Use Cases for VMware ESX 3.x DisallowSnapshotLUN and ESX 4.x Force-Mount

Posted on 21 Jan 2011 by Ray Heffer

If you are involved in DR for your organisations IT infrastructure and are replicating virtual machine VMFS datastores then you may be familiar with DisallowSnapshotLUN in ESX 3.x. Let’s start with a background on what these advanced settings are and why they are there.

Since virtualization changed the landscape for disaster recovery some time ago now, most businesses have embraced SAN storage replication for DR. This is old news now, but unless your SAN vendor integrates with something like VMware Site Recovery Manager then you will have a number of manual tasks involved in your DR recovery process.

When you replicate your datastores (or LUN’s) from site A to site B, you will be using an asynchronous mirror between these two sites. In other words, data blocks will be scheduled (or queued) for replication to site B. If it was synchronous then as data is written to disk, the write operation wouldn’t be acknowledged until it is written to both disks or arrays. Obviously synchronous replication would be far too slow across a WAN to a DR site.

In order to use the replicated LUN in your DR site, you will need to take a snapshot of the replicated LUN then present the snapshot LUN to your VMware ESX hosts in the DR site. Different SAN vendors will have slightly different methods and management capabilities, but the underlying requirement doesn’t change. Without modifying the advanced settings of your ESX 3.x host you will not be able to present your snapshot LUN (VMFS datastore) to the ESX host.

ESX 3.x: What you’ll need to do is set the following parameters:

EnableResignature=0 DisallowSnapshotLUN=0

By switching off DisallowSnapshotLUN, the ESX host will allow your replicated LUN to me added and it won’t re-signature the volume. By default DisallowSnapshotLUN is set to 1. You can find this setting in the Configuration tab of the ESX host > Advanced Settings.

ESX 4.x:

VMware already have a KB article on this here:

It says that you can just use the Add Storage wizard and it will display the VMFS label. If it’s not mounted then you can assume it is a snapshot (or volume copy) and can go ahead and mount it. Using the GUI will force-mount a VMFS volume and also make it persistent. In fact the advanced settings (EnableResignature and DisallowSnapshotLUN) are no longer present with ESX 4.x.

Useful KB Articles: