Specifications

Administration Guide
366 VMware, Inc.
Memory Overcommitment
In!many!consolidated!workloads,!it!is!rare!for!all!virtual!machines!to!be!actively!using!
all!of!their!memory!simultaneously.!Typically ,!some!virtual!machines!are!lightly!loaded,!
while!others!are!more!heavily!loaded,!and!relative!activity!lev els!generally!vary!over!
time.!In!such!cases,!it!might!be!reasonable!to!overcommit!memory
!to!reduce!hardware!
memory!requirements.
ESX!Server!transfers!memory!from!idle!virtual!machines!to!virtual!machines!that!
actively!need!more!memory!to!improve!memory!utilization.
You!can!also!specify!configuration!parameters!to!preferentially!devote!space!to!
important!virtual!machines.!
The!minimum!size!for!a!virtual!machine!defines!a!guaranteed!lower!bound!
on!the!
amount!of!memory!that!it!is!allocated,!even!when!memory!is!overcommitted.!You!can!
also!use!memory!shares!to!specify!the!relative!importance!of!different!virtual!machines.!
In!any!case,!you!should!configure!an!appropriate!minimum!size!for!each!virtual!
machine!to!ensure!that!each!virtual!machine!can!function
!effectively!(without!excessive!
paging),!even!when!all!virtual!machines!are!active!concurrently.
When!memory!is!scarce,!ESX!Server!dynamically!reclaims!space!from!some!virtual!
machines!based!on!importance!and!current!working!sets.!For!optimal!performance,!the!
server!attempts!to!reclaim!memory!from!a!virtual!machine!via!a!VMware‐supplied!
vmmemctl!module
!running!in!the!guest.!This!allows!the!guest!operating!system!to!
invoke!its!own!native!memory!management!policies,!causing!it!to!swap!to!its!own!
virtual!disk!only!when!necessary.!
ESX!Server!also!has!its!own!swap!file!and!may!also!swap!memory!from!a!virtual!
machine!to!the!ESX!
Server!swap!file!directly,!without!any!involvement!by!the!guest!
operating!system.
Example: Web Server Consolidation
Suppose!that!you!are!using!ESX!Server!to!consolidate!eight!nearly‐identical!Web!
servers!running!IIS!on!Windows!2000.!Each!Windows!2000!machine!is!configured!with!
512MB!of!memory.!The!native!memory!requirement!with!eight!physical!servers!is!8!*!
512MB!=!4GB.
To!consolidate!these!servers!as!virtual!machines,!24MB!
is!needed!for!the!server!
virtualization!layer!and!192MB!is!recommended!for!the!service!console.!Each!virtual!
machine!also!requires!an!additional!54MB!of!overhead!memory.!An!additional!6!
percent!should!be!added!to!account!for!the!minimum!free!memory!level.!Assuming!no!
overcommitment!and!no!benefits!from!memory!sharing,!the
!memory!required!for!
virtualizing!the!workload!is!24MB!+!192MB!+!(1.06!*!8!*!(512MB!+!54MB))!=!5016MB.!The!
total!overhead!for!virtualization!in!this!case!is!920MB.