Specifications

VMware, Inc. 347
Chapter 12 VMware ESX Server Resource Management
Setting Memory Minimum, Maximum, and Shares
You!can!set!a!memory!minimum,!memory!maximum,!and!shares!to!manage!memory!
resources!for!your!virtual!machines.!Memory!minimums!and!maximums!specify!
absolutes,!an!absolute!minimum!or!maximum!memory!usage!by!a!virtual!machine.!
Shares,!on!the!other!hand,!represent!relative!importance!or!priority.!You!set!shares!to!
specify!which
!virtual!machines!will!get!preferential!treatment!when!ESX!Server!is!
overcommitted.
For!example,!virtual!machine!A!has!a!minimum!memory!size!of!192MB,!and!a!
maximum!memory!size!of!256MB,!while!virtual!machine!B!has!a!minimum!memory!
size!of!256MB!and!a!maximum!memory!size!of!512MB.!
You!then!give
!virtual!machine!A!high!memory!shares!and!virtual!machine!B!normal!
memory!shares.!By!default,!the!setting!for!high!is!twice!that!of!normal,!or!four!times!
that!of!low.!For!example,!a!virtual!machine!with!high!shares!has!twice!as!many!shares!
as!a!virtual!machine!with!normal!
shares,!or!four!times!as!many!shares!as!a!virtual!
machine!with!low!shares.!To!change!these!defaults,!see“Service!Console!Commands”!
on!page 353.
ESX!Server!interprets!this!allocation!so!that!virtual!machine!A!will!never!have!less!than!
192MB!memory,!and!virtual!machine!B!will!never!have!less!than!256MB!memory,
!in!any!
situation.
However,!if!one!or!more!virtual!machines!are!not!actively!using!their!allocated!memory!
(for!example,!the!virtual!machines!are!idling),!ESX!Server!can!redistribute!a!portion!of!
unused!memory!proportionally,!based!on!the!virtual!machines’!memory!shares.!Active!
virtual!machines!benefit!when!extra!resources!are!available.!In
!this!example,!because!
virtual!machine!A!has!high!shares,!it!can!get!twice!as!much!memory!as!virtual!machine!
B!(low!shares),!subject!to!the!specified!memory!minimum!or!maximum.!
For!detailed!information!on!how!ESX!Server!dynamically!redistributes!memory,!see!
“Allocating!Memory!Dynamically”!on!page 348.
Admission Control Policy
VMware!ESX!Server!uses!an!admission!control!policy!to!ensure!that!sufficient!
unreserved!memory!and!swap!space!are!available!before!powering!on!a!virtual!
machine.!Memory!must!be!reserved!for!the!virtual!machine’s!guaranteed!minimum!
size;!additional!overhead!memory!is!required!for!virtualization.!Thus!the!total!required!
for!each!virtual!machine
!is!the!specified!minimum!plus!overhead.!
The!overhead!memory!size!is!determined!automatically;!it!is!typically!54MB!for!a!single!
virtual!CPU!virtual!machine,!and!64MB!for!a!dual‐virtual!CPU!SMP!virtual!machine.!
Additional!overhead!memory!is!reserved!for!virtual!machines!larger!than!512MB.!