[CentOS-virt] CentOS 6 kvm disk write performance
nux at li.nux.ro
Sat Aug 11 08:31:01 EDT 2012
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
> - 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?
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?
Sent from the Delta quadrant using Borg technology!
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