building a list of KVM workloads

jurgen.depicker at jurgen.depicker at
Wed Sep 14 07:49:03 UTC 2011

> From: "Serge E. Hallyn" <serge.hallyn at>
> To: ubuntu-server <ubuntu-server at>
> Cc: jurgen.depicker at, Mark Mims <mark.mims at>
> Date: 13/09/2011 17:07
> Subject: Re: building a list of KVM workloads
> Thanks, guys.  Unfortunately I'm having a harder time thinking through
> how to properly classify these by characteristics.  Here is an 
> attempt:
>   * source code hosting (github, gitosis, etc)
>     - characteristics?
>   * checkpointable (i.e. Mark's single point backup gitosis vms)
>     - qcow2 or qed based for snapshotting?
>   * web hosting
>     - characteristics?
>   * Network performance (hard to generalize)
>     - vpn
>     - various appliation layer/tiers
>     - characteristics?
>   * db hosting
>     - characteristics?
>   * desktop virtualization
>     - ideally, using spice?
Yes, but i haven't tried yet since installation is not 'standard' yet.

>     - should survive unexpected host reboots?
This is something REALLY important which, as far as i know, is better 
managed with RedHat too :-(.  I nearly died when I accidentally typed 
'reboot' in the wrong terminal (after which i installed molly-guard 
everywhere) and when i noticed there was no clean shutdown of the guests; 
more even: that reboot corrupted some of the running windows-Vms...
I did some research on that, but didn't find time to properly synthetise 
it and implement the stuff I found (basically, the init scripts used in 
redhat as far as i remember).

>   * windows workloads ?
>     - characteristics?
> I'll probably put these up on the wiki soon so we can all edit, but in
> the meantime if you have any suggestions for improving the grouping or
> filling in characteristics, please speak up.

I noticed that most of my load is due to cpu wait: disk IO I guess.  Most 
troubles with too much 'wait' are due to windows VMs.

All my VMs use qcow2.  There is an option, when you create the disk images 
manually, to 'preallocate', which is supposed to increase the performance 
a lot: -o preallocation=metadata :
From KVM I/O slowness on RHEL 6
So, if you are using Red Hat Enterprise Linux or Fedora Linux as the host 
operating system for you virtualization server and you plan to use the 
QCOW2 format, remember to manually create preallocated virtual disk files 
and to use a “none” cache policy (you can also use a “write-back” policy, 
but be warned that your guests will be more prone to data loss).
If you can confirm this article, then I guess this should be a default 
option when creating disk images from the GUI VMManager

> BTW - using VMS to host VMS *is* in fact doable, at least on AMD.  I've
> done it in the past just for debugging qemu bugs in several releases on
> a remote, borrowed AMD laptop.  It might be worth adding to the list,
> as it likely will have its own tuning requirements.
> thanks,
> -serge

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