fyi...here is some more info from development on the 3590 twin
tailing to multiple adsm servers.   Summary...

* it is the RESERVE causing the restriction,
* looks like a resolution has a dependency on SCSI 3 technology,
* sharing 3590 devices between multiple adsm servers on
different systems is not officially supported,
* however it should work if you can GUARANTEE the ports are
used at different times,
* BE CAREFUL

....greg tevis

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 There are holes in dual port support.  If a host has the device
 There are holes in dual port support.  If a host has the device
 reserved and a bus reset happens, it may go back and reestablish
 the reservation but during the window between the bus reset and
 the reestablishment of the reservation a host can come in the
 other port and reserve the device.  There is nothing ADSM can
 do to close this hole.  This and other more subtle problems
 having to do with Reserve deadlocks and unit attention reporting
 will not be there until we support PERSISTENT RESERVE, a
 SCSI 3 construct.  We have had several requests for a more solid
 dual port implementation.  The current plan is that support would
 come out with our first serial interface (FC-EL or FC-AL) which still
 doesn't have an official date.

 If the ports are being used at different times (guaranteed) either
 by tightly coupled hosts (like our control unit) or by some other
 means like a visual inspection by the operator then what we have
 should suffice for dual port support.


 
========================================================================400000228A
Date: 10 March 1997, 12:20:25 PST
From: GJTEVIS  at SJEVM5
To:   adsm-l at vm.marist.edu
Subject: 3590 attachment to multiple servers

Warning!  This is not a supported configuration.  Currently, the
official position is that the drives need to be dedicated to a
particular server.  The 3494 does not have any problem being
accessed by multiple servers.  And physically, the 3590 drives can
be attached to multiple systems...but they can not be shared by
multiple systems with impunity.  The problem is that an underlying
RESERVE macro does not currently operate  across systems.  Thus,
an adsm server can not be sure it has exclusive rights to a drive.
You can run like this but you open the door for problems..eg, one
adsm server is using one of its tapes and the other server requests
a scratch mount...the tape in the drive will look like a valid
scratch tape to the second server.

ADSM did recently put in an enhancement to allow dynamic sharing of
3590 drives within a 3494...but only for sharing between ADSM and other
applications on the same system (including a 2nd adsm server on the
same system).  This support is not for across system.

I would encourage those interested to open a 3590 requirement for this.
...greg tevis

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