Conference stkhlm::magtape

    Now, here's a thought:  instead of trying to jam SCSI adapters into my
    system box, should I pursue buying, say, a HSZ device, and connect the
    TL8nn SCSI devices to it?  Is this a good idea?
    
    Thanks!
    tl

         <<< Note 3728.2 by SUBSYS::TRAN "Straight <Left> Hitter.." >>>
                                 -< A mist.. >-

    
    It's really a mist about how many tape drives can connect to one SCSI
    adapter. Rule of thumb is using max adapter speed/max drive speed.
    
    For example, KZPSA max I/O in narrow mode is 10MB/s and
    		 TZ88 is max at 3MB/s 
    So for KZPSA you can hook up to 3 TZ88's on one bus.
    
    example 2, 	KZPSA max I/O in Wide mode is 20MB/s and
    		TZ89 is max at 10MB/s (actually ~9MB/s in testing) 
    So for KZPSA you can hook up to 2 TZ89's on one bus.
    
    But don't do that on KZPAA, KZPDA or P2SE.
    
    HSZ is good for multiple SCSI ports but not really boot speed, it needs to 
    connect to SCSI adapter on the computer like KZPSA. So, you might as
    well connect the library directly to KZPSA's.
    
    T.

    

	The problem with the HSZ is basically the same one as pointed
	out in .-1; bandwidth.  The HSZ40/HSZ50 are Fast/Wide SCSI
	and limited to 20 MB/sec bandwidth.  That's 4 or 5 TZ88s or
	two TZ89s.  If you just need connectivity and don't plan to
	run all the tapes at the same time, the HSZ isn't a bad
	way to go.  For Digital UNIX, MAKEDEV may not be up to making
	the special files for tapes at something other than LUN 0,
	but that could worked-around with mknod.

	I'm not sure any interesting software has qualified the robot
	on an HSZ.  I know that MRU hasn't qualified any of the TL8x0
	family.

	For what its worth, recent versions of Digital UNIX support up
	to 64 SCSI controllers.  I think a DEC 8400 can be configured
	with this many.

    Hi T
    
    Excellent information; I can now understand the idea behind the two
    drive per adapter rule.  If I were staying with our KZPSA+TZ88 in wide
    mode, I'd be all set for 3 drives per KZPSA.  Alas, I'm moving to
    TZ89s, so I'll need to figure 2 drives per KZPSA.
    
    We just did some math whose result said we needed to buy AS 8400s in
    order to optimally spread our drives.  {notice the sticker shock on my
    face}.  Given this I'm going to replace my AS 2000 with a AS 2100A rack
    mount, load 5 of the 8 PCI slots with KZPSA FWD SCSI adapters (the
    other 3 slots will have 2 FDDI and 1 RAID adapter), and be very planful
    about how I install and use 15 Tz89 drives across 5 adapters.  I know
    that 3 of the 15 will be dedicated to a very important but low volume
    need (SAP R/3 archived redo logs), so 12 drives on 5 adapters isn't so
    bad, I think.
    
    Thanks very much for the fast help; you really helped me make some
    difficult decisions with good information.
    
    tl

	Before getting two worried you may want to check with product
	management about SCSI adapters with multiple buses.  This will
	save a lot of slots.  

	If you are using a lot of drives there may not be very much 
	difference over a large backup between 2 and 3 tapes per bus.
	Each drive will have some amount of time that it is not
	transfering data, it isn't like we just start them and 
	run for hours at full speed.  If any of the backups are
	coming from other client systems the net will limit
	your data rates anyway.

    I want to connect two TL822s to an AlphaServer 2000 with two KZPSA-BB. 
    Is this possible?
    
    Thanks!
    tl

	The easy way of just cabling one tower to each bus it is certainly
	possible.  But that puts three TZ88s on a bus, which limit the
	bandwidth available.  From what I remember of the TL822 there is
	only one connector per drive, so it is hard to run a cable back
	out of the cabinet.  It may be possible though.  It helps that
	the cabling is differential and you have a lot of meters to play
	with.