Technical Note
VMware, Inc. 1
Virtual Disk Format 5.0
VMware ESXi and Hosted Products
Thedocumentdescribesthevirtualmachinedisk(VMDK)formatandcontainsthefollowingsections:
“VirtualDisksforVirtualMachines”onpage 1
“LayoutBasics”onpage 2
“TheDescriptorFile”onpage 3
“SimpleExtents”onpage 5
“ESXiHostSparseExtents”onpage 9
“Stream‐OptimizedCompressed”onpage 11
“Glossary”onpage 14
Virtual Disks for Virtual Machines
Whenavirtualmachine’soperatingsystemreadsandwritestovirtualdisk,itusesthesameinterfacesasfor
physicaldisk.VMwaredesignedtheVMDK(virtualmachinedisk)formattomimictheoperationofphysical
disk.VirtualdisksarestoredasoneormoreVMDKfilesonthehostcomputerorremotest
oragedevice,and
appeartotheguestoperatingsystemasstandarddiskdrives.
VMwareplatformproductsallsupporttheVMDKformat,withslightvariations.
HostedplatformproductssuchasVMwareWorkstationorVMwareFusionstoreVMDKfilesonafilesystem
providedbyanunderlyinghostoperatingsystem,eitherWindows,Linux,orMacOSX.
Dat
acenterplatformproductsstoreVMDKfileseitheronthelocalstorageofanESXihost,oronanetwork
connectedstoragedevice.OnESXihosts,VMDKfilesareusuallystoredonVMFS(virtualmachinefilesystem)
partitions,optimizedforlarge‐filestorage,butcanalsobestoredonNA
Spartitions(NFS).
VMFS‐3wasintroducedinESX3.0andstillsupportedforESX/ESXi4.0and4.1.VMFS‐4wasneverreleased.
VMwareintroducedVMFS‐5invSphere5,withenhancementsshowninTable 1.
Table 1. Comparison of VMFS-3 and VMFS-5
VMFS-3 VMFS-5
Thelargestextentforadiskvolumewas2TB. Extentlimitincreasedto~60TB,forlargervolumesincludingRDM.
MBR(masterbootrecord)partitiontype. GPT(GUIDpartitiontable)supports largerextents.
Blocksizewas1,2,4,or8MBforverylargefiles. Unified1MBblocksizesupportsverylargefiles>256GB.
Smallestsu
b‐blockwas64KB. 8KBsub‐blocksosmallfilesconsumelessspaceandgroweasily.
Maximumfilecountwas30,720. Supportfor>100,000filespervolume.
MaximumVMDKfilesizeis2TB. Samelimit.
MaximumnumberofsupportedLUNsis256. Samelimit.
LockingofentireLUNbySCSIreservation. Per‐sectorVAAIhardware‐assistedlockingre
ducesdiskcontention.
