Tagged Command Queuing (Was: RE: Tuning 2940)

Hi Mark and others

Tagged Command Queuing (TCQ) is a method for allowing multiple SCSI
initiators to send requests to SCSI devices and getting the right response
back to the right initiator. In a system such as Digital's OpenVMS, which
can support a shared SCSI bus (2 or 3 hosts and one set of disks / SCSI RAID
controller on the bus), it is the one function that "breaks" so many drive
manufacturers firmware. They often claim to be fully TCQ compliant (via the
SCSI inquiry replies), but generally aren't, and OpenVMS soon finds that
out. Sometimes you can set the TCQ response bit back to "off", just so the
brain-dead drive isn't lying (but this doesn't help OpenVMS any).

The only disks that Digital seems to trust are their own (e.g. RZ29 =
4.3Gb), which are re-badged Seagate drives with Digital Firmware in them
(they still come with the original Seagate documentation pamphlet).

As to how TCQ relates to most Unixes (and Linux in your case) I'm not sure.
I don't believe too many other OSes actually support it. OpenVMS seems to
stand alone in this regard, hence the reason why it stress tests SCSI
compliance far more than other operating systems which are more "lenient".

Regards

Jason
Who still likes OpenVMS a real lot, especially over most Unixes.

----------
From: Mark Dabrowski[SMTP:mark@ica.net]
Sent: Wednesday, 27 May 1998 11:06
To: squid-users@nlanr.net
Subject: Tuning 2940

I think I know how to enable tag queueing... What does it do anyways?

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Received on Tue May 26 1998 - 19:40:50 MDT