vSphere Storage - ESXi 5.1 - Documentation - VMware
vSphere Storage - ESXi 5.1 - Documentation - VMware
vSphere Storage - ESXi 5.1 - Documentation - VMware
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n Creating a template<br />
n Deploying a virtual machine from a template<br />
n Migrating a virtual machine with vMotion<br />
When metadata changes are made in a shared storage enviroment, VMFS uses special locking mechanisms to<br />
protect its data and prevent multiple hosts from concurrently writing to the metadata.<br />
VMFS Locking Mechanisms<br />
In a shared storage environment, when multiple hosts access the same VMFS datastore, specific locking<br />
mechanisms are used. These locking mechanism prevent multiple hosts from concurrently writing to the<br />
metadata and ensure that no data corruption occurs.<br />
VMFS supports SCSI reservations and atomic test and set (ATS) locking.<br />
SCSI Reservations<br />
VMFS uses SCSI reservations on storage devices that do not support hardware acceleration. SCSI reservations<br />
lock an entire storage device while an operation that requires metadata protection is performed. After the<br />
operation completes, VMFS releases the reservation and other operations can continue. Because this lock is<br />
exclusive, excessive SCSI reservations by a host can cause performance degradation on other hosts that are<br />
accessing the same VMFS. For information about how to reduce SCSI reservations, see the <strong>vSphere</strong><br />
Troubleshooting documentation.<br />
Atomic Test and Set (ATS)<br />
For storage devices that support hardware acceleration, VMFS uses the ATS algorithm, also called hardware<br />
assisted locking. In contrast with SCSI reservations, ATS supports discrete locking per disk sector. For<br />
information about hardware acceleration, see Chapter 21, “<strong>Storage</strong> Hardware Acceleration,” on page 215.<br />
Mechanisms that VMFS uses to apply different types of locking depend on the VMFS version.<br />
Table 17-2. Use of ATS Locking on Devices with Hardware Acceleration Support<br />
<strong>Storage</strong> Devices New VMFS5 Upgraded VMFS5 VMFS3<br />
Single extent ATS only ATS, but can revert to SCSI<br />
reservations<br />
Multiple extents Spans only over ATS-capable<br />
devices<br />
ATS except when locks on nonhead<br />
ATS, but can revert to<br />
SCSI reservations<br />
ATS except when locks<br />
on non-head<br />
In certain cases, you might need to turn off the ATS-only setting for a new VMFS5 datastore. For information,<br />
see “Turn off ATS Locking,” on page 157.<br />
Turn off ATS Locking<br />
When you create a VMFS5 datastore on a device that supports atomic test and set (ATS) locking, the datastore<br />
is set to the ATS-only mode. In certain circumstances, you might need to turn off the ATS mode setting.<br />
Turn off the ATS setting when, for example, your storage device is downgraded or firmware updates fail and<br />
the device no longer supports hardware acceleration. The option that you use to turn off the ATS setting is<br />
available only through the <strong>ESXi</strong> Shell. For more information, see the Getting Started with <strong>vSphere</strong> Command-Line<br />
Interfaces.<br />
Procedure<br />
u To turn off the ATS setting, run the following command:<br />
vmkfstools --configATSOnly 0 device<br />
Chapter 17 Working with Datastores<br />
<strong>VMware</strong>, Inc. 157
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