Aidan Finn, IT Pro

I’ve been doing some thinking about how to configure Dynamic Memory (DM) in a Windows Server 2012 Hyper-V cloud.  One of the traits of a cloud is self-service.  And that’s why some thought is required.

If you’re not familiar with Dynamic Memory, then have a look at the paper I wrote on the feature as it was in W2008 R2.  Then take a few minutes to update your knowledge of the changes to DM in WS2012. 

I’ve previously stated that you had to be careful with the Startup RAM setting in a cloud/hosting scenario.  The guest OS can only see the high-water mark of allocated RAM since the VM booted up.  For example:

• A VM is configured with 512 MB Startup RAM and 8 GB Maximum RAM
• The VM boots up and the guest OS sees 512 MB RAM
• The DMVSC integration component starts up
• Pressure for RAM increases in the guest OS and maybe the VM increases to 768 MB
• Pressure reduces and the VM is now using 612 MB

Imagine a customer who owns this VM, logs into it, downloads the SQL installer and runs setup.  Setup will fail because the installer requires 1 GB of RAM for SQL to run.  The guest OS can only see 768 MB – the high water mark since the VM booted up.  That’s a helpdesk call.  Now scale that out to hundreds or thousands of VMs.  Imagine all the helpdesk calls.  Sure; you’ve saved some money on RAM, but you’ve had to hire more people to close calls.  Trust me … no wiki or knowledge base article will sort it out.  I’ve been on the hosting service provider side of the fence and I’m a hosting customer too

So my advice for W2008 R2 was to set Startup RAM to be 1 GB of RAM.  Sure, lots of VMs remain idle in a cloud – you’d be amazed how many might never be logged into, even if there is a monthly invoice for them.  You’ve reduced the helldesk calls but you’re still using up RAM.

Enter Windows Server 2012 Hyper-V (and Hyper-V Server 2012) …

We have a new setting called Minimum RAM, allow a VM to balloon down to below the Startup RAM when the VM is idle.  All that idle RAM returns back to the host for reuse.  How about this now:

• The VM is configured with 1 GB RAM Startup RAM and 8 GM Maximum RAM
• Minimum RAM is set to 256 MB
• The VM powers up with 1 GB RAM, goes idle and balloons down to 320 MB RAM
• After a week, the customer logs into the VM, attempts to install SQL Server.  The RAM high-water mark is 1 GB and the SQL setup has no problems.

No helldesk calls there!  And it’s done without the loss of performance associate with RAM over-commitment and second level paging.

Copyright Warning

This blog post is the property of Aidan Finn (@joe_elway / http://www.aidanfinn.com) and may not be reused in any manner without prior consent of Aidan Finn. You may quote one paragraph from this blog post if you link to the original blog post.

Related posts:

1. Is Dynamic Memory Coming To Linux Virtual Machines on Hyper-V?
2. WS2012 – Enabling Services To Start When A VM Can’t Get Enough Dynamic Memory
3. Altaro Blog Post – Hyper-V Guest Design: Fixed vs. Dynamic VHD
4. Build A WS2012 Hyper-V Cloud Using PowerShell
5. WS2012 Hyper-V – Host Memory Sizing