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Do you defrag your virtual servers?

There have been some recent comments on Scott Drumond’s site (and others)  regarding defragging of virtual servers.  What do you think?  Do you defrag your virtual servers?  I’m personally torn.  I can see the value, but I am not sure if the costs are justified,  and I am not just talking about the monetary costs either.

We try to have a “what we do for one, we do for a thousand” mentality when it comes to our standard server builds.  Simply, this means that if we are going to declare a piece of software as a part of our standard base image all servers get it.  So far, it means that all servers get anti-virus protection, log monitoring, and a few others as a default.  In terms of dollars, it adds up to quite a pretty penny.  Virtualization makes it more expensive because I  have more server instances in my environment than I would have if every server was physical due to physical server costs.  Since a project doesn’t have to pay for hardware, it’s easier to ask for a server.  Some people call this sprawl, but I wouldn’t.  Sprawl connotates lack of control and we have well-defined controls in place.  No servers get deployed without adequate licensing and other resources.

Another cost is resource utilization.  If a server is busy defragging, it’s using CPU and disk resources.  Does this impact other virtual servers?  I would say yes, but I can’t say how much.  Your mileage will vary.  Yes, I can quantify direct resource utilization, but if my customers don’t notice the difference, does it really matter.    A 5% increase in CPU may have no impact on customer experience.  Fine then.  But what if they do notice a difference?  What if transaction times go up?  All the sudden that $xxx license may have just tripled in cost due to lost productivity.

Don’t forget to throw in the costs of environmentals.  If the host is busy, it’s generating heat.  If the host is busy, it’s using more electricity than it would be at idle (definitely true on Nehalem CPUs with power mgmt active).

Long story short, it’s not so simple as saying “defragging will improve vm performance by x”.  You need to figure out all the other ramifications.    My personal belief is to defrag those systems that clearly need it.  You’ll know which ones because you will either already have seen a significant performance degradation in them, or if you actively monitor your systems, you’re watching them begin to degrade.

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  • As an aside..most of the performance results being posted these days are based on running newer CPUs, newer storage, etc.  Older equipment will not fare the same.  Example, just because Virtual Center shows 50% CPU available doesn’t mean it’s really available.  An additional 5% load can be noticed by other virtual servers.  We’ve experienced it in my organization on 3yr old servers.  It’s not a problem on new equipment, but something we have to take into consideration when deploying guests onto older hosts.
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