Datasheet

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EXPLORING VMWARE VSPHERE 5

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. The goal of DRS is

twofold:

u At startup, DRS attempts to place each VM on the host that is best suited to run that VM at

that time.

u 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 ﬁ 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 conﬁ 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 conﬁ guration.

DRS performs these on-the-ﬂ y migrations without any downtime or loss of network con-

nectivity to the VMs by leveraging vMotion, the live migration functionality I 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.

Fewer Bigger Servers or More Smaller Servers?

Remember from Table 1.2 that VMware ESXi supports servers with up to 160 CPU cores (64 CPU cores

in vSphere 4.0) and up to 2 TB of RAM. With vSphere DRS, though, you can combine multiple smaller

servers for the purpose of managing aggregate capacity. This means that bigger, more powerful servers

might not be better servers for virtualization projects. These larger servers, in general, are signiﬁ -

cantly more expensive than smaller servers, and using a greater number of smaller servers (often

referred to as “scaling out”) may provide greater ﬂ exibility than a smaller number of larger servers

(often referred to as “scaling up”). The new vRAM licensing model for vSphere 5, discussed in the

“Licensing VMware vSphere” section, would also affect this decision. The key thing to remember

is that a bigger server isn’t necessarily a better server.

VSPHERE STORAGE DRS

vSphere Storage DRS, a major new feature of VMware vSphere 5, takes the idea of vSphere DRS

and applies it to storage. Just as vSphere DRS helps to balance CPU and memory utilization across a

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