Performance Tweaks

Note: Article about Proxmox VE 2.0

Introduction

This page is intended to be a collection of various performance tips/tweaks to help you get the most from your virtual servers.

KVM

USB Tablet Device

Disabling the USB tablet device in windows VMs can reduce idle CPU usage and reduce context switches. [1]

VirtIO

Use virtIO for disk and network for best performance.

Linux has the drivers built in

Windows requires drivers which can be obtained here: Fedora VirtIO Drivers

Disk Cache

Note: The information below is based on using raw volumes, other volume formats may behave differently.

cache=none seems to be the best performance and is the default in Proxmox 2.X.

cache=directsync seems similar in performance to cache=none

cache=writethrough should help speed up reads since it can use the host disk cache for reads but in practice it performs worse than cache=none

cache=writeback should help with reads and writes at the risk of loosing data, in practice it performs worse than cache=none.

caching adds additional data copies and bus traffic causing it to perform worse.

Application Specific Tweaks

Microsoft SQL Server

Trace Flag T8038

Setting the trace flag -T8038 will drastically reduce the number of context switches when running SQL 2005 or 2008.

To change the trace flag:

1. Open the SQL server Configuration Manager
2. Open the properties for the SQL service typically named MSSQLSERVER
3. Go to the advanced tab
4. Append ;-T8038 to the end of the startup parameters option

For additional references see: Proxmox forum