Can one LU be in multiple devices?

George Penokie gop at us.ibm.com
Thu Sep 30 13:15:03 PDT 2004


* From the T10 Reflector (t10 at t10.org), posted by:
* George Penokie <gop at us.ibm.com>
*
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Jim, 

The constraint is that one logical unit cannot span multiple target
devices when both those targets devices and the logical unit are in the
same domain. (This is a very simplified statement that is described in
much more gory detail in my previous note. 

I believe your example and the statement after is correct. 

You ask why it is this way. My question is what's wrong with the current
definition. As far as I know, that are now cases where it causes a
problem. Also, allowing it would cause problems in identifying and
locating specific target devices. 

Before you start trying to convince my I am wrong, STOP, it will do no
good. The standard is the way I have described it and SAM-3 is done. To
change SAM-4 you have to convince the CAP working group, not me,  not
only that a change is needed but you will have to write up all the
changes to the standards that would be required to implement the change.


Consider these notes an education as to how it is defined today and go
|from there, if you dare. 

Bye the way the t10 reflector has been dead for most of this thread.
When it comes up I will try to make sure this is all posted. As I see
that some who have been involved are not on this distribution list. 

Bye for now,
George Penokie

Dept 2C6  114-2 N212
E-Mail:    gop at us.ibm.com
Internal:  553-5208
External: 507-253-5208   FAX: 507-253-2880





Jim Hafner/Almaden/IBM 


09/29/2004 04:30 PM 

To
George Penokie/Rochester/IBM at IBMUS 

cc
"Mallikarjun C." <cbm at rose.hp.com>, "Reuter, Jim" <james.reuter at hp.com>,
Julian Satran <Julian_Satran at il.ibm.com>, "KRUEGER,MARJORIE
(HP-Roseville,ex1)" <marjorie.krueger at hp.com>, Paul.Vonbehren at sun.com,
T10 Reflector <t10 at t10.org> 

Subject
Re: Can one LU be in multiple devices? Link
<Notes:///062564740060D5BE/A24CF79FA927570285256356005827C4/E59B7D20F4A1
80E386256F1E006FE221> 

	





George, 

What I read now is that there is a constraint that one LU per domain,
but that doesn't preclude an LU from appearing in multiple domains.  Is
that right? 

But if so, what is the point of a restriction about one LU (uniquely
named) per domain.  What feature is gained by this restriction?   

Consider the following two entities: 
1) dual-headed controller with shared LUs that collectively form a
single target device (one name) 
2) dual-headed controller with shared LUs that form a pair of target
devices. 

In what you say, the second is outlawed, UNLESS they are on separate
domains (e.g., FC loops), in which case it's OK.   But what's the
difference functionally? Why does the domain pose any constraints on the
model? 

Thanks, (and I've never seen you so verbose before :-) 

Jim Hafner/Almaden/IBM
Tel: (408) 927-1892, Fax: (408) 927-3030 (t/l 457)
Email:hafner at almaden.ibm.com 



	George Penokie 


09/29/2004 02:15 PM 
        To:        Jim Hafner/Almaden/IBM at IBMUS 
        cc:        "Mallikarjun C." <cbm at rose.hp.com>, "Reuter, Jim"
<james.reuter at hp.com>, Julian Satran <Julian_Satran at il.ibm.com>,
"KRUEGER,MARJORIE (HP-Roseville,ex1)" <marjorie.krueger at hp.com>,
Paul.Vonbehren at sun.com, T10 Reflector <t10 at t10.org> 
        Subject:        Re: Can one LU be in multiple devices? Link
<Notes:///882563A90059E2B4/5E502A1BAAAF40CA85256197006C1A32/C38CF0850CB0
5C0C85256F1E006E1049> 



Jim, 

In SCSI there are names and identifiers. Identifiers are unique to the
SCSI domain. Names are required to be world wide unique (i.e., you
should only see the name of a SCSI device once no matter what). 

So the logical units name is worldwide unique therefore it doesn't
matter where it is, who is talking to, whether it is virtual, or real it
has to have a unique name. Now it can have more that one unique name but
that same set of names are always returned from that logical unit when
request regardless of which protocol asks for them. There is no rule
that prevents a single logical unit from appearing in more than one SCSI
domain. In fact, the name is how a management applications can tell it's
the same logical unit even though it is being accessed from different
SCSI domains. 

An underlaying physical logical unit will have one (set) unique name(s)
and any virtual (mapped) logical unit must have a different (set) of
unique name(s). 

You certainly like those SCSI target devices. But from the point of view
of how they look to different domains and the naming rules there is
really no difference between logical units and SCSI target devices.
Except it's a heck of a lot easier to get the logical units name. But if
you insist: 

A SCSI target devices name is worldwide unique therefore it doesn't
matter where it is, who is talking to, whether it is virtual, or real it
has to have a unique name. Now it can have more that one unique name but
that same set of names are always returned regardless of which protocol
asks for them. There is no rule that prevents a single SCSI target
device from appearing in more than one SCSI domain. 

In case you are wondering there is two ways to find out a SCSI target
devices name in SCSI but only one is guaranteed to work. You can do an
INQUIRY command and asked for VPD page 83h and hope the logical unit
sends you the SCSI target devices name. Then you can do that for every
logical unit in you know about and compare the SCSI target device names
(assuming they all return that information). Or you can send the INQUIRY
command and a REPORT LUNS command to a REPORT LUNS well known logical
unit and it is required to tell the SCSI target devices name and give
you a list of all the logical units in that SCSI target device. 

So now I suspect your next question will be: But if everything you say
is true can a single logical unit appear in two different SCSI target
devices if those SCSI target devices are in two different SCSI domains. 

The answer is yes and long as the SCSI domain rules are followed.
Because they are in different SCSI domains and SCSI rules currently only
apply to the SCSI domain. 

Bye for now,
George Penokie

Dept 2C6  114-2 N212
E-Mail:    gop at us.ibm.com
Internal:  553-5208
External: 507-253-5208   FAX: 507-253-2880





Jim Hafner/Almaden/IBM 


09/29/2004 03:09 PM 

To
George Penokie/Rochester/IBM at IBMUS 

cc
"Mallikarjun C." <cbm at rose.hp.com>, "Reuter, Jim" <james.reuter at hp.com>,
Julian Satran <Julian_Satran at il.ibm.com>, "KRUEGER,MARJORIE
(HP-Roseville,ex1)" <marjorie.krueger at hp.com>, Paul.Vonbehren at sun.com,
T10 Reflector <t10 at t10.org> 

Subject
Re: Can one LU be in multiple devices? Link
<Notes:///062564740060D5BE/A24CF79FA927570285256356005827C4/5CFB9927A337
B5EC86256F1E0064D86C> 

	




George, 

Thanks for the history lesson.  But I think you confused me. 

You state: 
The net of all of this is that: 
- a single logical unit in one SCSI domain may be mapped into several
logical units in another domain. 
- a single logical unit in a single target device in one SCSI domain may
be mapped into several logical units with any number of those logical
units residing on any number of target devices in another domain. 

BUT the rules for naming still apply. 
-All the logical unit names have to be worldwide unique. 
-All the target device names have to be worldwide unique. 

So, if a single logical unit (call it the source) is "mapped into
several logical units", and each logical unit name is WWU, does this
mean that the "mapped" LUs have to have a different name from the source
one?  If not, then it would appear that the same LU shows up in multiple
target devices.  If so, then there is no (standard) way from a
management perspective to tell that the mapped LUs have a source. 

Also, you seem to imply (if I read you right) that in order for an LU to
appear in another domain, it must be in some different target device.
This presumes then that a target device lives in only one domain -- but
can't a device have both FC, SPI and iSCSI interface and so appear in
multiple domains (or even two SPI interfaces on different SCSI buses)?
Or is this a case of "two devices" with the same target device name? 

Thanks, 

Jim Hafner/Almaden/IBM
Tel: (408) 927-1892, Fax: (408) 927-3030 (t/l 457)
Email:hafner at almaden.ibm.com 



	George Penokie 


09/29/2004 12:58 PM 
        To:        Jim Hafner/Almaden/IBM at IBMUS 
        cc:        "Mallikarjun C." <cbm at rose.hp.com>, "Reuter, Jim"
<james.reuter at hp.com>, Julian Satran <Julian_Satran at il.ibm.com>,
"KRUEGER,MARJORIE (HP-Roseville,ex1)" <marjorie.krueger at hp.com>,
owner-t10 at t10.org, Paul.Vonbehren at sun.com, T10 Reflector <t10 at t10.org> 
        Subject:        Re: Can one LU be in multiple devices? Link
<Notes:///882563A90059E2B4/5E502A1BAAAF40CA85256197006C1A32/7F7FF38D60E5
1CC285256F1E005059B9> 



I have been reading these notes, looking in the places that supposedly
justify the belief that a logical unit can be split across multiple
target devices. But so far all I have found is that, either target
devices are not talked about or that target is being used, either
directly or indirectly, to refer to a target port. 

So a little history lesson is in order. Up until a year or so the term
target was a fuzzy thing on one end of a wire (with an initiator on the
other). This fuzziness started to become a problem when the SCSI
architecture began expanding outside the parallel environment. To
resolve this the SCSI committee started to look into defining the
various SCSI objects (soon to become classes) more solidly. The first,
and most obvious, problem was that 90% of the time the term target or
initiator was used in the standards what was really being described was
the port on a target or initiator. As a result new terms were defined:
target port, target device, initiator port, and initiator device.  This
has resulted in every use of the term target and initiator to being
examined to determine if it is a port, a device, or something else. At
this point SAM-3, SBC-2, SPC-3, and SAS 1.1 have been examined and
corrected to the new terminology. 

Back to the main point. Under SAM-3 a target device contains logical
units, target ports, and has a name (it has other objects but that is a
different discussion). There is no provision in SAM-3 that would allow a
single logical unit to be split across multiple target devices and there
is no amount of SAM-3 word twisting that will change that. This is not
something that I believe is desirable or necessary to accomplish given
everything that I have read so far.   

Now, before you vitalization guys go crazy, keep reading. 

Vitalization of logical units has been implemented in SCSI long before
that term was in vogue. It worked under SCSI then and it still works.
The reason it works is because of how the SCSI domain works. For example
a RAID device or a bridge controller that receives SCSI operations
(i.e., a target device) and then retransmits the information received
using different SCSI operations (i.e., an initiator device) operates in
two SCSI domains. Each SCSI domain has a set of initiator ports, target
ports, logical units, and, if you insist, target devices. The
relationship between the all these objects within each domain is
strictly controlled by the SCSI standards. The relationship between the
objects between the SCSI domains is not currently defined by SCSI. 

The net of all of this is that: 
- a single logical unit in one SCSI domain may be mapped into several
logical units in another domain. 
- a single logical unit in a single target device in one SCSI domain may
be mapped into several logical units with any number of those logical
units residing on any number of target devices in another domain. 

BUT the rules for naming still apply. 
-All the logical unit names have to be worldwide unique. 
-All the target device names have to be worldwide unique. 

As a final note: SAM-3 is done and SAM-4 is starting. The most obvious
change in SAM-4 will be the use of UML diagrams. Another change is the
editor has is now myself. So if there is some new class that needs to be
defined (as Paul suggests) then someone will have to bring in a proposal
and get it accepted. 

Bye for now,
George Penokie

Dept 2C6  114-2 N212
E-Mail:    gop at us.ibm.com
Internal:  553-5208
External: 507-253-5208   FAX: 507-253-2880

<<<Filtered>>> 






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<br><font size=2 face="sans-serif">Jim,</font>
<br>
<br><font size=2 face="sans-serif">The constraint is that one logical unit
cannot span multiple target devices when both those targets devices and
the logical unit are in the same domain. (This is a very simplified statement
that is described in much more gory detail in my previous note.</font>
<br>
<br><font size=2 face="sans-serif">I believe your example and the statement
after is correct. </font>
<br>
<br><font size=2 face="sans-serif">You ask why it is this way. My question
is what's wrong with the current definition. As far as I know, that are
now cases where it causes a problem. Also, allowing it would cause problems
in identifying and locating specific target devices.</font>
<br>
<br><font size=2 face="sans-serif">Before you start trying to convince
my I am wrong, STOP, it will do no good. The standard is the way I have
described it and SAM-3 is done. To change SAM-4 you have to convince the
CAP working group, not me, &nbsp;not only that a change is needed but you
will have to write up all the changes to the standards that would be required
to implement the change. </font>
<br>
<br><font size=2 face="sans-serif">Consider these notes an education as
to how it is defined today and go from there, if you dare.</font>
<br>
<br><font size=2 face="sans-serif">Bye the way the t10 reflector has been
dead for most of this thread. When it comes up I will try to make sure
this is all posted. As I see that some who have been involved are not on
this distribution list.</font>
<br><font size=2 face="sans-serif"><br>
Bye for now,<br>
George Penokie<br>
<br>
Dept 2C6 &nbsp;114-2 N212<br>
E-Mail: &nbsp; &nbsp;gop at us.ibm.com<br>
Internal: &nbsp;553-5208<br>
External: 507-253-5208 &nbsp; FAX: 507-253-2880<br>
<br>
</font>
<br>
<br>
<br>
<table width=100%>
<tr valign=top>
<td width=40%><font size=1 face="sans-serif"><b>Jim Hafner/Almaden/IBM</b></font>
<p><font size=1 face="sans-serif">09/29/2004 04:30 PM</font>
<td width=59%>
<table width=100%>
<tr>
<td>
<div align=right><font size=1 face="sans-serif">To</font></div>
<td valign=top><font size=1 face="sans-serif">George Penokie/Rochester/IBM at IBMUS</font>
<tr>
<td>
<div align=right><font size=1 face="sans-serif">cc</font></div>
<td valign=top><font size=1 face="sans-serif">"Mallikarjun C."
<cbm at rose.hp.com&gt;, "Reuter, Jim" <james.reuter at hp.com&gt;,
Julian Satran <Julian_Satran at il.ibm.com&gt;, "KRUEGER,MARJORIE
(HP-Roseville,ex1)" <marjorie.krueger at hp.com&gt;, Paul.Vonbehren at sun.com,
T10 Reflector <t10 at t10.org&gt;</font>
<tr>
<td>
<div align=right><font size=1 face="sans-serif">Subject</font></div>
<td valign=top><font size=1 face="sans-serif">Re: Can one LU be in multiple
devices?</font>Link</table>
<br>
<table>
<tr valign=top>
<td>
<td></table>
<br></table>
<br>
<br>
<br><font size=2 face="sans-serif">George,</font>
<br>
<br><font size=2 face="sans-serif">What I read now is that there is a constraint
that one LU per domain, but that doesn't preclude an LU from appearing
in multiple domains. &nbsp;Is that right?</font>
<br>
<br><font size=2 face="sans-serif">But if so, what is the point of a restriction
about one LU (uniquely named) per domain. &nbsp;What feature is gained
by this restriction? &nbsp; </font>
<br>
<br><font size=2 face="sans-serif">Consider the following two entities:</font>
<br><font size=2 face="sans-serif">1) dual-headed controller with shared
LUs that collectively form a single target device (one name)</font>
<br><font size=2 face="sans-serif">2) dual-headed controller with shared
LUs that form a pair of target devices.</font>
<br>
<br><font size=2 face="sans-serif">In what you say, the second is outlawed,
UNLESS they are on separate domains (e.g., FC loops), in which case it's
OK. &nbsp; But what's the difference functionally? Why does the domain
pose any constraints on the model? </font>
<br>
<br><font size=2 face="sans-serif">Thanks, (and I've never seen you so
verbose before :-)</font>
<br><font size=2 face="sans-serif"><br>
Jim Hafner/Almaden/IBM<br>
Tel: (408) 927-1892, Fax: (408) 927-3030 (t/l 457)<br>
Email:hafner at almaden.ibm.com</font>
<br>
<br>
<br>
<table width=100%>
<tr valign=top>
<td>
<td><font size=1 face="sans-serif"><b>George Penokie</b></font>
<p><font size=1 face="sans-serif">09/29/2004 02:15 PM</font>
<td>
<br><font size=1 face="sans-serif">&nbsp; &nbsp; &nbsp; &nbsp; To:
&nbsp; &nbsp; &nbsp; &nbsp;Jim Hafner/Almaden/IBM at IBMUS</font>
<br><font size=1 face="sans-serif">&nbsp; &nbsp; &nbsp; &nbsp; cc:
&nbsp; &nbsp; &nbsp; &nbsp;"Mallikarjun C." <cbm at rose.hp.com&gt;,
"Reuter, Jim" <james.reuter at hp.com&gt;, Julian Satran <Julian_Satran at il.ibm.com&gt;,
"KRUEGER,MARJORIE (HP-Roseville,ex1)" <marjorie.krueger at hp.com&gt;,
Paul.Vonbehren at sun.com, T10 Reflector <t10 at t10.org&gt;</font>
<br><font size=1 face="sans-serif">&nbsp; &nbsp; &nbsp; &nbsp; Subject:
&nbsp; &nbsp; &nbsp; &nbsp;Re: Can one LU be in multiple devices?</font>Link</table>
<br>
<br><font size=2 face="sans-serif">Jim,</font>
<br>
<br><font size=2 face="sans-serif">In SCSI there are names and identifiers.
Identifiers are unique to the SCSI domain. Names are required to be world
wide unique (i.e., you should only see the name of a SCSI device once no
matter what). </font>
<br>
<br><font size=2 face="sans-serif">So the logical units name is worldwide
unique therefore it doesn't matter where it is, who is talking to, whether
it is virtual, or real it has to have a unique name. Now it can have more
that one unique name but that same set of names are always returned from
that logical unit when request regardless of which protocol asks for them.
There is no rule that prevents a single logical unit from appearing in
more than one SCSI domain. In fact, the name is how a management applications
can tell it's the same logical unit even though it is being accessed from
different SCSI domains.</font>
<br>
<br><font size=2 face="sans-serif">An underlaying physical logical unit
will have one (set) unique name(s) and any virtual (mapped) logical unit
must have a different (set) of unique name(s).</font>
<br>
<br><font size=2 face="sans-serif">You certainly like those SCSI target
devices. But from the point of view of how they look to different domains
and the naming rules there is really no difference between logical units
and SCSI target devices. Except it's a heck of a lot easier to get the
logical units name. But if you insist:</font>
<br>
<br><font size=2 face="sans-serif">A SCSI target devices name is worldwide
unique therefore it doesn't matter where it is, who is talking to, whether
it is virtual, or real it has to have a unique name. Now it can have more
that one unique name but that same set of names are always returned regardless
of which protocol asks for them. There is no rule that prevents a single
SCSI target device from appearing in more than one SCSI domain. </font>
<br>
<br><font size=2 face="sans-serif">In case you are wondering there is two
ways to find out a SCSI target devices name in SCSI but only one is guaranteed
to work. You can do an INQUIRY command and asked for VPD page 83h and hope
the logical unit sends you the SCSI target devices name. Then you can do
that for every logical unit in you know about and compare the SCSI target
device names (assuming they all return that information). Or you can send
the INQUIRY command and a REPORT LUNS command to a REPORT LUNS well known
logical unit and it is required to tell the SCSI target devices name and
give you a list of all the logical units in that SCSI target device. </font>
<br>
<br><font size=2 face="sans-serif">So now I suspect your next question
will be: But if everything you say is true can a single logical unit appear
in two different SCSI target devices if those SCSI target devices are in
two different SCSI domains. </font>
<br>
<br><font size=2 face="sans-serif">The answer is yes and long as the SCSI
domain rules are followed. Because they are in different SCSI domains and
SCSI rules currently only apply to the SCSI domain.</font>
<br><font size=2 face="sans-serif"><br>
Bye for now,<br>
George Penokie<br>
<br>
Dept 2C6 &nbsp;114-2 N212<br>
E-Mail: &nbsp; &nbsp;gop at us.ibm.com<br>
Internal: &nbsp;553-5208<br>
External: 507-253-5208 &nbsp; FAX: 507-253-2880<br>
<br>
</font>
<br>
<br>
<br>
<table width=100%>
<tr valign=top>
<td width=40%><font size=1 face="sans-serif"><b>Jim Hafner/Almaden/IBM</b></font>
<p><font size=1 face="sans-serif">09/29/2004 03:09 PM</font>
<td width=59%>
<table width=100%>
<tr>
<td>
<div align=right><font size=1 face="sans-serif">To</font></div>
<td valign=top><font size=1 face="sans-serif">George Penokie/Rochester/IBM at IBMUS</font>
<tr>
<td>
<div align=right><font size=1 face="sans-serif">cc</font></div>
<td valign=top><font size=1 face="sans-serif">"Mallikarjun C."
<cbm at rose.hp.com&gt;, "Reuter, Jim" <james.reuter at hp.com&gt;,
Julian Satran <Julian_Satran at il.ibm.com&gt;, "KRUEGER,MARJORIE
(HP-Roseville,ex1)" <marjorie.krueger at hp.com&gt;, Paul.Vonbehren at sun.com,
T10 Reflector <t10 at t10.org&gt;</font>
<tr>
<td>
<div align=right><font size=1 face="sans-serif">Subject</font></div>
<td valign=top><font size=1 face="sans-serif">Re: Can one LU be in multiple
devices?</font>Link</table>
<br>
<table>
<tr valign=top>
<td>
<td></table>
<br></table>
<br>
<br><font size=2 face="sans-serif">George,</font>
<br>
<br><font size=2 face="sans-serif">Thanks for the history lesson. &nbsp;But
I think you confused me. </font>
<br>
<br><font size=2 face="sans-serif">You state: </font>
<br><font size=2 face="sans-serif">The net of all of this is that:</font>
<br><font size=2 face="sans-serif">- a single logical unit in one SCSI
domain may be mapped into several logical units in another domain. </font>
<br><font size=2 face="sans-serif">- a single logical unit in a single
target device in one SCSI domain may be mapped into several logical units
with any number of those logical units residing on any number of target
devices in another domain. </font>
<br>
<br><font size=2 face="sans-serif">BUT the rules for naming still apply.
</font>
<br><font size=2 face="sans-serif">-All the logical unit names have to
be worldwide unique.</font>
<br><font size=2 face="sans-serif">-All the target device names have to
be worldwide unique.</font>
<br>
<br><font size=2 face="sans-serif">So, if a single logical unit (call it
the source) is "mapped into several logical units", and each
logical unit name is WWU, does this mean that the "mapped" LUs
have to have a different name from the source one? &nbsp;If not, then it
would appear that the same LU shows up in multiple target devices. &nbsp;If
so, then there is no (standard) way from a management perspective to tell
that the mapped LUs have a source.</font>
<br>
<br><font size=2 face="sans-serif">Also, you seem to imply (if I read you
right) that in order for an LU to appear in another domain, it must be
in some different target device. &nbsp;This presumes then that a target
device lives in only one domain -- but can't a device have both FC, SPI
and iSCSI interface and so appear in multiple domains (or even two SPI
interfaces on different SCSI buses)? &nbsp; Or is this a case of "two
devices" with the same target device name?</font>
<br>
<br><font size=2 face="sans-serif">Thanks, </font>
<br><font size=2 face="sans-serif"><br>
Jim Hafner/Almaden/IBM<br>
Tel: (408) 927-1892, Fax: (408) 927-3030 (t/l 457)<br>
Email:hafner at almaden.ibm.com</font>
<br>
<br>
<br>
<table width=100%>
<tr valign=top>
<td>
<td><font size=1 face="sans-serif"><b>George Penokie</b></font>
<p><font size=1 face="sans-serif">09/29/2004 12:58 PM</font>
<td>
<br><font size=1 face="sans-serif">&nbsp; &nbsp; &nbsp; &nbsp; To:
&nbsp; &nbsp; &nbsp; &nbsp;Jim Hafner/Almaden/IBM at IBMUS</font>
<br><font size=1 face="sans-serif">&nbsp; &nbsp; &nbsp; &nbsp; cc:
&nbsp; &nbsp; &nbsp; &nbsp;"Mallikarjun C." <cbm at rose.hp.com&gt;,
"Reuter, Jim" <james.reuter at hp.com&gt;, Julian Satran <Julian_Satran at il.ibm.com&gt;,
"KRUEGER,MARJORIE (HP-Roseville,ex1)" <marjorie.krueger at hp.com&gt;,
owner-t10 at t10.org, Paul.Vonbehren at sun.com, T10 Reflector <t10 at t10.org&gt;</font>
<br><font size=1 face="sans-serif">&nbsp; &nbsp; &nbsp; &nbsp; Subject:
&nbsp; &nbsp; &nbsp; &nbsp;Re: Can one LU be in multiple devices?</font>Link</table>
<br>
<br><font size=2 face="sans-serif">I have been reading these notes, looking
in the places that supposedly justify the belief that a logical unit can
be split across multiple target devices. But so far all I have found is
that, either target devices are not talked about or that target is being
used, either directly or indirectly, to refer to a target port. </font>
<br>
<br><font size=2 face="sans-serif">So a little history lesson is in order.
Up until a year or so the term target was a fuzzy thing on one end of a
wire (with an initiator on the other). This fuzziness started to become
a problem when the SCSI architecture began expanding outside the parallel
environment. To resolve this the SCSI committee started to look into defining
the various SCSI objects (soon to become classes) more solidly. The first,
and most obvious, problem was that 90% of the time the term target or initiator
was used in the standards what was really being described was the port
on a target or initiator. As a result new terms were defined: target port,
target device, initiator port, and initiator device. &nbsp;This has resulted
in every use of the term target and initiator to being examined to determine
if it is a port, a device, or something else. At this point SAM-3, SBC-2,
SPC-3, and SAS 1.1 have been examined and corrected to the new terminology.
</font>
<br>
<br><font size=2 face="sans-serif">Back to the main point. Under SAM-3
a target device contains logical units, target ports, and has a name (it
has other objects but that is a different discussion). There is no provision
in SAM-3 that would allow a single logical unit to be split across multiple
target devices and there is no amount of SAM-3 word twisting that will
change that. This is not something that I believe is desirable or necessary
to accomplish given everything that I have read so far. &nbsp;</font>
<br>
<br><font size=2 face="sans-serif">Now, before you vitalization guys go
crazy, keep reading.</font>
<br>
<br><font size=2 face="sans-serif">Vitalization of logical units has been
implemented in SCSI long before that term was in vogue. It worked under
SCSI then and it still works. The reason it works is because of how the
SCSI domain works. For example a RAID device or a bridge controller that
receives SCSI operations (i.e., a target device) and then retransmits the
information received using different SCSI operations (i.e., an initiator
device) operates in two SCSI domains. Each SCSI domain has a set of initiator
ports, target ports, logical units, and, if you insist, target devices.
The relationship between the all these objects within each domain is strictly
controlled by the SCSI standards. The relationship between the objects
between the SCSI domains is not currently defined by SCSI. </font>
<br>
<br><font size=2 face="sans-serif">The net of all of this is that:</font>
<br><font size=2 face="sans-serif">- a single logical unit in one SCSI
domain may be mapped into several logical units in another domain. </font>
<br><font size=2 face="sans-serif">- a single logical unit in a single
target device in one SCSI domain may be mapped into several logical units
with any number of those logical units residing on any number of target
devices in another domain. </font>
<br>
<br><font size=2 face="sans-serif">BUT the rules for naming still apply.
</font>
<br><font size=2 face="sans-serif">-All the logical unit names have to
be worldwide unique.</font>
<br><font size=2 face="sans-serif">-All the target device names have to
be worldwide unique.</font>
<br><font size=2 face="sans-serif"><br>
As a final note: SAM-3 is done and SAM-4 is starting. The most obvious
change in SAM-4 will be the use of UML diagrams. Another change is the
editor has is now myself. So if there is some new class that needs to be
defined (as Paul suggests) then someone will have to bring in a proposal
and get it accepted.</font>
<br>
<br><font size=2 face="sans-serif">Bye for now,<br>
George Penokie<br>
<br>
Dept 2C6 &nbsp;114-2 N212<br>
E-Mail: &nbsp; &nbsp;gop at us.ibm.com<br>
Internal: &nbsp;553-5208<br>
External: 507-253-5208 &nbsp; FAX: 507-253-2880<br>
<br>
<<<Filtered>>></font>
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