More on Report LUNs command

George Penokie GOP at
Thu Dec 12 12:42:53 PST 1996

* From the SCSI Reflector (scsi at, posted by:
* "George Penokie" <GOP at RCHVMP3.VNET.IBM.COM>
Alright, Ralph has dragged me off the sidelines with his last note. So
I will attempt to clear up what appears to be some confusion about
LUNs and peripheral devices. (I have found that usually these
attempts generate more questions than answers but here goes anyway).

Generally I agree with Ralph's note except in the areas noted below.

A LUN is a Logical Unit Number that is the address of a Logical Unit
(LU). So a LUN is not an object or a thing it is an address of an
object or thing.

Every Logical Unit is defined as having a particular type (e.g.
block-access device, sequential-access device, array controller
device, etc.).  These Logical Unit types are called Peripheral Device
Types and are defined in the Inquiry command.  If you don't know the
peripheral device type of a logical unit you don't know what commands
can be sent to it.

The point is that the list of LUNs returned in the Report LUNs
command is nothing more than a list of addresses of Logical Units the
application can talk to.  To determine the peripheral device types of
Logical Units the application must issue Inquiry commands to all the
Logical Units.

Now if one of the Inquiry commands returns a peripheral device type of
Array Controller then the address methods field within the LUN, as
defined in the SCC standard, can be used to determine which of the
Logical Units within the addressed Target represent logical Logical
Units (i.e.  Volume Sets).  The remaining LUNs will be the addresses
of the physical Logical Units (e.g.  disk drives, tape drives, etc.)
the Target allows commands to be issued to by the application.

There is allot more that can be said about addressing Array Controller
devices but it is all in the SCC standard. I think (hope, wish) the
above at least clears things up a little.

Bye for now,
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