System information
Each virtual disk that a virtual machine can access through one of the virtual SCSI controllers resides in the
VMware Virtual Machine File System (VMFS) datastore, NFS-based datastore, or on a raw disk. From the
standpoint of the virtual machine, each virtual disk appears as if it were a SCSI drive connected to a SCSI
controller. Whether the actual physical disk device is being accessed through parallel SCSI, iSCSI, network, or
Fibre Channel adapters on the host is transparent to the guest operating system and to applications running
on the virtual machine.
Figure 1-1 gives an overview of storage virtualization. The diagram illustrates storage that uses VMFS and
storage that uses raw device mapping (RDM).
Figure 1-1. SAN Storage Virtualization
VMFS
ESX/ESXi
HBA
VMware virtualization layer
.vmdk
LUN1 LUN2 LUN5
RDM
SCSI controller
virtual disk 2virtual disk 1
virtual machine
1
Virtual Machine File System
In a simple configuration, the disks of virtual machines are stored as files within a Virtual Machine File System
(VMFS). When guest operating systems issue SCSI commands to their virtual disks, the virtualization layer
translates these commands to VMFS file operations.
ESX/ESXi hosts use VMFS to store virtual machine files. With VMFS, multiple virtual machines can run
concurrently and have concurrent access to their virtual disk files. Since VMFS is a clustered file system,
multiple hosts can have a shared simultaneous access to VMFS datastores on SAN LUNs. VMFS provides the
distributed locking to ensure that the multi-host environment is safe.
You can configure a VMFS datastore on either local disks or SAN LUNs. If you use the ESXi host, the local disk
is captured and used for the VMFS datastore during the host's first boot.
A VMFS datastore can map to a single SAN LUN or stretch over multiple SAN LUNs. You can expand a
datastore while virtual machines are running on it, either by growing the datastore or by adding a new extent.
The VMFS datastore can be extended over 32 physical storage extents of the same storage type.
Fibre Channel SAN Configuration Guide
12 VMware, Inc.