Installation guide

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VMware ESX Server Administration Guide
Managing Disk Bandwidth
ESX Server provides dynamic control over the relative amount of disk bandwidth
allocated to each virtual machine. You can control disk bandwidth separately for each
physical disk or logical volume. The system manages the allocation of disk bandwidth
to virtual machines automatically based on allocation parameters and system load.
This is done in a way that maintains fairness and tries to maximize throughput.
You may specify initial disk bandwidth allocation values for a virtual machine in its
configuration file. You may also modify disk bandwidth allocation parameters
dynamically using the VMware Management Interface, the procfs interface on the
service console or the VMware Scripting API.
Reasonable defaults are used automatically when you do not specify parameters
explicitly. However, if you plan to run a virtual machine that will have disk-intensive
workloads, such as a database, or file server, then you may want to increase its disk
shares.
Information about current disk bandwidth allocations and other status is available via
the management interface, the procfs interface on the service console and the
VMware Scripting API.
Allocation Policy
ESX Server uses a modified proportional-share allocation policy for controlling disk
bandwidth per virtual machine. This policy attempts to control the disk bandwidth
used by a virtual machine to access a disk while also trying to maximize throughput to
the disk.
Disk bandwidth shares entitle a virtual machine to a fraction of the bandwidth to a
disk or LUN. For example, a virtual machine that has twice as many shares as another
for a particular disk is entitled to consume twice as much bandwidth to the disk,
provided that they are both actively issuing commands to the disk.
Bandwidth consumed by a virtual machine is represented in consumption units. Every
SCSI command issued to the disk effectively consumes one unit by default and
additional units proportional to the size of the data transfer associated with the
command.
Throughput to the disk is maximized through the use of a scheduling quantum for
disk requests from a virtual machine to a disk. A virtual machine is allowed to issue a
number of requests to a disk (the scheduling quantum) without being preempted by
another virtual machine. The issuing of a multiple requests without preemption is
applicable only if these requests access sequential sectors on the disk.