Conference ssag::ask_ssag

    
    	re:   .3
    
    		I looked in [public.docs], and found 6 versions of the
    EK-HZFAM-UG.PS	Does anyone know which of these is the most recent?
    Also, does it cover anything about the HSZ50?  I'd rather not waste
    bandwidth and disk space trying to find out.  Not to mention printing
    them out.
    
    		thanks, Glenn 

    
    Hey Glen.......dir/date might work?
    
    Hows things?
    
    chet

    
    	Hi chet.  Things good!  In MKO heading for ZKO.  
    
    	dir/date.......   DOH!!!!!!!  Been 3 years since I've even thought
    	about VMS, much less touched it!  Let's see now, how do I get the
    	parameters on this file?????   8*)
    
    	Anyhoo, these versions are 1994.  Three years ago.  Does anyone
    	know where todays 'versions' of the same thing are?  Such as HSZ50
    	users guide, and/or any other technical pieces relating to disk
    	subsystems.  I do have quite a bit now from the SOC and
    	StorageWorks web sites.  I know there's a ton of info out there,
    	I just can't find what I want.
    
    		wish list
    	- up to date SCSI overview.  right up to Ultra.....
    	- overview of DEC approach to designing disk matrices (?)
    	- config guidelines
    	- performance vs configuration discussions.  # controllers, etc....
      
    	In other words, I'm trying to figure out what to use to build the
    	system that I'm asked for, BUT, I want to know why I'm using the
    	parts.  Why is one controller better than the other?  How do I make
    	this thing 'rock' and get the performance that I can use to get an
    	edge in the market?  
    
    	One of the problems we're seeing is that we seem to be I/O bound.
    	
    
    	that's my story, and I'm sticking to it  8*)
    
    		Glen(n)
    

    
    	Update.  I did access the whatsa.????  web site mentioned, and
    there's a lot there.
    
    	In trying to sort out some of the physical differences, I came up
    with the following.  Is this true?
    
    
    			8 Bit		16 bit
    
    ULTRA SCSI		20MB/sec	40MB/sec
    
    FAST 		(Fast Scsi)	(Fast Wide Scsi)
    			10MB/sec	20MB/sec
    
    SCSI				(Wide Scsi)
    			 5MB/sec	10MB/sec
    
    	This is the way that I think I understand the differences.  Am I
    missing anything here?
    
    		thanks, Glenn
    

Those numbers are correct.  HOWEVER, remember too that 
those are the bus bandwidth numbers.  So, consider if
you got 2 disks with exactly the same platters, heads,
and spindle motor.  The only difference is one has the
"SCSI 5Mb/sec" transfer engine, and the other has the
"ULTRA wide 40Mb/sec" transfer engine.

Is the performance of these 2 disks any different?  The
answer is NO.  Their performance is exactly the same.
The difference is in the bus idle time.  The bus that
has the "ULTRA wide 40Mb/sec" disk will have 8 times more
bus idle time (because the data still moves at the same
rate to/from the media).

Now consider mutiple units on the same bus.  IF it took 2
of the SCSI 5Mb/sec units to saturate that bus (0 idle time),
then it would require 16 of the ULTRA 40Mb/sec units to
saturate their bus (these devices would simply use up that
extra bus idle time).

But, this is also sort of a contrived example just to show
a point since in reality, all the new ULTRA disks also have
faster motors, faster analog electronics (the heads), and
increased density; which all together means they move the
data faster (not only on the bus, but to/from the media as
well).

Also remember that those numbers don't account for any of
the scsi overhead.  The "real" bandwidth numbers are slightly
lower than the published "maximums".

	Fred Knight