Specifications

Administration Guide
350 VMware, Inc.
Swapping!is!used!to!forcibly!reclaim!memory!from!a!virtual!machine!when!no!
vmmemctl!driver!is!available.!This!might!be!the!case!if!the!vmmemctl!driver!was!never!
installed,!has!been!explicitly!disabled,!is!not!running!(for!example,!while!the!guest!
operating!system!is!booting),!or!is!temporarily!unable!to!reclaim
!memory!quickly!
enough!to!satisfy!current!system!demands.!Standard!demand!paging!techniques!swap!
pages!back!in!when!the!virtual!machine!needs!them.
Use!the!vmmemctl!approach!for!optimum!performance.!Swapping!is!a!reliable!
mechanism!of!last!resort!that!the!system!uses!to!reclaim!memory!only!when!necessary.
Swap Space and Guest Operating Systems
If!you!overcommit!memory!with!ESX!Server,!be!sure!your!guest!operating!systems!have!
sufficient!swap!space.!This!swap!space!must!be!greater!than!or!equal!to!the!difference!
between!the!virtual!machine’s!maximum!and!minimum!sizes.!
To!prevent!virtual!machine!failure,!increase!the!swap!size!in!your!virtual!machines:
! Windows!guest!operating!systems!–!Refer!to!swap!space!as!“paging!files.”!Some!
Windows!operating!systems!try!to!increase!the!size!of!paging!files,!if!sufficient!free!
disk!space!is!available.
Refer!to!your!Windows!documentation!or!search!the!Windows!help!files!for!
“paging!files.”!Follow!the!instructions!for!changing!the!
size!of!the!virtual!memory!
paging!file.
! Linux!guest!operating!system!–!Refers!to!swap!space!as!“swap!files.”!For!
information!on!increasing!swap!files,!see!to!the!mkswap!(sets!up!a!Linux!swap!area)!
and!swapon!(enables!devices!and!files!for!paging!and!swapping)!man!pages!in!
your!Linux!guest!operating!system.
Guest!operating!systems!with!large!
memory!and!small!virtual!disks!(for!example,!a!
virtual!machine!with!3.6GB!RAM!and!a!2!GB!virtual!disk)!are!more!susceptible!to!this!
problem.
Sharing Memory Across Virtual Machines
Many!ESX!Server!workloads!present!opportunities!for!sharing!memory!across!virtual!
machines.!For!example,!several!virtual!machines!may!be!running!instances!of!the!same!
guest!operating!system,!have!the!same!applications!or!components!loaded,!or!contain!
common!data.!In!such!cases,!ESX!Server!uses!a!proprietary!transparent!page!sharing!
CAUTION If!memory!is!overcommitted,!and!the!guest!operating!system!is!configured!
with!insufficient!swap!space,!the!guest!operating!system!in!the!virtual!
machine!may!fail.