Subject: RE: [Ips] MC/S vs multiple sessions

From: "John Hufferd" <[email-masked]>
Date: Wed, 1 Nov 2006 06:25:12 -0800

Because of I-T-L Nexus issues, most wedge drivers (such as MPIO) do not
send I/O for the same LUN down different Sessions.  Even if they do,
they need to ensure that no I/O for the same LUN travels down another
Session while I/O for that same LUN is pending on the first session.

With Multiple Connections per Session (MC/S), there are no such
restrictions.  Therefore the Initiator can send commands for the same
LUN down both connections at a rate that is only limited by the ability
of the storage controller to handle it.  It is also true that with MPIO
commands for other LUs can occur on the other Paths, but this may or may
not be the process which is most acceptable to the total environment.  

What this means is that performance is generally better with MC/S, but
the degree of performance improvement will depend on the design of the
Wedge Driver (MPIO) and the quantity of Multiple I/Os to the same LUN.

.

.

.

John L Hufferd

Sr. Executive Director of Technology

Brocade Communications Systems, Inc

[email-masked]

Office Phone: (408) 333-5244; eFAX: (408) 904-4688

Alt Office Phone: (408) 997-6136; Cell: (408) 627-9606

________________________________

From: Qi, Yanling [mailto:[email-masked]] 
Sent: Wednesday, November 01, 2006 8:43 AM
To: [email-masked]
Subject: [Ips] MC/S vs multiple sessions

Assume my iSCSI target device has two network portals and my initiator
device has two network portals as well.  I could create two sessions or
create one session with two connections between the initiator and
target. I do have MPIO service on the initiator host that provides IO
load balance cross the two sessions. I could not see the differences
from the viewpoint of IO availability.  Which implementation is better
in term of IO performance?

Thanks,

Yanling

Yanling Qi

Engenio Storage Group - LSI Logic

512-794-3713 (Office)

512-794-3702 (Fax)

[email-masked]

_______________________________________________
Ips mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/ips

---

From: Julian Satran <[email-masked]>
Date: Wed, 1 Nov 2006 18:14:17 +0200

To the above - only a tiny caveat - your iSCSI driver (initiator and 
target) must really use both connections. Some drivers do it better than 
others and even use the connections to asure a modicum of QoS. The 
restriction that most MPIOs introduce as John mentioned is due to the need 
of many applications to keep ordering.

Julo

"John Hufferd" <[email-masked]> 
01/11/06 16:25

To
"Qi, Yanling" <[email-masked]>, <[email-masked]>
cc

Subject
RE: [Ips] MC/S vs multiple sessions

Because of I-T-L Nexus issues, most wedge drivers (such as MPIO) do not 
send I/O for the same LUN down different Sessions.  Even if they do, they 
need to ensure that no I/O for the same LUN travels down another Session 
while I/O for that same LUN is pending on the first session.
 
With Multiple Connections per Session (MC/S), there are no such 
restrictions.  Therefore the Initiator can send commands for the same LUN 
down both connections at a rate that is only limited by the ability of the 
storage controller to handle it.  It is also true that with MPIO commands 
for other LUs can occur on the other Paths, but this may or may not be the 
process which is most acceptable to the total environment. 
 
What this means is that performance is generally better with MC/S, but the 
degree of performance improvement will depend on the design of the Wedge 
Driver (MPIO) and the quantity of Multiple I/Os to the same LUN.
 
.
.
.
John L Hufferd

Assume my iSCSI target device has two network portals and my initiator 
device has two network portals as well.  I could create two sessions or 
create one session with two connections between the initiator and target. 
I do have MPIO service on the initiator host that provides IO load balance 
cross the two sessions. I could not see the differences from the viewpoint 
of IO availability.  Which implementation is better in term of IO 
performance?
 
Thanks,
 
Yanling

Yanling Qi

---

From: Santosh Rao <[email-masked]>
Date: Fri, 3 Nov 2006 09:53:57 -0800 (PST)

I'm not aware of any need to ensure no I/O to the same LUN travels down
another session (I-T-L nexus) while I/O is pending for the LUN on the
first session (I-T nexus).

Not sure which drivers are being referred to here when you say "most wedge
drivers do not send I/O for the same LUN down different sessions (I-T
nexi)."

Most MPIO drivers I am aware of will all send I/Os down different
I-T-L nexi of a lun to perform dynamic load balancing.

Perhaps you were referring to I/Os to the same LBA range of a LUN.
Serializing I/Os to the same LBA ranges of a LUN is the job of the upper
layer application that owns the content residing on the lun such as a
filesystem, database, etc.

To answer Yanling's question, I think both solutions are equally feasible
and the debate over MPIO vs. MC/S has been a long one in the past with
marginal benefits from MC/S compared to utilizing existing MPIO
infrastructure, avoiding command ordering bottlenecks, avoiding the
creation of multiple layers performing very similar functionality.

Thanks,
Santosh

---

From: "Eddy Quicksall" <[email-masked]>
Date: Mon, 6 Nov 2006 00:18:48 -0500

I think what John and Julian were pointing out is that if the ordered or 
head of queue attributes are used along with simple attributes then using 
multiple sessions may not work properly. For the cases where MPIO is load 
balancing, the initiator will have to be sure it manually synchronizes the 
ordered and head of queue attributes with simple attributes. Also if the 
Queue Algorithm Modifier in the Control Mode Page is set to 0 then MPIO, 
doing writes to the same block over alternate sessions, will need to 
manually synchronize the writes.

I suspect that all MPIO writers are aware of these issues.

Eddy

----- Original Message ----- 
From: "Santosh Rao" <[email-masked]>
To: "John Hufferd" <[email-masked]>
Cc: <[email-masked]>; "Qi, Yanling" <[email-masked]>; "Santosh Rao" 
<[email-masked]>
Sent: Friday, November 03, 2006 12:53 PM
Subject: RE: [Ips] MC/S vs multiple sessions