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white paper

Considerations when Virtualizing High I/O

WorkloadsTitle: Considerations when Vir tualizing High I/O Workloads

Author(s): Xtravirt (Paul Buckle)

Target Audience: Technical - Intermediate

Current Revision: 1.0 (July 2009)

First Published: July 2009

Product(s): General Virtualization

UID: XD10108

Content Overview:

Typical High I/O Workloads

Virtualizing High I/O Workloads, options and considerations

1.0 IntroductionA server consolidation project typically starts with an

assessment o the existing physical estate, resulting

in not only the identication o the most suitable

virtualization candidates, but conversely, those which

have greater resource demands. Oten, though not

exclusively, these are ound to be those machines that

provide an organizations enterprise-class services, such

as databases, e-mail, and ERP.

While these potential bottlenecks cannot be ignored,

a combination o advances in hardware technology

and a creative approach to conguration o the

virtualized environment can enable the virtualization

o such machines. An awareness o these options

and considerations will help make the subsequent

virtualization o high I/O workloads a success.

2.0 Typical High I/O Workloads E-mailServers Almost always exhibit very highdisk I/O, simply as a result o the nature o the

product.

Databases - Oten exhibit high disk I/O as data is

read at a rapid rate rom a limited number o les.

TerminalServers - Can exhibit high disk I/O as

the hosted applications almost constantly write

to disk.

FileServers - Can exhibit high network and disk

I/O, particularly i anti-virus sotware is installed.

3.0 Options and Considerations

3.1 Use virtualization-aware host serversOver recent years, CPU and chipset vendors have

introduced a rat o technologies that improve the

perormance o virtualized workloads; Intel has

developed the Intel-VT amily o technologies and

AMD, AMD-V.

All work on the principle o making the hardware more

aware o the environment in which it operates, allowing

it to co-operate with the virtualization sotware,

reducing the load on the hypervisor and hence the

perormance overhead on virtual machines. This in turn

widens the range o workloads or which virtualization

is an option. This is one option that really should not be

overlooked; ensure all your host hardware is Intel-VT or

AMD-V capable.

3.2 Use suitably specied I/O cardsRegardless o how eective virtualization sotware is,

equivalent unctionality can always be perormed more

eciently in hardware/rmware. This principle not

only led to the introduction o the above hardware

assist technologies, but also in the development o

enhancements to I/O cards. This includes eatures such

as jumbo rame support and TCP Ofoad Engine ( TOE),

as well as the traditional increments to operational

speed. Consider using I/O cards with such capabilities

in host servers, ensuring that the virtualization platorm

also supports the card and eature set.

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3.3 Use storage with suitable perormance

capabilities

Modern virtualization platorms support the use o awide variety o storage types and while the dierences

in levels o per ormance continue to diminish over time,

it remains critical to ensure the storage is able to meet

the demands o the workloads it will be supporting. For

example, a bre channel-based SAN back-ended with

15K RPM disks will oer much higher I/O rates than

NFS-based storage on 7.2K RPM SATA disks.

Thats not to say that the same storage platorm has to

be used or all virtualized workloads; using the highest

perorming platorm throughout can be prohibitively

expensive and so a tiered storage approach would

typically be adopted.

3.4 Dedicate host I/O resources to the virtual

machineBy its very nature, virtualization results in the sharing

o host resources between multiple virtual machines.

In the vast majority o cases does not cause any issues;

indeed its rom this approach that the principal benets

o virtualization are derived.

However, when a virtual machine has consistently high

I/O demands, it may be constrained by the portion o

I/O bandwidth that the virtualization platorm makes

available to it. Consider conguring the virtualization

platorm so that the relevant resource, a network card

or example, is dedicated to the virtual machine in

question.

3.5 Provide the guest operating system with

direct access to the I/O hardware

A recent development is the ability to give a virtualmachines guest operating system direct access to the

host I/O hardware, bypassing the virtualization layer

and avoiding the associated overhead altogether. This

is achieved through a combination o hardware and

virtualization platorm eatures, such as Intel VT-d orAMD IOMMU and VMware VMDirectPath. However,

it should be noted that the use o such pass-through

abilities has an impact on the portability o the virtual

machine and it is eectively becomes tied to that

specic host server.

3.6 Use paravirtualised device driversParavirtualized device drivers are developed to be

virtualization-aware and consequently result in lower

host CPU utilization and greater I/O throughput.

Typically, the vir tualization platorm has to emulate thehost I/O devices that are presented to a guest operating

system and this emulation can be very slow, resulting in

poor perormance.

With a paravirtualized device driver, a disk device or

network card will continue to appear as a normal to the

guest operating system but because the device driver

is aware o the virtualization layer and able to interact

with it directly (without emulation), disk and network

subsystems can operate at near native speeds. Note

that not all guest operating systems support the use or

paravirtualized device drivers.

3.7 Dedicate a host to the VMThe ultimate extension to the approach o dedicating a

specic resource to a virtual machine is to dedicate the

entire host to it. This technique is principally employed

to dedicate the entire compute resource (CPU and

RAM) o the host to a single virtual machine, but it

obviously has the same eect or all I/O resource (disk

and network) too. The result is that the per ormance o

the virtual machine is only aected to the extent o the

overhead imposed by the virtualization platorm.

Online resources for the virtualization industryWe simplify complex concepts and make them valuable, practical

assets. Free white papers, articles, how-to-guides, e-books,

presentation tools and much more.

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Useul Links1. Intel Virtualization Technology, http://www.intel.com/

technology/virtualization/technology.htm?iid=tech_vt+tech

2. AMD Virtualization Technology, http://www.amd.com/us/

products/technologies/virtualization/Pages/amd-v.aspx

3. Intel Virtualization Technology or Directed I/O (VT-d), http://

www.intel.com/technology/itj/2006/v10i3/2-io/1-abstract.htm

4. I/O Virtualization and AMDs IOMMU, http://developer.amd.com/documentation/articles/pages/892006101.aspx

5. Jumbo rames, http://en.wikipedia.org/wiki/Jumbo_rame

6. TCP Ofoad Engine, http://en.wikipedia.org/wiki/TCP_

Ofoad_Engine

7. Virtualizing Server Workloads, http://www.amd.com/us-en/

assets/content_type/DownloadableAssets/AMD_WP_

Virtualizing_Server_Workloads-PID.pd

TagsVirtualizing, High I/O, I /O, Workloads

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With modern sotware this overhead can be as low as 5

per cent, and this can be reduced urther through the

use o paravirtualized drivers, making it inconsequential

in the majority o cases. The natural trade os in this

approach are the whether the benets o virtualizationoutweigh the cost o virtual inrastructure licensing,

administration etc. The point o view on this could vary

signicantly between dierent companies.

4.0 ConclusionDevelopments in server hardware and advances in

virtualization technology have made it possible to

virtualize an increasingly diverse range o workloads.

In summary, those workloads that were previously

excluded as virtualization candidates due to their highI/O should be reconsidered.

This concludes the white paper.

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