As a cluster file system, VMFS lets multiple ESX/ESXi hosts access the same VMFS datastore concurrently.

To ensure that multiple servers do not access the same virtual machine at the same time, VMFS provides on-disk locking. To coordinate access to VMFS internal file system information, ESX/ESXi uses SCSI reservations on the entire LUN.

Sharing a VMFS Datastore Across ESX/ESXi Hosts shows several ESX/ESXi systems sharing the same VMFS volume.

Sharing a VMFS Datastore Across ESX/ESXi Hosts
This image shows three ESX/ESXi hosts connected to the same VMFS volume and storing their virtual disks on this volume.

Because virtual machines share a common VMFS datastore, it might be difficult to characterize peak-access periods or to optimize performance. You must plan virtual machine storage access for peak periods, but different applications might have different peak-access periods. VMware recommends that you load balance virtual machines over servers, CPU, and storage. Run a mix of virtual machines on each server so that not all experience high demand in the same area at the same time.