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Multi-site SAN with single subnet and vSphere

Chris Wigglesworth
Occasional Advisor

Multi-site SAN with single subnet and vSphere

We are planning a multi-site SAN with about 30 storage nodes arranged in 3 clusters to support a number of applications including, vSphere, MS SQL and Oracle clusters. We have two machine rooms about 400m apart connected by 10 GbE running over single-mode fibre. The latency between the rooms measured by a ping is about 130 microseconds. We intend for the two machines rooms to each be considered a site, with a third machine room hosting a FOM to provide quorum.

We would like to use a single subnet so we can take advantage of vSphere multi-pathing. Reading through these forums it has been suggested that using a single subnet with a multi-site SAN configuration causes problems with the VIP not migrating between the sites quickly enough and causing issues for systems that are particularly sensitive to connectivity loss, such as databases and Exchange.

Would this still be the case with a low latency link between sites such as ours?
4 REPLIES
Joshua Small_2
Valued Contributor

Re: Multi-site SAN with single subnet and vSphere

Consider this: a major networking issue on one side will take out the other side.

If you're going to this much effort, do it properly and have two subnets.
Chris Wigglesworth
Occasional Advisor

Re: Multi-site SAN with single subnet and vSphere

We have certainly considered the risks of using a single subnet, however there are also advantages in avoiding routing and staying with a layer 2 storage network.
The question is whether the P4000 architecture gives us a choice and will migrate the VIP quickly enough in a site failure scenario to avoid issues with databases etc., or whether it demands the we use separate subnets in a multi-site SAN scenario to avoid connectivity loss issues.
Dirk Trilsbeek
Valued Contributor

Re: Multi-site SAN with single subnet and vSphere

as far as i know vSphere can't route iSCSI traffic, so in a multisite setup with two subnets the vSphere hosts would probably not failover to the SAN VIP in the other subnet if the local nodes (or the connection to these nodes) fail.
Chris Wigglesworth
Occasional Advisor

Re: Multi-site SAN with single subnet and vSphere

I think vSphere can route iSCSI traffic, but not easily when using iSCSI Multipathing. I think it can be done as explained in this blog entry http://virtualgeek.typepad.com/virtual_geek/2009/09/a-multivendor-post-on-using-iscsi-with-vmware-vsphere.html, but it is not recommended.