Friday, June 29, 2012


Here's a few concepts I've been studying.  A TPG (Target Portal Group) is basically a method of allowing the server to communicate to your storage system via iSCSI on multiple interfaces and multiple connections on a single session.  This means you can enable MPIO by having multiple TPGs.  Here's the dummy breakdown:
- Each interface (virtual or real) can only be part of one TPG
- Each TPG can have multiple interfaces
- Each session can have multiple connections
- Each connection can only be in one session.
- Each session can only communicate through one TPG

A great use of this is when a server has multiple virtual OS's and therefore needs multiple connections to the same storage system.  If you have multiple paths, you need ALUA.

When understanding ALUA (Asymmetric Logical Unit Access), it's helpful to know it's also called Target Port Group Support.  Basically it's a protocol for determining the best path from the server to the LUN (hence the LU in ALUA).  This protocol is standardized to work with any vendor's iSCSI hardware.

ISNS is basically DNS for iSCSI, but a little smarter in that it also understands TPG's and helps systems find each other that way.

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