[CentOS-virt] CentOS 6 kvm disk write performance

[Philip Durbin]

Nice post, Julian. It generated some feedback at http://irclog.perlgeek.de/crimsonfu/2012-08-10 and a link to http://rhsummit.files.wordpress.com/2012/03/wagner_network_perf.pdf

Phil

On Aug 10, 2012, at 8:46 AM, Julian price <centos.org at julianprice.org.uk> wrote:

> I have 2 similar servers. Since upgrading one from CentOS 5.5 to 6, disk write performance in kvm guest VMs is much worse.
> 
> There are many, many posts about optimising kvm, many mentioning disk performance in CentOS 5 vs 6.  I've tried various changes to speed up write performance, but northing's made a significant difference so far:
> 
> - Install virtio disk drivers in guest
> - update the host software
> - Update RAID firmware to latest version
> - Switch the host disk scheduler to deadline
> - Increase host RAM from 8GB to 24GB
> - Increase guest RAM from 2GB to 4GB
> - Try different kvm cache options
> - Switch host from ext4 back to ext3
> - Set noatime on the virtual disk image file
> Note: There is no encryption or on-access virus scanner on any host or guest.
> 
> Below are some the block write figures in MB/s from bonnie++ with various configurations:
> 
> First, figures for the hosts show that the CentOS 6 server is faster:
> 
> 54    CentOS 5 Host
> 50    CentOS 5 Host
> 69    CentOS 6 host
> 70    CentOS 6 host
> 
> Figures for a CentOS 6 guest running on the CentOS 5 host show that the performance hit is less than 50%:
> 
> 30    CentOS 6 guest on CentOS 5 host with no optimisations
> 27    CentOS 6 guest on CentOS 5 host with no optimisations
> 32    CentOS 6 guest on CentOS 5 host with no optimisations
> 
> Here are the figures a CentOS 6 guest running on the CentOS 6 host with various optimisations.  Even with these optimisations, performance doesn't come close to the un-optimised guest running on the CentoOS 5 host:
> 
>  5   No optimisations (i.e. same configuration as on CentOS 5)
>  4   deadline scheduler
>  5   deadline scheduler
> 15   noatime,nodiratime
> 14   noatime,nodiratime
> 15   noatime
> 15   noatime + deadline scheduler
> 13   virtio
> 13   virtio
> 10   virtio + noatime
>  9   virtio + noatime
> 
> The CentOS 6 server has a better RAID card, different disks and more RAM, which might account for the better CentOS 6 host performance.  But why might the guest write performance be so much worse?
> 
> Is this a known problem?  If so, what's the cause?    If not, is there a way to locate the problem rather than using trial and error?
> 
> Thanks,
> Julian
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