TASK SET FULL Definition(s) in FC-TAPE and SAM-2

gop at us.ibm.com gop at us.ibm.com
Wed Mar 17 09:28:12 PST 1999


* From the T10 Reflector (t10 at symbios.com), posted by:
* gop at us.ibm.com
*
Joe,
The is a significant difference on how an initiator is supposed to respond
to a BUSY vs a TASK SET FULL status.

A BUSY only tells the initiator that the command could not be received by
the target and the initiator should try again later. There is no indication
as to how much later from the target.

A TASK SET FULL also tells the initiator that the command could not be
received by the target and that in initiator should not try again until it
receives a command complete indication from a currently outstanding
command. So in the case of a TASK SET FULL there is an indication from the
target as to when the initiator has a chance of having the command
accepted.

This difference is important because the way most initiators respond to
BUSY is to immediately resend the command which has the effect of clogging
up the SCSI bus with needless activity. Where as TASK SET FULL causes the
initiators to hold back resending the command thus reducing traffic on the
SCSI bus.

Bye for now,
George Penokie

Dept PPV  114-2 N212
E-Mail:    gop at us.ibm.com
Internal:  553-5208
External: 507-253-5208   FAX: 507-253-2880




JoeBre at Exabyte.COM on 03/17/99 10:38:40 AM

To:   t10 at Symbios.COM, fc at nsg0.network.com
cc:    (bcc: George Penokie/Rochester/IBM)
Subject:  TASK SET FULL Definition(s) in FC-TAPE and SAM-2





* From the T10 Reflector (t10 at symbios.com), posted by:
* JoeBre at Exabyte.COM
*
Replies inline...


> -----Original Message-----
> From: Dave Peterson [mailto:dap at nsg0.network.com]
> Sent: Tuesday, March 16, 1999 1:36 PM
> To: t10 at symbios.com; fc at nsg0.network.com
> Subject: TASK SET FULL Definition(s) in FC-TAPE and SAM-2
>
>
> Howdy All,
> FC-TAPE currently states in clause 9.2 for TASK SET FULL:
>
> - TASK SET FULL (if Tagged Queuing is used or ULP resources are
unavailable)
>
> SAM-2r10 states in clause 5.2:
>
> TASK SET FULL. This status shall be implemented if the logical unit
supports the creation
> of tagged tasks (see 4.9). This status shall be returned when the logical
unit receives a
> command and does not have enough resources to enter the associated task
in
the task set.
>
> SAM-2r10 states in clause 4.8:
>
> A Task Set is composed of zero or more Untagged Tasks or a combination of
zero or more
> Tagged Tasks and zero or more Untagged Tasks.
>
> So it seems to me TASK SET FULL should be implemented regardless.

<Breher>
I am unclear as to what information TASK SET FULL conveys that BUSY does
not, in the case that Tagged Queing is not implemented. I would therefore
prefer that a target not be required to implement TASK SET FULL in this
case.

Accordingly, perhaps 9.2 should say:
- TASK SET FULL (if Tagged Queuing is implemented)
</Breher>

> Another issue: what is the proper SCSI Status to return if:
>
> a) the lun does not support tagged tasks, is processing a command
received
|from one Initiator (has no reservation active), has enough resources to
receive another command and enter it into the Task Set,
> and receives another command from a different Initiator.
>
> My preference would be a BUSY.

<Breher>
Shouldn't the lun just go ahead and process the new command from the other
intiator (queue it for execution)? Isn't it the responsibility of the
initiators to Reserve the lun if they are doing something that can be
disturbed by another initiator?
</Breher>

> b) the lun does not support tagged tasks, is processing a command
received
|from one Initiator (has no reservation active), does not have enough
resources to receive another command and enter it into the
> Task Set,  and receives another command from a different Initiator.
>
> I would expect a TASK SET FULL.

<Breher>
As outlined above, I would expect a BUSY.
</Breher>

>
> Comments?
>
> --
> ===================================================================
> Dave Peterson                     phone : 612-391-1008
> Principal Engineer
> StorageTek Network Business Group email: dap at network.com

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