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MC/S
MC/S (Multiple Connections per Session) is a feature of iSCSI protocol, which allows combining several
connections inside a single session for performance and failover purposes. In this way, I/O can be sent on
any TCP/IP connection to the target. If one connection fails, another connection can continue processing I/O
without interruption to the application.
Difference
MC/S is implemented on iSCSI level, while MPIO is implemented on the higher level. Hence, all MPIO
infrastructures are shared among all SCSI transports, including Fiber Channel, SAS, etc. MPIO is the most
common usage across all OS vendors. The primary difference between these two is which level the
redundancy is maintained. MPIO creates multiple iSCSI sessions with the target storage. Load balance and
failover occurs between the multiple sessions. MC/S creates multiple connections within a single iSCSI
session to manage load balance and failover. Notice that iSCSI connections and sessions are different than
TCP/IP connections and sessions. The above figures describe the difference between MPIO and MC/S.
There are some considerations when user chooses MC/S or MPIO for multi-path.
1.
If user uses hardware iSCSI off-load HBA, then MPIO is the only one choice.
2.
If user needs to specify different load balance policies for different LUNs, then MPIO should be used.
3.
If user installs anyone of Windows XP, Windows Vista or Windows 7, MC/S is the only option since
Microsoft MPIO is supported Windows Server editions only.
