long as you can
	keep the MSCP server at the other end busy, you'll get bits
	about as fast as the wire will allow.  If the wire is SDI that's
	at most about 2.2 MB/sec.  If the wire is DSSI then it's around
	4 MB/sec.

	p.s.  For reference I'm not a VMS person.  About the only
	thing I use it for is to read VAXnotes and occasionally
	run VAX Document.

> -- 
> In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
> Domain:	chris at cs.umd.edu	Path:	uunet!mimsy!chris

-- 
Alan Rollow				alan at nabeth.enet.dec.com