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.

     host don't do cache. 
     guest disk cache is writeback
     Warn : like writeback, you can loose datas in case of a powerfailure
     you need to use barrier option in your linux guest fstab if kernel < 2.6.37 to avoid fs corruption in case of powerfailure.       
     

cache=writethrough

     host do read cache
     guest disk cache mode is writethrough
     Writethrough make a fsync for each write. So it's the more secure cache mode, you can't loose data. It's also the slower.
   

cache=directsync.

     host don't do cache.
     guest disk cache mode is writethrough
     similar to writethrough, a fsync is made for each write.
    

cache=writeback

     host do read/write cache
     guest disk cache mode is writeback
     Warn : you can loose datas in case of a powerfailure
     you need to use barrier option in your linux guest fstab if kernel < 2.6.37 to avoid fs corruption in case of powerfailure.
      

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

For read cache memory: try to add more memory in your guest, they already do the job with their buffer cache

cache=directsync and writethrough can be fast, if you have a san or raid controller with cache with battery by exemple

Avoid to use cache=directsync and writethrough with qcow2 files.

some interestings articles : barriers : http://monolight.cc/2011/06/barriers-caches-filesystems/ cache mode and fsync : http://www.ilsistemista.net/index.php/virtualization/23-kvm-storage-performance-and-cache-settings-on-red-hat-enterprise-linux-62.html?start=2

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