vSphere Storage - ESXi 5.1 - Documentation - VMware

vSphere Storage
Sharing a VMFS Datastore Across Hosts
As a cluster file system, VMFS lets multiple ESXi hosts access the same VMFS datastore concurrently. You can
connect up to 128 hosts to a single VMFS datastore.
Figure 17-1. Sharing a VMFS Datastore Across Hosts
host
A
host
B
VMFS volume
virtual
disk
files
host
C
VM1 VM2 VM3
disk1
disk2
disk3
To ensure that the same virtual machine is not accessed by multiple servers at the same time, VMFS provides
on-disk locking.
Sharing the same VMFS volume across multiple hosts offers the following advantages:
n You can use VMware Distributed Resource Scheduling (DRS) and VMware High Availability (HA).
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. In case of a failure, 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. To support
vMotion between ESXi 5 and version 3.x or 4.x hosts, virtual machines must be located on VMFS3 volumes.
For information about migrating virtual machines, see the vCenter Server and Host Management
documentation.
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 a file's attributes
n Powering a virtual machine on or off
n Creating or deleting a VMFS datastore
n Expanding a VMFS datastore
156 VMware, Inc.