Subject: [Ips] MC/S vs multiple sessions

From: "Qi, Yanling" <[email-masked]>
Date: Wed, 1 Nov 2006 06:42:41 -0700

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]

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From: "Qi, Yanling" <[email-masked]>
Date: Wed, 1 Nov 2006 08:47:53 -0700

Thanks John. See my question about load balance below.

________________________________

From: John Hufferd [mailto:[email-masked]] 
Sent: Wednesday, November 01, 2006 8:25 AM
To: Qi, Yanling; [email-masked]
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.

[Qi, Yanling] I don't think I understand the I_T_L nexus issue. I assume
the issue is independent to transport protocols. The device server on
the target side should serialize all tasks from different target ports
(or sessions). I don't understand why I/O can not go to the same LUN at
the same time cross different I-T-L nexuses. Are you concerning IO
out-of-order? For a cluster file system configuration, IOs from
different nodes may issue IOs at the same time (read). Thank you.

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

Thanks,

Yanling

Yanling Qi

512-794-3713 (Office)

512-794-3702 (Fax)

[email-masked]