6.5.1

n
ESXi system swaps out a page from a virtual machine to a server swap le without any involvement by
the guest operating system. Each virtual machine has its own swap le.
Memory Balloon Driver
The memory balloon driver (vmmemctl) collaborates with the server to reclaim pages that are considered least
valuable by the guest operating system.
The driver uses a proprietary ballooning technique that provides predictable performance that closely
matches the behavior of a native system under similar memory constraints. This technique increases or
decreases memory pressure on the guest operating system, causing the guest to use its own native memory
management algorithms. When memory is tight, the guest operating system determines which pages to
reclaim and, if necessary, swaps them to its own virtual disk.
Figure 6‑1. Memory Ballooning in the Guest Operating System
1
2
3
memory
memory
memory
swap space
swap space
N You must congure the guest operating system with sucient swap space. Some guest operating
systems have additional limitations.
If necessary, you can limit the amount of memory vmmemctl reclaims by seing the sched.mem.maxmemctl
parameter for a specic virtual machine. This option species the maximum amount of memory that can be
reclaimed from a virtual machine in megabytes (MB). See “Set Advanced Virtual Machine Aributes,” on
page 118.
Using Swap Files
You can specify the location of your guest swap le, reserve swap space when memory is overcommied,
and delete a swap le.
ESXi hosts use swapping to forcibly reclaim memory from a virtual machine when the vmmemctl driver is not
available or is not responsive.
n
It was never installed.
n
It is explicitly disabled.
n
It is not running (for example, while the guest operating system is booting).
n
It is temporarily unable to reclaim memory quickly enough to satisfy current system demands.
vSphere Resource Management
36 VMware, Inc.