Installation guide

C H A P T E R 9 Storage and File Systems
313
Sharing the SCSI Bus
Normally, VMware ESX Server enforces locking and does not allow two virtual
machines to access the same virtual disk (VMFS file) at the same time. If a second
virtual machine tries to access a VMFS file, it gets an error and does not power on.
However, it is often useful to have more than one virtual machine share a disk in order
to provide high availability. This configuration is commonly used for disk-based
failover, in which one machine takes over running an application when the primary
machine fails. The data required for the application is typically stored on a shared disk,
so the backup machine can immediately access the necessary data when the failover
occurs. See Configuration for Clustering on page 329 for complete information on
clustering with ESX Server.
The bus sharing setting is used to determine if virtual machines are allowed to access
the same virtual disk simultaneously.
Setting Bus Sharing Options
Use the VMware Management Interface to change the bus sharing settings for each
virtual machine that will access the same virtual disk simultaneously.
There are three bus sharing options.
• None: Disks cannot be shared by other virtual machines
• Virtual: Disks can be shared by virtual machines on same server
• Physical: Disks can be shared by virtual machines on any server
To enable sharing of virtual disks, choose Virtual or Physical. All virtual disks on the
specified virtual bus will be sharable and have the specified mode.
If the bus sharing is Virtual, only virtual machines on the same physical machine will
be able to share disks. This setting allows for a “cluster-in-a-box” configuration, in
which all members of a high-availability cluster are on the same physical machine.
This setup is useful for providing high availability when the likely failures are due to
software or administrative errors.
If the bus sharing is Physical, virtual machines on different physical machines will be
able to share disks. In this case, the VMFS holding the virtual disks must be on a
physically shared disk, so all of the physical machines can access it. This setup is useful
for providing high availability when the likely failures also include hardware errors.
When a shared disk is used for high availability purposes, the current machine that is
running the application and using the shared data often reserves the disk using a SCSI
command.