It is a shared file system that multiple ESXi hosts can read and write to, simultaneously. VMFS is used to store all aspects of the VM including but not limited to: VMDK disks, snapshots, configuration data and swap data.
When a VM wants to read or write to a disk located on the SAN, the following must take place:
The guest operating system issues a read or write command to the virtual disk attached to the VM.
A SCSI command is issued to the SCSI driver for the virtual disk and then the virtual SCSI controller processes the command.
The virtual SCSI controller forwards the command to the VMkernel.
The VMkernel does the following:
Locates the VMDK file on the VMFS volume that the VM is attempting to read or write to
Maps the SCSI command to the block on the virtual disk
Maps the block on the physical disk to the block that corresponds to on the virtual disk
Builds and sends a SCSI command with the true physical block location on the VMFS datastore
The HBA installed on...