Non-Persistent Disks with VMs?

There are some features of VMware deployments using VMs that don’t get the press they deserve, and hence aren’t used as much as they ought to be. Non-persistent disks are (in my opinion) such a feature.

When creating a new VM or editing an existing VM, under each disk’s settings are Advanced options for that disk – Independent: persistent or non-persistent. The functions of these options are described in the ‘Advanced’ dialog box for the options:

– Independent (Independent disks are not affected by snapshots)

– Independent – Persistent (Changes are immediate and permanently written to the disk).

– Independent – Nonpersistent (Changes to the disk are discarded when you power off or restore a snapshot).

So, why do I care about these disk options? Think of it as ‘starting a VM in exactly the same state everytime – irrespective of what was done during the last session….’

Use Case: Test / Development VMs.

As a VMware Administrator, when you are deploying VMs for testing and/or development purposes, despite the option of snapshots, clones and templates – there are some instances where a streamlined base OS is required to test particular functionality without the intermediate steps of snapshots or VM clones. Administrators can build a VM with a minimal or streamlined guest build, then freeze the configuration of the VM by changing the disk type to Independent – Nonpersistent. With this freeze in place, a VM acts as a point-in-time deployment against which installs, application tests and other development work can be done within the guest with a guaranteed OS baseline. Whilst this solution isn’t as clean as VM snapshotting, as a quick testbed for deployments and OS patches, it’s perfect.

There are also applications for non-persistent disks in VMware View deployments, where instances are deployed against linked-clones.

Limits / Restrictions?

There are some limits of using nonpersistent disks, namely they are not supported with Storage vMotion in vSphere 5. For a VM to be migrated to a different datastore, the disks must be in either persistent mode or configured as an RDM.

Jeremy loves all things technology! Has been in IT for years, loves Macs (but doesn't preach to others about their virtues), loves virtualization (and does shout about it's virtues), and sometimes skis, bikes and directs amateur plays!

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