SPC - Report Luns command

ROWEBER at acm.org ROWEBER at acm.org
Wed Dec 11 19:05:08 PST 1996

* From the SCSI Reflector (scsi at symbios.com), posted by:
* ROWEBER at acm.org
Unless George Penokie (Mr. SCC) jumps all over this, I think it pretty well
covers the questions about the Report Luns command.

First, it is critical to note that Report Luns is designed to take the place
of a testing each possible LUN for a response and command's design is centered 
around that very limited function.  When there were only 8 possible LUNs, 
the application client could be expected to issue a Test Unit Ready to
every LUN, to see if it was accessible.  With 2**64 possible LUNs, that
technique is impractical, since it would taken nearly forever.  The design
of the Report Luns command assumes that a Test Unit Ready command will be
issued to determine a LUN's status and an Inquiry command will be issue to
determine a LUN's properties.

Second, it is important to note that LUN 0 is assumed to be responsive and
informative on all SCSI-3 devices.  In the absence of this assumption, the
Report Luns command is of no use, because there is no reliable place to
which it can be sent.  However, the assumption also means that an Inquiry
command (and possibly other commands) can be sent to LUN 0.

>From the Inquiry command, the application client can determine the device 
type of the SCSI device at that bus ID.

Device type is critical because:

   - All device types except array devices have only "LUN devices"
     (such devices do not have "peripheral devices" or "volume set devices")

   - Array type devices have both "peripheral devices" and "volume set 
     devices", but array devices do not have "LUN devices".  Finally,
     "peripheral devices" can be differentiated from "volume set devices"
     by inspecting their logical unit numbers in accordance with the
     rules described in SCC (SCSI-3 Controller Commands).

It also is important to note that SCC defines commands for accessing informa-
tion about all logical unit devices defined within a storage array.  The SCC
commands provide the control over what information is returned that Larry Chen
requested in the original mail.

Although it may not be immediately obvious, the features requested by Larry 
Chen are already available in SCSI.  In most cases, only one type of LUN is
reported by the Report Luns command.  In the only exception to that rule,
commands other than Report Luns provide the requested selection capability.
Therefore, no changes are required.


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