6.5.1

A similar result is obtained when determining Memory Shared and Memory Shared Common for the host.
n
Memory Shared for the host is the sum of each virtual machine's Memory Shared. Calculate shared
memory by looking at each virtual machine's guest physical memory and counting the number of
blocks that have arrows to machine memory blocks that themselves have more than one arrow pointing
at them. There are six such blocks in the example, so Memory Shared for the host is 24 KB.
n
Memory Shared Common is the amount of machine memory shared by virtual machines. To determine
common memory, look at the machine memory and count the number of blocks that have more than
one arrow pointing at them. There are three such blocks, so Memory Shared Common is 12 KB.
Memory Shared is concerned with guest physical memory and looks at the origin of the arrows. Memory
Shared Common, however, deals with machine memory and looks at the destination of the arrows.
The memory metrics that measure guest physical memory and machine memory might appear
contradictory. In fact, they are measuring dierent aspects of a virtual machine's memory usage. By
understanding the dierences between these metrics, you can beer use them to diagnose performance
issues.
Memory Reliability
Memory reliability, also known as error isolation, allows ESXi to stop using parts of memory when it
determines that a failure might occur, as well as when a failure did occur.
When enough corrected errors are reported at a particular address, ESXi stops using this address to prevent
the corrected error from becoming an uncorrected error.
Memory reliability provides beer VMkernel reliability despite corrected and uncorrected errors in RAM. It
also enables the system to avoid using memory pages that might contain errors.
Correcting an Error Isolation Notification
With memory reliability, VMkernel stops using pages that receive an error isolation notication.
The user receives an event in the vSphere Web Client when VMkernel recovers from an uncorrectable
memory error, when VMkernel retires a signicant percentage of system memory due to a large number of
correctable errors, or if there are a large number of pages that are unable to retire.
Procedure
1 Vacate the host.
2 Migrate the virtual machines.
3 Run memory related hardware tests.
About System Swap
System swap is a memory reclamation process that can take advantage of unused memory resources across
an entire system.
System swap allows the system to reclaim memory from memory consumers that are not virtual machines.
When system swap is enabled you have a tradeo between the impact of reclaiming the memory from
another process and the ability to assign the memory to a virtual machine that can use it. The amount of
space required for the system swap is 1GB.
Memory is reclaimed by taking data out of memory and writing it to background storage. Accessing the
data from background storage is slower than accessing data from memory, so it is important to carefully
select where to store the swapped data.
Chapter 6 Administering Memory Resources
VMware, Inc. 43