eaw@Alliant.COM (Eric Woudenberg) (10/31/90)
Does anyone know if it is possible to start multiple CDBs between one initiator and one target in SCSI-2 (so that, for instance, a drive may do seek ordering?) I imagine this would require the ability to specify a transaction tag during the disconnect/reconnect messages. It does not seem possible in SCSI-1. Is it possible in SCSI-2? Eric Woudenberg
ts@cup.portal.com (Tim W Smith) (11/01/90)
Yes. When you select the target with ATN, you can follow the identify with a queue tag message. This is a two byte message. The second byte is an identifier that identifies the request. The target is allowed to queue such requests. When the target reselects, it will send this tag back so you can tell which I/O is continuing. The first byte of the message tells how the request is to be placed in the queue. There are three possible values: HEAD OF QUEUE The request goes to the head of the queue SIMPLE The request goes in the queue. Requests can be reordered by target subject to the conditions below. ORDERED The request goes in the queue. All SIMPLE or ORDERED requests that are already in the queue will be completed before this request. All subsequent SIMPLE or ORDERED requests that arrive after this one will be completed after it. Tim Smith
neese@adaptx1.UUCP (11/03/90)
>/* ---------- "Command Queueing in SCSI-2" ---------- */ >Does anyone know if it is possible to start multiple CDBs between one >initiator and one target in SCSI-2 (so that, for instance, a drive may >do seek ordering?) I imagine this would require the ability to >specify a transaction tag during the disconnect/reconnect messages. >It does not seem possible in SCSI-1. Is it possible in SCSI-2? In the SCSI-1 specification this was not allowed. It did make provisions for a target to accept commands from more than one initiator, but no ordering was spec'd. In the SCSI-2 spec, a new *optional* feature called tagged queueing is implemented that allows a target to receive more than one command from an initiator. It also makes provisions for sorting and proioritizing commands in the queue. It does take both initiator and target to implement this. But remember, it is an option in the SCSI-2 spec, so you need to look for it specifically in the product literature for a device. FYI: Our 1740 EISA does implement the tag queue message when in native (1740) mode. Roy Neese Adaptec Senior SCSI Applications Engineer UUCP @ uunet!utacfd!cs.utexas.edu! {nominil,merch,cpe}!adaptex!neese uunet!mlite!adaptex!neese
kc@hprnd.rose.hp.com (Kurt Chan) (11/14/90)
In addition, the consensus of the last ANSI working group in Austin was that it was our intent to allow multiple *active* I/O processes to occur in a target, as long as the target had multiple independent sense areas to maintain REQUEST SENSE information for each process. What this means is that you are permitted by the standard to make forward progress on queued process 2 even though queued process 1 was received first and may be mid-way through completion (of course, data integrity remains a vendor-specific problem). We are working on wording which may appear in a Technical Information Bulletin to clarify this intent. Kurt Chan Hewlett-Packard Roseville Networks Division