In a shared storage environment, when multiple hosts access the same VMFS datastore, specific locking mechanisms are used. These locking mechanism prevent multiple hosts from concurrently writing to the metadata and ensure that no data corruption occurs.

VMFS supports SCSI reservations and atomic test and set (ATS) locking.

VMFS uses SCSI reservations on storage devices that do not support hardware acceleration. SCSI reservations lock an entire storage device while an operation that requires metadata protection is performed. 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 on other hosts that are accessing the same VMFS. For information about how to reduce SCSI reservations, see the vSphere Troubleshooting documentation.

For storage devices that support hardware acceleration, VMFS uses the ATS algorithm, also called hardware assisted locking. In contrast with SCSI reservations, ATS supports discrete locking per disk sector. For information about hardware acceleration, see Storage Hardware Acceleration.

Mechanisms that VMFS uses to apply different types of locking depend on the VMFS version.

Use of ATS Locking on Devices with Hardware Acceleration Support

Storage Devices


Upgraded VMFS5


Single extent

ATS only

ATS, but can revert to SCSI reservations

ATS, but can revert to SCSI reservations

Multiple extents

Spans only over ATS-capable devices

ATS except when locks on non-head

ATS except when locks on non-head

In certain cases, you might need to turn off the ATS-only setting for a new VMFS5 datastore. For information, see Turn off ATS Locking.