In a shared storage environment, when multiple hosts access the same VMFS datastore, specific locking mechanisms are used. These locking mechanisms prevent multiple hosts from concurrently writing to the metadata and ensure that no data corruption occurs.
Depending on its configuration and the type of underlying storage, a VMFS datastore can use different types of locking mechanisms. It can exclusively use the atomic test and set locking mechanism (ATS-only), or use a combination of ATS and SCSI reservations (ATS+SCSI).
For storage devices that support T10 standard-based VAAI specifications, VMFS provides ATS locking, also called hardware assisted locking. The ATS algorithm supports discrete locking per disk sector. All newly formatted VMFS5 and VMFS6 datastores use the ATS-only mechanism if the underlying storage supports it, and never use SCSI reservations.
When you create a multi-extent datastore where ATS is used, vCenter Server filters out non-ATS devices. This filtering allows you to use only those devices that support the ATS primitive.
In certain cases, you might need to turn off the ATS-only setting for a VMFS5 or VMFS6 datastore. For information, see Change Locking Mechanism to ATS+SCSI.
A VMFS datastore that supports the ATS+SCSI mechanism is configured to use ATS and attempts to use it when possible. If ATS fails, the VMFS datastore reverts to SCSI reservations. In contrast with the ATS locking, the 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.
Datastores that use the ATS+SCSI mechanism include VMFS5 datastores that were upgraded from VMFS3. In addition, new VMFS5 or VMFS6 datastores on storage devices that do not support ATS use the ATS+SCSI mechanism.
If your VMFS datastore reverts to SCSI reservations, you might notice performance degradation caused by excessive SCSI reservations. For information about how to reduce SCSI reservations, see the vSphere Troubleshooting documentation.