The answer to a question that rarely
seems to come up 'Should I Use Windows Server 2008 MCS (Multiple
Connected Session) or MPIO for the iSCSI Storage Connection?', depends a
lot on if there is a supported DSM (Device Specific Module) for the iSCSI storage vendor
in the particular scenario (the Microsoft DSM might come with manufacturer support.) The following post aims to illustrate
the differences between MCS and MPIO with configuration examples
using either 4-way MCS, or 4-way MPIO, from a Windows 2008 Server
with 4 x 1 Gbps dedicated iSCSI interfaces.
MCS is a Many to One Mapping
In the following example, the MCS has
been configured (via iSCSI Initiator Properties > Targets tab >
Properties > Sessions tab > MCS) with 4 different source
portals and only one target portal (it is not possible to select more
than one target portal.) This could be used where there is no
supported DSM, and the Windows 2008 source uses its 4 x 1 Gbps iSCSI
NICs as source portal interfaces, and the Storage target portal
is either a virtual interface of port aggregated 1 Gbps NICs (e.g. a
Multi-mode VIF on a NetApp FAS2040 utilizing 4 x 1 Gbps ethernet
interfaces), or a 10 Gbps interface.
Fig. 1: Multiple Connected Session
from 4 source portals to 1 target portal.
Fig. 2: Windows 2008 Server with 4
iSCSI interfaces, connecting via MCS to 1 interface on the Storage.
Note: Needs to be verified with the
storage provider regards whether MCS is supported.
MPIO can be a Many to One or Many
to Many Mapping
With a supported DSM. From the example
lab Windows 2008 Server, the configuration could either use one
target portal, and create multiple paths to that target from the
dedicated iSCSI NICs (again, the target portal would either be a
virtual interface of port aggregated 1 Gbps NICs or a 10 Gbps
interface.)
Fig. 3: iSCSI Initiator configured
with 1 target portal.
Or could use 4 target portals and
establish separate paths from each source portal (e.g. a NetApp
FAS2040 with 4 x 1 Gbps ethernet interfaces, each interface configured with a
separate IP address.)
Fig. 4: iSCSI Initiator configured
with 4 target portals.
Fig. 5: Windows 2008 Server with 4
interfaces, connecting to 4 interfaces on the Storage.
Note: The Multipath I/O feature must
first be installed –
Fig. 6: Windows 2008 Server features
– Multipath I/O.
– and then the DSM can be installed,
and MPIO Multi-Paths can be discovered via MPIOCPL.exe from
Administrative Tools.
Fig. 7: MPIOCPL.exe icon in
Administrative Tools.
Fig. 8: MPIO Properties > DSM
Install.
Conclusion
Admittedly, I have not had much reason
to use MCS in practice (would greatly appreciate to hear comments
from people that use it,) and with MPIO being more widely supported,
this seems to be the way to go if can obtain a supported DSM. Knowing
about the option is handy – another weapon to keep in the arsenal
of solutions. There is an excellent Microsoft and NetApp White Paper
available on the internet entitled “iSCSI 10 Gigabit Ethernet
Performance Tuning With Windows Server 2008, Hyper-v and the NetApp
FAS 3070” which identifies throughput advantages of MCS over MPIO,
at least up to four connections.
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