VMware ESX Server 2.0Features | Documentation | Knowledge Base | Discussion ForumsNormally, 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 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. 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.
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. Also, if the bus sharing is Physical, commands that reserve, reset or release a shared virtual disk are transmitted through to the physical disk, so other machines sharing the disk can properly detect when a virtual disk has been reserved or reset. Therefore, when you are sharing disks among virtual machines across physical machines for high availability purposes, it is often best to put only a single VMFS with a single virtual disk on each shared disk — that is, have only one virtual disk per physical disk. In such a configuration, each virtual disk can be reserved and released independently. To change the bus sharing setting, complete the following steps:
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