6.7

You can distribute virtual machines across different physical servers. That means you run a mix of
virtual machines on each server, so that not all experience high demand in the same area at the
same time. If a server fails, you can restart virtual machines on another physical server. If the failure
occurs, the on-disk lock for each virtual machine is released. For more information about VMware
DRS, see the vSphere Resource Management documentation. For information about VMware HA,
see the vSphere Availability documentation.
n
You can use vMotion to migrate running virtual machines from one physical server to another. For
information about migrating virtual machines, see the vCenter Server and Host Management
documentation.
To create a shared datastore, mount the datastore on those ESXi hosts that require the datastore access.
VMFS Metadata Updates
A VMFS datastore holds virtual machine files, directories, symbolic links, RDM descriptor files, and so on.
The datastore also maintains a consistent view of all the mapping information for these objects. This
mapping information is called metadata.
Metadata is updated each time you perform datastore or virtual machine management operations.
Examples of operations requiring metadata updates include the following:
n
Creating, growing, or locking a virtual machine file
n
Changing attributes of a file
n
Powering a virtual machine on or off
n
Creating or deleting a VMFS datastore
n
Expanding a VMFS datastore
n
Creating a template
n
Deploying a virtual machine from a template
n
Migrating a virtual machine with vMotion
When metadata changes are made in a shared storage environment, VMFS uses special locking
mechanisms to protect its data and prevent multiple hosts from concurrently writing to the metadata.
VMFS Locking Mechanisms
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).
vSphere Storage
VMware, Inc. 159