Information Technology Reference
In-Depth Information
hardware to serve a common goal is the same, the technology, coni guration, and feature sets
are quite different between VMware ESXi clusters and Windows Server clusters.
Aggregate Capacity and Single Host Capacity
Although we say that a DRS cluster is an implicit aggregation of CPU and memory capacity, it's
important to keep in mind that a VM is limited to using the CPU and RAM of a single physical
host at any given time. If you have two ESXi servers with 32 GB of RAM each in a DRS cluster, the
cluster will correctly report 64 GB of aggregate RAM available, but any given VM will not be able
to use more than approximately 32 GB of RAM at a time.
An 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 VMs assigned to the cluster (keeping in mind that
any given VM can only use resources from one host; see the sidebar titled “Aggregate Capacity
and Single Host Capacity”). The goal of DRS is twofold:
At startup, DRS attempts to place each VM on the host that is best suited to run that VM at
that time.
◆
While a VM is running, DRS seeks to provide that VM with the required hardware
resources while minimizing the amount of contention for those resources in an effort to
maintain balanced utilization levels.
◆
The i rst part of DRS is often referred to as
intelligent
placement
. DRS can automate the place-
ment of each VM 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 VM at that moment.
DRS isn't limited to operating only at VM startup, though. DRS also manages the VM's loca-
tion while it is running. For example, let's say three servers have been coni gured in an ESXi
cluster with DRS enabled. When one of those servers begins to experience a high contention for
CPU utilization, DRS detects that the cluster is imbalanced in its resource usage and uses an
internal algorithm to determine which VM(s) should be moved in order to create the least imbal-
anced cluster. For every VM, DRS will simulate a migration to each host and the results will
be compared. The migrations that create the least imbalanced cluster will be recommended or
automatically performed, depending upon DRS's coni guration.
DRS performs these on-the-l y migrations without any downtime or loss of network connec-
tivity to the VMs by leveraging vMotion, the live migration functionality we described earlier.
This makes DRS extremely powerful because it allows clusters of ESXi hosts to dynamically
rebalance their resource utilization based on the changing demands of the VMs running on that
cluster.
