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EXPLORING VMWARE VSPHERE 4 7
has more available CPU. More details on the VMware VMotion feature and its requirements are
provided in Chapter 10, ‘‘Managing Resource Allocation.’’
Storage VMotion builds on the idea and principle of VMotion, further reducing planned
downtime with the ability to move a virtual machine’s storage while the virtual machine is still
running. Deploying VMware vSphere in your environment generally means that lots of shared
storage—Fibre Channel or iSCSI SAN or NFS—is needed. What happens when you need to
migrate from an older storage array to a newer storage array? What kind of downtime would
be required?
Storage VMotion directly addresses this concern. Storage VMotion moves the storage for a
running virtual machine between datastores. Much like VMotion, Storage VMotion works without
downtime to the virtual machine. This feature ensures that outgrowing datastores or moving to a
new SAN does not force an outage for the affected virtual machines and provides administrators
with yet another tool to increase their flexibility in responding to changing business needs.
VMware Distributed Resource Scheduler
Now that I’ve piqued your interest with the introduction of VMotion, let me introduce VMware
Distributed Resource Scheduler (DRS). If you think that VMotion sounds exciting, your anticipa-
tion will only grow after learning about DRS. DRS, simply put, is a feature that aims to provide
automatic distribution of resource utilization across multiple ESX/ESXi hosts that are configured
in a cluster. The use of the term cluster often draws IT professionals into thoughts of Microsoft
Windows Server clusters. However, ESX/ESXi clusters are not the same. The underlying con-
cept of aggregating physical hardware to serve a common goal is the same, but the technology,
configuration, and feature sets are different between ESX/ESXi clusters and Windows Server
clusters.
Aggregate Capacity and Single Host Capacity
Although I say that a DRS cluster is an implicit aggregation of CPU and memory capacity, it’s impor-
tant to keep in mind that a virtual machine is limited to using the CPU and RAM of a single physical
host at any given time. If you have two ESX/ESXi servers with 32GB of RAM each in a DRS cluster,
the cluster will correctly report 64GB of aggregate RAM available, but any given virtual machine will
not be able to use more than approximately 32GB of RAM at a time.
An ESX/ESXi cluster is an implicit aggregation of the CPU power and memory of all hosts
involved in the cluster. After two or more hosts have been assigned to a cluster, they work in
unison to provide CPU and memory to the virtual machines assigned to the cluster. The goal of
DRS is twofold:
◆ At startup, DRS attempts to place each virtual machine on the host that is best suited to run
that virtual machine at that time.
◆ While a virtual machine is running, DRS seeks to provide that virtual machine with
the required hardware resources while minimizing the amount of contention for those
resources in an effort to maintain good performance levels.
The first part of DRS is often referred to as intelligent placement. DRS can automate the placement
of each virtual machine as it is powered on within a cluster, placing it on the host in the cluster
that it deems to be best suited to run that virtual machine at that moment.