Information Technology Reference
In-Depth Information
What happens when you run three more VMs, each coni gured with 4 GB of RAM? Each of
the additional VMs will request 4 GB of RAM from the ESXi host. At this point, four VMs will
be accessing the physical memory, and you will have allocated all 16 GB of memory to the VMs.
ESXi has now run out of a critical resource (memory).
What happens when you launch a i fth VM? Will it run? The short answer is yes, and some
of the key technologies that enable administrators to overcommit memory—that is, to allocate
more memory to VMs than is actually installed in the VMware ESXi host—are quite advanced.
Because these technologies are integral to understanding how memory allocation works with
VMware ESXi, let's take a look at these technologies and how they work.
Understanding ESXi Advanced Memory Technologies
VMware ESXi supports a number of different technologies for advanced memory management.
As a result of these advanced memory management technologies, at the time of this writing,
VMware ESXi is the only commercially available hypervisor on the market capable of perform-
ing memory overcommitment in a manner that is guest OS agnostic.
ESXi Does Not Require Guest OS Involvement
h ere are other commercially available hypervisors that off er the ability to overcommit memory,
but these products support that functionality only for certain guest OSes.
VMware ESXi employs i ve different memory-management technologies to make sure the
physical server's RAM is utilized as efi ciently as possible: idle memory tax, transparent page
sharing, ballooning, memory compression, and swapping.
While it doesn't cover everything, for anyone interested in more in-depth and detailed infor-
mation on some of these memory-management technologies, we strongly recommend reading
Memory
Resource
Management
in
VMware
ESX
Server
by Carl A. Waldspurger, available online at
the following location:
Idle Memory Tax
Before VMware ESXi actively starts making changes to relieve memory pressure, it ensures that
VMs do not actively horde memory by “charging” more for the idle memory. Up to 75 percent of
the memory allocated to each VM can be borrowed to service another VM by IMT. This setting
is coni gurable on a per-VM basis within the Advanced VM settings (see Chapter 9), although
in most cases this is not necessary and not recommended unless there is a specii c requirement.
Inside the guest OS, VMware Tools will use its balloon driver to understand which memory
blocks are allocated but idle and, therefore, available to be used elsewhere. The balloon driver is
also used in a more active fashion, which we will explain shortly.
Transparent Page Sharing
The next memory-management technology ESXi uses is
transparent
page
sharing
(
TPS
), in which
identical memory pages are shared among VMs to reduce the total number of memory pages
consumed. The hypervisor computes hashes of the contents of memory pages to identify pages
