Resource allocation for Virtual Machines

Ever since I started transitioning our production systems to my ESX cluster, I’ve been fascinated how visible resource utilization has become.  Or to put it another way, how blind I was before.  I’ve also been interested to hear about the natural tendency of many Administrator’s to over allocate resources to their VM’s.  Why does it happen?  Who’s at fault?  From my humble perspective, it’s a party everyone has shown up to.

• Developers & Technical writers
• IT Administrators
• Software Manufacturers
• Politics

Developers & Technical Writers
Best practices and installation guides are usually written by Technical Writers for that Software Manufacturer.  They are provided information by whom else?  The Developers.  Someone on the Development team will take a look-see at their Task Manager or htop in Linux, maybe PerfMon if they have extra time.  They determine (with a less than thorough vetting process) what the requirement should be, and then pass it off to the Technical Writer.  Does the app really need two CPU’s or does that just indicate it’s capable of multithreading?  Or both?  …Or none of the above?  Developers are the group that seems to be most challenged at understanding the new paradigm of virtualization, yet are the one’s to decide what the requirements are.  Some barely know what it is, or dismiss it as nothing more than a cute toy that won’t work for their needs.  It’s pretty fun to show them otherwise, but frustrating to see their continued suspicions of the technology.

IT Administrators (yep, me included)
Take a look at any installation guide for your favorite (or least favorite) application or OS.  Resource minimums are still written for hardware based provisioning.   Most best practice guides outline memory and CPU requirements within the first few pages.  Going against recommendations on page 2 generally isn’t good IT karma.  It feels as counterintuitive as trying to breathe with your head under water.  Only through experience have I grown more comfortable with the practice.  It’s still tough though.

Software Manufacturers
Virtualization can be a sensitive matter to Software Manufactures.  Some would prefer that it doesn’t exist, and choose to document it and license it in that way.  Others will insist that resources are resources, and why would they ever recommend their server application can run with just 768MB of RAM and a single CPU core if there was even a remote possibility of it hurting performance.

Politics
Let’s face it.  How much is Microsoft going to dive into memory recommendations for an Exchange Server when their own virtualization solution does not support some of the advanced memory handling features that VMWare supports?  The answer is, they aren’t.  It’s too bad, because their products run so well in a virtual environment.  Politics can also come from within.  IT departments get coerced by management, project teams or departments, or are just worried about SLA’s of critical services.  They acquiesce to try to keep everyone happy.

What can be done about it.
Rehab for everyone involved.  Too ambitious?  Okay, let’s just try to improve Installation/Best Practices guides from the Software Manufactures.

• Start with two or three sets of minimums for requirements.  Provisioning the application or OS on a physical machine, followed by provisioning on a VM accommodating a few different hypervisors.
• Clearly state if the application is even capable of multithreading.  That would eliminate some confusion on whether you even want to consider two or more vCPU’s  on a VM.  I suspect many red faces would show up when software manufactures admit to their customers they haven’t designed their software to work with more than one core anyway.  But this simple step would help Administrators greatly.
• For VM based installations, note the low threshold amount for RAM in which unnecessary amounts of disk paging will begin to occur.   While the desire is to allocate as little resources as needed, nobody wants disk thrashing to occur.
• For physical servers, one may have a single server playing a dozen different roles.  Minimums sometimes assume this, and they will throw in a buffer to pad the minimums – just in case.  With a VM, it might be providing just a single role.  Acknowledge that this new approach exists, and adjust your requirements accordingly.

Wishful thinking perhaps, but it would be a start.  Imagine the uproar (and competition) that would occur if a software manufacturer actually spec’d a lower memory or CPU requirement when running under one hypervisor versus another?  …Now I’m really dreaming.

IT Administrators have some say in this too.  Simply put, the IT department is a service provider.  Your staff and the business model are your customers.  As a Virtualization Administrator, you have the ability to assert your expertise on provisioning systems to provide a service, and it work as efficiently as possible.  Let them define what the acceptance criteria for the need they have, and then you deal with how to make it happen.

Real World Numbers
There are many legitimate variables that make it difficult to give one size fits all recommendations on resource requirements.  This makes it difficult for those first starting out.  Rather than making suggestions, I decided I would just summarize some of my systems I have virtualized, and how the utilization rates are for a staff of about 50 people, 20 of them being Software Developers.  These are numbers pulled during business hours.  I do not want to imply that these are the best or most efficient settings.  In fact, many of them were  “first guess” settings that I plan on adjusting later.  They might offer you a point of reference for comparison, or help in your upcoming deployment.

To complicate matters a bit, you might observe different behaviors on some OS’s (XP versus Vista/2008 versus Windows 7/2008R2, or SQL 2005 versus SQL 2008) in their willingness to pre populate RAM.  Give SQL 2008 4GB of RAM, and it will want to use it even if it isn’t doing much.   You might notice this when looking at relatively idle VM’s with different OS’s, where some have a smaller memory footprint than others.   At the time of this writing, none of my systems were running Windows 2008 R2, as it wasn’t supported on ESX 3.5 as I was deploying them.

Some of these numbers are a testament to ESX’s/vSphere’s superior memory management handling and CPU scheduling.  Memory ballooning, swapping, Transparent Page Sharing all contribute to pretty startling efficiency.

I have yet to virtualize my CRM, web, mail relay, and miscellaneous servers, so I do not have any good data yet for these types of systems.  Having just upgraded to vSphere in the last few days, this also clears the way for me to assign multiple vCPU’s to the code compiling machines (as many as 20 VM’s).  The compilers have switches that can toggle exactly how many cores end up being used, and our Development Team needs these builds compiled as fast as possible.  That will be a topic for another post.

Lessons Learned
I love having systems isolated to performing their intended function now.  Who wants Peachtree crashing their Email server anyway?  Those administrators working in the trenches know that a server that is serving up a single role is easy to manage, way more stable, and doesn’t cross contaminate other services.  In a virtual environment, it’s  worth any additional costs in OS licensing or overhead.

When the transition to virtualizing our infrastructure began, I thought our needs, and our circumstances would be different than they’ve proven to be.  Others claim extraordinary consolidation ratios with virtualization.  I believed we’d see huge improvements, but those numbers wouldn’t possibly apply to us, because (chests puffed out) we needed real power.  Well, I was wrong, and so far, we really are like everyone else.

Helpful links

• Memory in ESX 3.x
  http://www.vmware.com/pdf/esx3_memory.pdf
• CPU ready time in ESX 3.x
  http://www.vmware.com/pdf/esx3_ready_time.pdf
• CPU Scheduler in vSphere
  http://www.vmware.com/files/pdf/perf-vsphere-cpu_scheduler.pdf
• Monitoring paging in Windows
  http://www.computerperformance.co.uk/HealthCheck/Memory_Health.htm http://support.microsoft.com/kb/139609  (note:  this is a relatively old KB, but demonstrates how easily performance counters by themselves can be downright misleading if you aren’t careful)
• Impact of Memory Reservations
  http://frankdenneman.wordpress.com/2009/12/08/impact-of-memory-reservation/
• vCPU scaling of SQL 2005 VMs under ESX 3.5
  http://www.delltechcenter.com/page/Performance+of+SQL+Server+2005+on+ESX+Server+3.5
• Idle Memory Tax in ESX
  http://www.boche.net/blog/index.php/2009/01/29/idle-memory-tax/