Very interesting question here... I have in the past done lots of research
on this issue for where I am currently working (although my assumptions are
based on LTO-1 which only has one uplex FC connector).
Your theory about connecting the three drives in a loop is technically sound
except is an pig to cable up. Assuming when I say FC connector means the
in/out ports of a pair then you would need to connect a simplex lead from
the out of the server HBA to the in of drive1, then a simplex lead from the
out of drive1 to drive2 etc... etc... until the out of driveX is connected
to the in on the server HBA. The biggest and most important reason for not
doing this, is ta taking any single drive down for maintenance would be mean
all the drive on the loop would have to be unavailable.
If price is still the key factor then you could either establish install an
FC HBA per drive or install a FC hub (instead of a switch). A hub is very
different and simpelr than a switch in that it allows you to loop your
devices together such as creating a loop in my previous example, but it is
clever enough to maintain loop interity should a tape drive be taken offline
for maintenance. The probems with hubs (which doesn't appear to be a concern
to you) is that the hub limits the total speed of the loop to the slower
device on the loop. for example a hub connecting 1 * 1Gb HBA and 3 * 1Gb
drives ill only run at 1Gb max throughput.
A switch on the other hand will allow maximum throughput for each connecting
device (so total bandwidth of an 8port 2Gb switch is 16Gbps). The other
advantage of a switch is if you have a mixture of FC-AL (tape drives) and
point-to-point devices (SAN storage, HBA's etc) in that the switch is able
to create a FC-AL loop for each drive connection and a point-to-point
connection for each connected HBA/ disk device, and make the lot talk
transparently together.
Hope this sheds some light on the subject!
Many thanks
Andy Wilcox
UNIX Systems Administrator
Aquila Networks
-----Original Message-----
From: Sam Johnson [mailto:Sam.Johnson AT FERNUNI-HAGEN DOT DE]
Sent: 30 December 2003 09:53
To: ADSM-L AT VM.MARIST DOT EDU
Subject: Do I need a hub for LTO2 drives in my 3584 Library ?
I have installed an IBM 3584-L32 Library with 3 FC LTO2 drives. I assumed
that
since the drives are FC-AL, I could attach them to a single FC HBA in my TSM
Server,
but my reseller say I need a switch, and the IBM Redbook "The IBM LTO
Ultrium
Tape Libraries Guide" (SG24-5946) states:
The device can be attached directly to SAN switches or FC-AL hubs.
Ignoring the questions of availabilty and performance, why can't I
simple connect the first drive to my server, the second to the first,
and the third to the second ?
Sam Johnson Sam.Johnson AT FernUni-Hagen DOT de
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