[CentOS-virt] CentOS 6 kvm disk write performance

Nux!
Sat, 11 Aug 2012 12:31:01 -0000

On 10.08.2012 13:46, Julian price 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

How are you running KVM machines? Through libvirt? Have you made any
changes to the cgroups?  What are you using as disks, raw files, lvm?

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Nux!