Issues to Consider When Sharing Disks

Do not try to share a disk among multiple running virtual machines that are not collocated on the same host. The disk file itself may be located remotely, but the virtual machines must be running together on the same GSX Server host. If you try to share a disk among virtual machines located on different hosts, data could be corrupted or lost.

Do not share a disk on SCSI bus 0. This bus is usually used for the boot disk. If you share the boot disk, you run the risk of corrupting it, as the boot program is not aware that the disk is being shared and can write to the disk regardless of whether or not it is being shared. It is far more secure to use SCSI reservation on a data disk located on a different bus.

If only one running virtual machine is using a given disk, and it is running applications that do not use SCSI reservation, then the disk's performance might be degraded slightly.

At this time, if one virtual machine does not have SCSI reservation enabled for its virtual disk, but another virtual machine does have SCSI reservation enabled for the same virtual disk, GSX Server does allow the disk to be shared. However, any virtual machine not configured for SCSI reservation that tries to access this disk concurrently can cause corruption or data loss on the shared disk. VMware recommends you take care when sharing disks.

If you need to shrink or defragment the virtual disk (which can be done only with a growable virtual disk), first disable SCSI reservation and make sure the virtual disk is not being used by any other virtual machine.

To disable SCSI reservation for all SCSI disks in a virtual machine, open the configuration file and comment out or remove the scsi<x>.sharedBus = "virtual" line and make sure the disk.locking line is set to "true".

If you want to disable SCSI reservation for only a specific SCSI disk on a shared bus, change the scsi<x>:<y>.shared = "true" line in the configuration file to scsi<x>:<y>.shared = "false"; you can also comment out the line.

In a Windows virtual machine, some disk errors are recorded in the Windows event log in normal operation. These error messages have a format similar to
"The driver detected a controller error on \Device\Scsi\BusLogic3"

The errors should appear in the log periodically only on the passive node of the cluster and should also appear when the passive node is taking over during a failover. The errors are logged because the active node of the cluster has reserved the shared virtual disk. The passive node periodically probes the shared disk and receives a SCSI reservation conflict error.