Q Freezing Protocol Needed

["Sudhanshu Mittal (TRIPACE/Zoetermeer)" <smittal@tripace.com>]

Hello All,
	I have run into a problem on my iSCSI design. The problem concerns
"Q Freezing" mechanism in Novell NetWare and UnixWare Operating Systems.

	According to specifications of OS, in case of hardware error with
any command (There is a list of errors), execution of further commands on
that target should be stopped to facilitate error recovery mechanism of OS.
In conventional HBA, it was no problem since commands are queued on HBA and
it is the job of HBA to send next command to target; and in case of error,
it will simply stop sending commands to that target till error condition has
been cleared by OS. But in case of iSCSI, that technique cannot work since
commands are queued on target and once command has left NIC, there is
nothing it can do to stop the execution. ISCSI has made Auto Sense mandatory
but that does not cover any error recovery mechanism that OS may wish to
deploy.

	In addition, there is another specification - OS may ask HBA to
freeze Q after execution of a command and HBA should not send any command to
target from its queue till it receives clearance from OS. This is probably
more for diagnostic purposes (I have not seen any actual case so far) but it
is part of OS specifications and needs to be complied with. Here again, I am
not able to find any proper mechanism to achieve this. One way could be for
NIC to examine whether any such request has been made by OS and stop sending
commands but I do not like this method. Also, there are specifications like
"Q Flush" and "Task Abort". In case of conventional HBA, these were no
problems but here it becomes difficult to implement these.

	Unless I am missing something very basic, no mechanism has been
provided for these since these do not form part of T10/SAM-2 specifications.
But a mechanism is needed to achieve this so that an implementer can achieve
OS specifications.

Sincerely,

Sudhanshu Mittal

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