Redpaper
Alex Osuna
John Foley Jr
IBM System Storage N series
MPIO Support—Frequently Asked
Questions
This IBM® Redpaper addresses commonly asked questions about planning and
implementing Multi-Path I/O support with the IBM System Storage™ N series. It is not meant
as a replacement for product documentation but rather an aid.
© Copyright IBM Corp. 2006. All rights reserved.
ibm.com/redbooks
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MPIO (Multi-Path I/O) information
Multi-Path I/O has been around for some time in SnapDrive® and is fairly popular in the Fibre
Channel arena. With the release of SnapDrive 3.2 and Microsoft®’s iSCSI Initiator 2.0, MPIO
is now available for iSCSI as well as for FCP implementations.
MPIO is available for both the software initiator and HBAs such as the QLogic 4010/4010c.
The basic premise behind MPIO is to provide a redundant path to a given LUN. If it is
implemented correctly, it can mitigate the issues that are associated with a Single Point of
Failure. Typically, these types of failures will be switch or HBA failures. A cable failure is also
possible but those are pretty rare.
Because the majority of IBM N series customers are iSCSI, this answer will center on those
discussions. A Fibre Channel implementation is very similar. The same concepts apply and
for the most part, only the hardware changes. There are some FCP specific entries within this
paper.
In Figure 1, the network and associated hardware is properly configured for MPIO but that
alone is not enough. The Initiators and SnapDrive must be configured as well. If we just tell
SnapDrive to create a session and a LUN on the filer, there will be only one logical path to the
LUNs despite the network configuration.
.
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© 2005 IBM Corpo
Figure 1 MPIO
Does IBM N series support the Microsoft MPIO driver for iSCSI connections?
Microsoft MPIO driver is supported via iSCSI. The N series Device Specific Module (DSM)
component of the iSCSI MPIO Device Driver is packaged in SnapDrive for Windows®.
Order SnapDrive for Windows (8050) and its requisite SnapDrive for Windows Media® Kit
(8051). You do not need any of the FCP features.
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IBM System Storage N series MPIO Support Frequently Asked Questions
So how do I enable MPIO for iSCSI or FCP?
MPIO is enabled when SnapDrive is installed. If an MPIO license is entered during the
installation of SnapDrive, it will load the MPIO drivers.
Note: The MPIO drivers will load even if the underlying SAN/iSAN infrastructure is not
configured to support it.
There is a chargeable feature code (8052) ONTAP DSM for Windows MPIO that has to be
added when MPIO is needed for FCP access.
I'm not doing MPIO now but might in the future. Should I go ahead and install the MPIO
drivers?
No, do not configure SnapDrive for MPIO until the underlying infrastructure is ready. With the
drivers loaded, SnapDrive will look for alternate paths to the LUNs.
Can I configure iSCSI MPIO with SnapDrive 3.1, 3.1.1 or 3.1.1R1?
No, iSCSI MPIO is not supported and will not work with any version of SnapDrive prior to 3.2.
What else do I need?
򐂰 Microsoft iSCSI 2.0 or later. iSCSI MPIO was not supported in earlier releases.
򐂰 iSCSI configuration: This configuration uses iSCSI HBAs or the Microsoft iSCSI Software
Initiator to support MPIO between a host and a storage system.
򐂰 The storage system has two GbE network adapters, and the host has one of the following
configurations:
– Two or more iSCSI HBAs
– The Microsoft iSCSI Software Initiator and two GbE NICs
IBM System Storage N series MPIO Support—Frequently Asked Questions
3
During the installation of Microsoft iSCSI 2.0, I see a check box for MPIO. Should I
select that?
No, absolutely not. It will cause problems with I/O. SnapDrive will provide the MPIO layer so
this should not be selected during the installation of MSiSCSI 2.0.
Figure 2 Microsoft iSCSI
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IBM System Storage N series MPIO Support Frequently Asked Questions
How can I tell whether the system has the Microsoft iSCSI MPIO DSM loaded or the
N series MPIO DSM loaded?
Look at the following registry key (Figure 3; if MPIO + Microsoft Generic dsm is installed):
HKLM\System\CurrentControlSet\Services\msiscdsm\
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© 2005 IBM Corporation
Figure 3 Microsoft DSM
Figure 4 shows MPIO + ntapdsm (Snap Drive):
HKLM\Service\CurrentControlSet\Services\ntapdsm\
Figure 4 ntapdsm key
IBM System Storage N series MPIO Support—Frequently Asked Questions
5
I have installed all of the software according to the instructions and created my LUNs.
Am I done?
No, not yet. Just installing the software and creating LUNs will work, but each LUN will only be
served by a single path. In other words, you are not configured for MPIO yet.
OK, what do I do now to get iSCSI MPIO configured?
See Figure 5 and compare it to Figure 1 on page 2. Note that there is a session established
for 10.61.66.122 to 10.61.65.65 but there is no session for the other connection. This must be
configured, as shown in Figure 6 on page 7 through Figure 10 on page 9.
Figure 5 SnapDrive
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IBM System Storage N series MPIO Support Frequently Asked Questions
Figure 6 iSCSI management
IBM System Storage N series MPIO Support—Frequently Asked Questions
7
The IP address provided here can be any reachable interface on the filer. SnapDrive will
query the filer and enumerate all available interfaces in the next window.
Figure 7 Providing IBM N series identification
The new session is being established between 10.61.66.121 and 10.61.64.25 (Figure 8).
Figure 8 Session establishment
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IBM System Storage N series MPIO Support Frequently Asked Questions
Click Next and Finish, and there are now two separate iSCSI sessions between the host and
filer.
Figure 9 iSCSI sessions
If we examine the Path Management container under the LUN, we see one active and one
passive path.
Figure 10 Paths
IBM System Storage N series MPIO Support—Frequently Asked Questions
9
I have a pair of HBAs installed with MPIO configured. Should my throughput increase
as a result or is one HBA there just for failover?
At this time, all of our MPIO strategies are Active Passive. That means that the customer
might see some improvements in throughput, depending on the configuration.
If using two HBAs in the host that are connected to two or more LUNs, SnapDrive will try to
balance the paths such that there is an even number of LUNs over each path. Whether that
increases throughput depends on how busy those LUNs are.
Here is an example. I have four LUNs on my filer volumes: Exchdb1.lun, Exchdb2.lun,
Exchlog1.lun, and Exchlog2.lun.
Say that those LUNs are connected in this order:
J: → Exchdb1.lun
K: → Exchdb2.lun
L: → Exchlog1.lun
M: → Exchlog2.lun
Then SnapDrive will configure the two paths as follows:
J: → Exchdb1.lun Active/Passive
K: → Exchdb2.lun Passive/Active
L: → Exchlog1.lun Active/Passive
M: → Exchlog2.lun Passive/Active
Notice how SnapDrive alternates the Active/Passive path for each LUN. At first glance, it
seems that this would provide us with an ideal throughput. But what if J: and L: are
responsible for 90% of the traffic? If that is the case, one Initiator would be pushing much
more traffic than the other. We would be better off to flip L: to Passive/Active and balance the
load manually across initiators.
An upcoming release of SnapDrive will be smarter about this and do weighted paths and
dynamic load balancing. Until then, it is static.
Multi-Path I/O has been around for some time in SnapDrive and is fairly popular in the Fibre
Channel arena. With the release of SnapDrive 3.2 and Microsoft’s iSCSI Initiator 2.0, MPIO is
now available for iSCSI implementations as well. MPIO is available for both the software
initiator and HBAs such as the QLogic 4010/4010c.
The basic premise behind MPIO is to provide a redundant path to a given LUN. If
implemented correctly, it can mitigate the issues that are associated with a Single Point of
Failure. Typically, these are switch or HBA failures. A cable failure is possible but those are
pretty rare.
I have added another FCP Initiator to my host but SnapDrive does not indicate paths
from it to the target filer. Do I have to disconnect and reconnect each LUN to get
SnapDrive to “see” the new paths?
Disconnecting and reconnecting each LUN will work, but that is the hard way to resolve this.
See Figure 11 on page 11. Note that the only path appears through the initiator 37:ba:9c.
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IBM System Storage N series MPIO Support Frequently Asked Questions
Figure 11 Path display
The solution is to right-click Path Management and select Add/Remove Initiator.
Figure 12 Add/Remove Initiator
Note that the system that is used in this example is a Microsoft Cluster, so I need to add the
new Initiator from each node. Simply open a cluster node at the top and then select the
IBM System Storage N series MPIO Support—Frequently Asked Questions
11
unused initiator. Click the arrow to move it to the Connected Initiators box. Repeat the
process for all other nodes in the cluster. Click OK and SnapDrive automatically adds the new
paths to the LUN.
Figure 13 Initiator Management
Figure 14 Path configuration
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IBM System Storage N series MPIO Support Frequently Asked Questions
When the process is complete, the path listing will resemble Figure 15. Repeat this process
for all LUNs.
Figure 15 Path LIsting
Does MPIO work with iSCSI and FCP boot LUNs on Windows? In other words, if I boot
off of a LUN on a filer, can I then perform MPIO on that LUN?
MPIO will work on a boot LUN, but only when the Windows OS is up and running. Currently a
server with two QLogic HBAs can have one configured to boot from a LUN on a filer. Upon
connection, the administrator can use SnapDrive to establish a session to the filer through the
second QLogic HBA. Then the administrator can look at the Path Management section for the
boot LUN to see the second Passive connection established for the LUN.
Note: MPIO at the BIOS level is not possible; the administrator must select one HBA to
boot from. After the Windows OS is fully operational and SnapDrive (and MPIO) are
running, then MPIO can be established properly.
IBM System Storage N series MPIO Support—Frequently Asked Questions
13
Other recommendations for MPIO
Build an underlying network topology that will support MPIO. This might include multiple
interfaces on the host, filer, or both. It also might include more than one switch. See Figure 16
as a example.
We still strongly recommend separate and dedicated networks for iSCSI traffic. The best
solution is a simple crossover cable from the host to the filer. If a switch must be used, we
recommend a non-routed VLAN.
.
Figure 16 Single host direct-attached to an IBM N series using either FCP or iSCSI over MPIO
The team that wrote this Redpaper
This Redpaper was produced by a team of specialists from around the world working at the
International Technical Support Organization, Poughkeepsie Center.
Alex Osuna is a Project Leader at the International Technical Support Organization, Tucson
Arizona Center. He writes extensively and teaches IBM classes about all areas of storage. He
has more than 28 years in the I/T industry working for United States Air Force, IBM, and
Tivoli® in computer maintenance, field engineering, service planning, Washington Systems
Center, business and product planning, advanced technical support, and systems
engineering focusing on storage hardware and software. He has more than 10 certifications
from IBM, Microsoft, and Red Hat.
John Foley is the worldwide product marketing manager for IP-attached storage solutions
from IBM. John has 18 years of experience in the fields of networking and storage and has
been involved in driving IBM IP storage solutions for the past six years.
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IBM System Storage N series MPIO Support Frequently Asked Questions
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