When storage devices do not support the hardware acceleration, ESXi hosts use the SCSI reservations mechanism when performing operations that require a file lock or a metadata lock in VMFS. SCSI reservations lock the entire LUN. Excessive SCSI reservations by a host can cause performance degradation on other servers accessing the same VMFS.
Excessive SCSI reservations cause performance degradation and SCSI reservation conflicts.
Several operations require VMFS to use SCSI reservations.
- Creating, resignaturing, or expanding a VMFS datastore
- Powering on a virtual machine
- Creating or deleting a file
- Creating a template
- Deploying a virtual machine from a template
- Creating a new virtual machine
- Migrating a virtual machine with VMotion
- Growing a file, such as a thin provisioned virtual disk
To eliminate potential sources of SCSI reservation conflicts, follow these guidelines:
- Serialize the operations of the shared LUNs, if possible, limit the number of operations on different hosts that require SCSI reservation at the same time.
- Increase the number of LUNs and limit the number of hosts accessing the same LUN.
- Reduce the number snapshots. Snapshots cause numerous SCSI reservations.
- Reduce the number of virtual machines per LUN. Follow recommendations in Configuration Maximums.
- Make sure that you have the latest HBA firmware across all hosts.
- Make sure that the host has the latest BIOS.
- Ensure a correct Host Mode setting on the SAN array.
For information about handling SCSI reservation conflicts on specific storage arrays, see the VMware knowledge base article at http://kb.vmware.com/kb/1005009.