Rodney Buike - Founder and original lazy admin.

Daniel Nerenberg - Lazy admin 2.0

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MPIO, MCS, IPv6 and iSCSI

Sorry for the alphabet soup title but I thought some explanation was due in regards to iSCSI redundancy options and issues you may have with IPv6.

Multi-path I/O (MPIO) and Multiple Connections per Session (MCS) are two options you have to provide load balancing and redundancy to your iSCSI connections.  MPIO and MCS are the same but different.  MPIO leverages Device Specific Modules (DSM) to manage the requests over multiple paths.

MCS is part of the iSCSI protocol and allows for teaming of iSCSI connections.  In order for this to work your SAN vendor must support it and your virtualization platform must support it as well.  VMWare supports MPIO but not MCS and Hyper-V supports both.

Whichever you choose is not relevant but there is something you should be aware of in regards to MCS.  MCS does not support using both IPv4 and IPv6 with the Microsoft iSCSI Initiator.  If you have IPv6 enabled on the iSCSI NIC(s) you will find it takes a very long time for the initiator to connect to the target. 

This screenshot was taken after the issue was fixed but if you are seeing Reconnecting…. and it is taking a long time (more than a few seconds) to change to Connected you most likely have IPv6 enabled on the iSCSI NIC(s).

To resolve simply open up the NIC properties for each iSCSI NIC on all servers connected to the SAN and uncheck the IPv6 option.

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