vsphere-esxi-vcenter-server-55-storage-guide

Chapter 16 Working with Datastores• Creating a template• Deploying a virtual machine from a template• Migrating a virtual machine with vMotionWhen metadata changes are made in a shared storage enviroment, VMFS uses special locking mechanismsto protect its data and prevent multiple hosts from concurrently writing to the metadata.VMFS Locking MechanismsIn a shared storage environment, when multiple hosts access the same VMFS datastore, specific lockingmechanisms are used. These locking mechanism prevent multiple hosts from concurrently writing to themetadata and ensure that no data corruption occurs.VMFS supports SCSI reservations and atomic test and set (ATS) locking.SCSI ReservationsVMFS uses SCSI reservations on storage devices that do not support hardware acceleration. SCSIreservations lock an entire storage device while an operation that requires metadata protection isperformed. After the operation completes, VMFS releases the reservation and other operations can continue.Because this lock is exclusive, excessive SCSI reservations by a host can cause performance degradation onother hosts that are accessing the same VMFS. For information about how to reduce SCSI reservations, seethe vSphere Troubleshooting documentation.Atomic Test and Set (ATS)For storage devices that support hardware acceleration, VMFS uses the ATS algorithm, also called hardwareassisted locking. In contrast with SCSI reservations, ATS supports discrete locking per disk sector. Forinformation about hardware acceleration, see Chapter 24, “Storage Hardware Acceleration,” on page 235.Mechanisms that VMFS uses to apply different types of locking depend on the VMFS version.Table 16‐2. Use of ATS Locking on Devices with Hardware Acceleration SupportStorage Devices New VMFS5 Upgraded VMFS5 VMFS3Single extent ATS only ATS, but can revert to SCSIreservationsATS, but can revert toSCSI reservationsMultiple extentsSpans only over ATS-capabledevicesATS except when locks on nonheadATS except when lockson non-headIn certain cases, you might need to turn off the ATS-only setting for a new VMFS5 datastore. Forinformation, see “Turn off ATS Locking,” on page 137.Turn off ATS LockingWhen you create a VMFS5 datastore on a device that supports atomic test and set (ATS) locking, thedatastore is set to the ATS-only mode. In certain circumstances, you might need to turn off the ATS modesetting.Turn off the ATS setting when, for example, your storage device is downgraded or firmware updates failand the device no longer supports hardware acceleration. The option that you use to turn off the ATS settingis available only through the ESXi Shell. For more information, see the Getting Started with vSphere Command-Line Interfaces.ProcedureuTo turn off the ATS setting, run the following command:vmkfstools --configATSOnly 0 deviceVMware, Inc. 137