Virtualization Oversubscription - What’s so scary? Memory Oversubscription (6 of 20)

As stated in my previous blog having set out a few ground rules we can now look at memory oversubscription.

So how can we over-subscribe memory?

Free Space - Just like disks have a lot of free space, servers typically run with free space in memory. 

Then there are a number of tricks that the hypervisor can do to find even more savings.

·        Page Sharing (Deduplication)

·        Balloon Driver (VMware)

·        Reservations

·        Shares

We'll start with a look at:

Transparent Page Sharing

When two or more Virtual Machines have the same pages of data in memory, VMware can store a single copy and present it to all the VMs.  Should a VM alter a shared memory page, a copy will be created by VMware and presented to that VM.

An Example is shown in the graphic below:

VM1 starts and allocates some unique memory.

VM2 starts and allocates some unique memory.

VM1 allocates memory for a standard windows dll.

VM2 also allocates memory for the same standard windows dll.

VMware maps both systems memory to the same page in RAM.

On Wednesday I'll explain more about the VMware Balloon Driver.

In the meantime register for access to some of our great Resources, white papers, on-demand webinars and more.

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Phil Bell

Consultant