Which pieces do or do not persist in an Azure Cloud Service Web Role?

Question

My understanding of the VMs involved in Azure Cloud Services is that at least some parts of it are not meant to persist throughout the lifetime of the service (unlike regular VMs that you can create through Azure).

This is why you must use Startup Tasks in your ServiceDefinition.csdef file in order to configure certain things.

However, after playing around with it for a while, I can't figure out what does and does not persist.

For instance, I installed an ISAPI filter into IIS by logging into remote desktop. That seems to have persisted across deployments and even a reimaging.

Is there a list somewhere of what does and does not persist and when that persistence will end (what triggers the clearing of it)?

Answer 1

See http://blogs.msdn.com/b/kwill/archive/2012/10/05/windows-azure-disk-partition-preservation.aspx for information about what is preserved on an Azure PaaS VM in different scenarios.

In short, the only things that will truly persist are things packaged in your cscfg/cspkg (ie. startup tasks). Anything else done at runtime or via RDP will eventually be removed.

Answer 2

See - How to: Update a cloud service role or deployment - in most cases, an UPDATE to an existing deployment will preserve local data while updating the application code for your cloud service.

Be aware that if you change the size of a role (that is, the size of a virtual machine that hosts a role instance) or the number of roles, each role instance (virtual machine) must be re-imaged, and any local data will be lost.

Also if you use the standard deployment practice of creating a new deployment in the staging slot and then swapping the VIP, you will also lose all local data (these are new VMs).