Friday, February 10, 2012

VMware clusters and multiple SANs


Most large organisations have two SANS and many larger organisation still have two or more data centres.

Lets just start with this is not a cloud, it is a data centre or separate SAN, OK that is off my chest. Bloody private cloud, this and that, don't get me started, “Why I oughta…”

You can utilise this architecture with VMware ESX/vSphere as active-active or active-passive, but I don't see value in active-passive so lets not speak of it again, it is not cheap, not effective and not quick. So back to Active-Active.

To build a single ESX cluster over 2 SANs (or 2 data centres) is basically the same, but as you reach out of the single data centre you need some extra configurations such as:

  • An IP network with a minimum bandwidth of 622 Mbps is required.
  • The maximum latency between the two VMware servers cannot exceed 5 milliseconds (ms).
  • The source and destination ESX servers must have a private VMotion network on the same IP subnet and broadcast domain.
  • The IP subnet on which the virtual machine resides must be accessible from both the source and destination ESX servers.
  • The data storage location including the boot device used by the virtual machine must be active and accessible by both the source and destination VMware ESX servers at all times.
  • Access from vCenter to all ESX servers is needed.
  • The two SANs to be supported it must have synchronous data replication (sometimes and expensive add on).

The value of this is protection from a SAN failure and all hardware used. If this is across data centres then you also have DR knocked on the head (your mileage may vary).

There are some issues such as a power outage in DC1 will cause all the VMs to restart in DC2 via HA so it is not true protection from massive outages. Secondly if you use vMotion to move the servers to the other nodes in DC2 the SAN writes are now subject to the latency of this link, so there can be a performance penalty. Additionally DRS may need to be manually updated to be aware of this configuration.

This is how it would look (minus some SAN switches etc.)

ESX clusters

Here are the VMware prerequisites:

Here is Cisco discussing this:

Here is a good blog on this:

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