Wiley VMware Infrastructure 3 For Dummies 978-0-470-27793-5 Manual De Usuario

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Inherent flaws are transferable. Flaws in a virtual infrastructure design 
can affect all virtual machines. This, in turn, will affect all users of those 
machines. Watch where you make your trade-offs, and design your 
system with ample capacity to avoid this risk.

  

Some apps can be troublesome for time syncs. For example, I have one 
application that throws off the virtual machine’s time sync. You can 
compensate for this by synching the virtual machine’s time to the ESX 
(Engagement Simulation Exercise) host. (An ESX host is a server that 
your virtual machines run on. It provides access to all the hardware 
resources your virtual machines share.) In turn, sync your ESX host to 
your network’s time source.

Symmetrical multiprocessing 

and why you care

Commonly, physical machines use multiple processors. In fact, you’d be 
hard pressed to find a server that doesn’t come with at least a dual-core 
processor. Each core is treated as a separate CPU, so a machine using a 
single, dual-core CPU is taking advantage of symmetrical multiprocessing 
(SMP). In the physical world, multiple CPUs can greatly aid in processing 
speed. Things are a little different in the virtual world, however.

Your ESX host will most likely have multiple processors with multiple cores. 
Whenever your virtual machine needs the CPU, the VMkernel (covered in 
the next section) can send the work to any CPU in your ESX host. Your single 
CPU virtual machine is, therefore, getting benefits similar to SMP without 
even knowing it.

 

If you have a license for SMP, you can assign multiple processors to a virtual 
machine. However, just because you can do something doesn’t mean that you 
should. Dual, virtual CPU machines force co-scheduling of physical processors.

With co-scheduling, if one physical processor is scheduled, a second one is 
as well. This can take resources away from your other virtual machines. 
Additionally, if Process 1 on virtual CPU1 is waiting too long for Process 2 on 
virtual CPU2 to finish, both processes might get scheduled out (finish their 
allotted share of CPU time and lose the processor until their next turn) before 
completion. This can negate the benefits of using multiple CPUs.

 

Best practices dictate adding multiple CPUs to a virtual machine only if you 
can prove an increase in performance. This is because multiple virtual 
CPUs can have some negative side effects:

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