Can one LU be in multiple devices?

George Penokie gop at us.ibm.com
Tue Sep 28 06:10:43 PDT 2004


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

A logical unit can only exist in a single target device and each logical
unit has it's own device server.  This is intentional and should become
more clear as the UML drawings are developed for SAM-4 (look for
04-023r2 to appear within the next few weeks). 

A logical unit may be accessed through multiple target ports by multiple
application clients. Or, if you prefer, a single logical unit can be
accessed using any number of I_T nexuses. 

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





"Mallikarjun C." <cbm at rose.hp.com> 
Sent by: owner-t10 at t10.org 


09/27/2004 07:53 PM 

To
T10 Reflector <t10 at t10.org> 

cc
Julian Satran <Julian_Satran at il.ibm.com>, "KRUEGER,MARJORIE
(HP-Roseville,ex1)" <marjorie.krueger at hp.com>, "Reuter, Jim"
<james.reuter at hp.com> 

Subject
Can one LU be in multiple devices?

	





* From the T10 Reflector (t10 at t10.org), posted by:
* "Mallikarjun C." <cbm at rose.hp.com>
*
iSCSI allows one "network entity" to host multiple SCSI devices. Since 
all the LU inventory is accesible in the same physical box, each LU can 
actually be a member of multiple SCSI devices and there are some 
advantages in doing that.  A similar situation exists for 
multi-networked devices and virtualization boxes.

I'd like to solicit input from this list on this question: is it 
SAM-3-compliant to have one LU appear in multiple devices?

Jim Hafner and I had an offline email discussion on this.  Jim's 
position was that LUs appearing in multiple devices is legal, because 
(paraphrasing him; Jim, please add/correct):
- It is technically no different from dual-controller arrays (same LU 
can be presented on each I_T nexus, and the nexus ID doesn't include the

device identifier).
- Storage virtualizers do this today as well.
- Target Reset task management function leads to different results if 
the LU is contained in two devices versus one device, but the recent 
obsolescence of Target Reset helps.

I tended to concur with him, but after looking through the SAM-3 
drawings, I am not sure.  Here's my reasoning:

1. SAM-3's hierarchy diagrams show containment relationships (3.6.1 
makes this clear).

2. Figure 11 and Figure 14 (in 4.7.2 & 4.8 respectively) together imply 
to me that a device server is contained within a SCSI device.

3. From the same figures, the containment relationship between the 
target device object & the device server object is 1-to-1.  IOW, Figure 
11 does not show a Logical Unit being contained in more than one SCSI 
target device (more precisely, the SCSI target device box is not
shaded).

Putting these things together, having one LU (i.e. with all the 
composition shown in Figure 14) show up as a contained object in 
multiple discrete SCSI target device instances seems disallowed.

Now the questions are:  Is my reading of the drawings correct?  If so, 
is this prohibition (that each LU belongs to one and only SCSI device) 
intended?

Comments would help.  Thanks.
-- 
Mallikarjun

Mallikarjun Chadalapaka
Networked Storage Architecture
Network Storage Solutions
Hewlett-Packard MS 5668
Roseville CA 95747
cbm [at] rose.hp.com




*
* For T10 Reflector information, send a message with
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<br><font size=2 face="sans-serif">Mallikarjun,</font>
<br>
<br><font size=2 face="sans-serif">A logical unit can only exist in a single
target device and each logical unit has it's own device server. &nbsp;This
is intentional and should become more clear as the UML drawings are developed
for SAM-4 (look for 04-023r2 to appear within the next few weeks). </font>
<br>
<br><font size=2 face="sans-serif">A logical unit may be accessed through
multiple target ports by multiple application clients. Or, if you prefer,
a single logical unit can be accessed using any number of I_T nexuses.</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>
</font>
<br>
<br>
<br>
<table width=100%>
<tr valign=top>
<td width=40%><font size=1 face="sans-serif"><b>"Mallikarjun C."
<cbm at rose.hp.com&gt;</b> </font>
<br><font size=1 face="sans-serif">Sent by: owner-t10 at t10.org</font>
<p><font size=1 face="sans-serif">09/27/2004 07:53 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">T10 Reflector <t10 at t10.org&gt;</font>
<tr>
<td>
<div align=right><font size=1 face="sans-serif">cc</font></div>
<td valign=top><font size=1 face="sans-serif">Julian Satran <Julian_Satran at il.ibm.com&gt;,
"KRUEGER,MARJORIE (HP-Roseville,ex1)" <marjorie.krueger at hp.com&gt;,
"Reuter, Jim" <james.reuter at hp.com&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">Can one LU be in multiple
devices?</font></table>
<br>
<table>
<tr valign=top>
<td>
<td></table>
<br></table>
<br>
<br>
<br><font size=2><tt>* From the T10 Reflector (t10 at t10.org), posted by:<br>
* "Mallikarjun C." <cbm at rose.hp.com&gt;<br>
*<br>
iSCSI allows one "network entity" to host multiple SCSI devices.
Since <br>
all the LU inventory is accesible in the same physical box, each LU can
<br>
actually be a member of multiple SCSI devices and there are some <br>
advantages in doing that. &nbsp;A similar situation exists for <br>
multi-networked devices and virtualization boxes.<br>
<br>
I'd like to solicit input from this list on this question: is it <br>
SAM-3-compliant to have one LU appear in multiple devices?<br>
<br>
Jim Hafner and I had an offline email discussion on this. &nbsp;Jim's <br>
position was that LUs appearing in multiple devices is legal, because <br>
(paraphrasing him; Jim, please add/correct):<br>
- It is technically no different from dual-controller arrays (same LU <br>
can be presented on each I_T nexus, and the nexus ID doesn't include the
<br>
device identifier).<br>
- Storage virtualizers do this today as well.<br>
- Target Reset task management function leads to different results if <br>
the LU is contained in two devices versus one device, but the recent <br>
obsolescence of Target Reset helps.<br>
<br>
I tended to concur with him, but after looking through the SAM-3 <br>
drawings, I am not sure. &nbsp;Here's my reasoning:<br>
<br>
1. SAM-3's hierarchy diagrams show containment relationships (3.6.1 <br>
makes this clear).<br>
<br>
2. Figure 11 and Figure 14 (in 4.7.2 &amp; 4.8 respectively) together imply
<br>
to me that a device server is contained within a SCSI device.<br>
<br>
3. From the same figures, the containment relationship between the <br>
target device object &amp; the device server object is 1-to-1. &nbsp;IOW,
Figure <br>
11 does not show a Logical Unit being contained in more than one SCSI <br>
target device (more precisely, the SCSI target device box is not shaded).<br>
<br>
Putting these things together, having one LU (i.e. with all the <br>
composition shown in Figure 14) show up as a contained object in <br>
multiple discrete SCSI target device instances seems disallowed.<br>
<br>
Now the questions are: &nbsp;Is my reading of the drawings correct? &nbsp;If
so, <br>
is this prohibition (that each LU belongs to one and only SCSI device)
<br>
intended?<br>
<br>
Comments would help. &nbsp;Thanks.<br>
-- <br>
Mallikarjun<br>
<br>
Mallikarjun Chadalapaka<br>
Networked Storage Architecture<br>
Network Storage Solutions<br>
Hewlett-Packard MS 5668<br>
Roseville CA 95747<br>
cbm [at] rose.hp.com<br>
<br>
<br>
<br>
<br>
*<br>
* For T10 Reflector information, send a message with<br>
* 'info t10' (no quotes) in the message body to majordomo at t10.org<br>
</tt></font>
<br>
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