Much of the beauty of dm-multipath in Red Hat is the chance to combine the redundancy and stability of traditional IP networking with the performance of fibre channel connectivity, culminating in a network connection that can withstand the failure of so many points it almost makes the head spin. This can be accomplished by the use of iSCSI and a TOE network interface, a dual port (minimum) FC-HBA, a properly configured dm-multipath, and a storage system that can export block storage across both mediums (IP network and FC..which is most systems, unless you are living in the year 2000).
Here is the basic premise:
1) Fibre Channel connections are your primary block transport. Set the initiator group on the storage unit as normal, export the blocks, and connect LUN's as normal.
2) Set up an iSCSI initiator to the storage unit and export the same LUN out the target of your storage system. Initiate the connection on both network ports out of the iSCSI TOE card.
3) Set up DM-Multipath. Out of the box, DM-Multipath will create a pseudo-device based on the SCSI_ID of each attached LUN and create a round-robin connection to each LUN (in count, you should have 6 total devices - 2 FC primary paths, 2 FC secondary paths, and 2 iSCSI IP network paths).
4) Set the 4 FC paths (primary and secondary) up accordingly with policies that let's the primary path be used during service time, the secondary path be a potential failover path in the event that either of the 2 primary paths are down, and a final policy that sets the IP network connections to the iSCSI target as dual tertiary paths that will be the final path of last resort.
Hang in there with me! I know that this sounds complicated, but over the next few posts, I'll be explaining each step in detail in a way that is easy for even a beginner to understand. I know that much of this sounds like a drunk man trying to explain why he is wearing a pink tutu in a park at 11am, but trust me when I say it is not that difficult.
At the end of this series, you can build a storage network that can withstand a charging rhino running from Godzilla (ok, maybe not a rhino, but certainly a koala bear wreaking havoc munching on FC cables or networks...if you have koala bears in your data center..after all..who doesn't?).
Until then, happy file and block exporting!Share on Facebook