[Veritas-bu] Tape Drive & Disk sharing HBAs - revisited

[jim fred]

Hi

We all know that people have been telling us for years that you should never
share tape drives and disk on the HBAs.

These days the idea that the HBA could become the bottle neck for data
transmission are becoming less.

That leaves the other reason of  tape FC-SCSI commands have adverse affects
upon the SAN operations of disks.   The has been sparsely and vaguely
documented to the point where it  may be more mythology than reality with
modern SAN, disk and tape technology.

Anybody attempted sharing emterprise grade disk [arrays] & LTO3 tape.  OS
not important but Windows 2003 would be interesting.

Anybody come across anything more definitive that these references ? .....

1. IBM Redbook sg246268 Implementing IBM Tape in Linux Widows p201+.
    I queried IBM to provide more info....no reply form them.

2. The following came from X-Info [Qlogic OEM]  Qlogic weren't interested
but flipped my questions to two of their OEMs to answer  (no referecne
cited) :

Tape resets can affect other devices on the SAN because when a tape does its
reset, it will also reset its FC connection (logs out and logs back into the
fabric nameserver of the switch).  When a device logs out and back into the
switch, the switch will send out an RSCN (registered state change
notification) to the rest of the ports on the switch letting other FC
devices know that there was a change in the fabric.  When the RSCN is
received by the initiators (servers and workstations), they in turn must log
out and back in to see what has changed on the nameserver.   This process of
logging out and in can cause issues with the host OS (particularly with
Windows) because of the delays incurred during this logout/in.  During
periods of high IO, the OS may just loose connection to the drives, or it
may kernel panic/blue screen the OS.

Qlogic switches have a feature called IOStreamguard that prevents RSCN's
from going to initiators that don't need to see them (i.e., the server does
not have an active connection to the tape, or the tape is not in its zone).
Other vendors switches can use zoning to restrict this as much as possible,
but RSCN's can still propagate outside the zones affected.

Also, tape resets are not as prevalent now as they used to be.  The older
SCSI tape drives that were connected to the SAN via SCSI to FC Bridge were
more of a problem that the newer native FC tape drives.  The native FC
drives are kinder to the fabric, and do not do resets unless they are
actually needed.  Despite the advances made with the native FC tape drives,
there still can be issues, and that is why there is the recommendation of
using a separate fabric for the tape subsystem.

Regards Jim





veritas-bu at mailman.eng.auburn.edu
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