Specifications

Table Of Contents
VMware, Inc. 149
Chapter 6 Performance Tuning and the VMware Server Host
Disk Options
The!various!disk!options!(SCSI!versus!IDE)!and!types!(virtual!or!physical)!affect!
performance!in!a!number!of!ways.
Overall,!SCSCI!disks!are!faster!than!IDE!disks!that!uses!direct!memory!access!(DMA)!
However,!in!certain!situations,!such!as!single!threaded!disk!access,!an!IDE!disk!that!
uses!DMA!is!as!
fast!as!a!SCSI!disk.!Inside!a!virtual!machine,!SCSI!disks!and!IDE!disks!
that!use!direct!memory!access!(DMA)!have!approximately!the!same!performance.!If!
supported,!VMware!recommends!that!you!enable!DMA!in!SCSI!disks.!IDE!disks!can!be!
very!slow!in!a!guest!operating!system!that!is!not!set
!to!use!DMA.!
The!easiest!way!to!configure!a!Linux!guest!to!use!DMA!for!IDE!drive!access!is!to!install!
VMware!Tools!(VM!>!Install!VMware!Tools).!Among!other!things,!the!installation!
process!automatically!sets!IDE!virtual!drives!to!use!DMA.
In!Windows!Server!2003,!Windows!XP!and!Windows
!2000,!DMA!access!is!enabled!by!
default.!The!method!for!changing!the!setting!varies!with!other!Windows!operating!
systems.!See!the!following!technical!notes!for!details.
! “Disk!Performance!in!Windows!NT!Guests!on!Multiprocessor!Hosts”
! “Windows!95!and!Windows!98!Guest!Operating!System!Performance!Tips”
When!a!snapshot!exists,!virtual!disks!often!have!very!good!performance!for!random!or!
nonsequential!access.!But!they!can!potentially!become!so!fragmented!that!performance!
is!affected.!In!order!to!defragment!the!disk,!you!must!first!remove!the!snapshot!
(Snapshot!>
!Remove!Snapshot).
When!no!snapshot!exists,!physical!disks!and!preallocated!virtual!disks!both!use!flat!
files!that!mimic!the!sequential!and!random!access!performance!of!the!underlying!disk.!
When!a!snapshot!exists!and!you!have!made!changes!since!powering!on!the!virtual!
machine,!any!access!to!those!changed!files!performs!
at!a!level!similar!to!the!
performance!of!a!virtual!disk!that!does!not!have!all!space!allocated!in!advance.!If!you!
remove!the!snapshot,!performance!is!again!similar!to!that!of!the!underlying!disk.
Overall,!if!no!snapshot!exists!and!you!are!using!physical!disks!or!preallocated!virtual!
disks,!you!
see!somewhat!better!performance!than!that!provided!by!other!
configurations.!
Disk!writes!can!be!slower!for!virtual!disks!that!do!not!have!all!space!allocated!in!
advance.!You!can!improve!performance!for!these!disks!by!defragmenting!them!from!
the!virtual!machine!settings!editor.!Choose!VM!>!Settings,!select!the!disk!
you!want!to!
defragment,!then!click!Defragment.