Download VMware vSphere 4 Technical information

Performance Best Practices for VMware vSphere 4.0
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ESX 4.0 introduces virtual hardware version 7. By creating virtual machines using this hardware version,
or upgrading existing virtual machines to this version, a number of additional capabilities become
available. Some of these, such as VMXNET3 and PVSCSI, can improve performance for many workloads.
This hardware version is not compatible with previous versions of ESX, however, and thus if a cluster of
ESX hosts will contain some hosts running previous versions of ESX, the virtual machines running on
hardware version 7 will be constrained to run only on the ESX 4.0 hosts. This could limit VMotion choices
for DRS or DPM.
VMware, Inc.