[Xen-API] XCP - Performance Issues with Disk-I/O for small block-sizes as well as poor memory i/o

["Balg, Andreas" <a.balg@xxxxxxxx>]

Hello everybody,

during some extensive benchmarking for an evaluation of virtualization 
technologies we found some issues with i/o-performance and also memory 
bandwidth of XCP-0.5 running
on a quite up-to-date and performant Dell-R610 Server (2 x Xeon E5620, 16G RAM,
4 x SATA 15K HDD's - RAID 5):

We ran the tests on plain hardware, using CentOS 5.5 with Xen-Kernel in a 
single VM as well as 7 VMs (Guests restricted to 2 VCPUs, 2GB RAM) at the same 
time:

- Especially for small block sizes (below 32k) the Disk-I/O is very poor. 

to give some figures:  The same Benchmark 
 
"time iozone -az -i0 -i1 -i2 -i8 -Rb results.xls"

Runs around 3 Minutes on the bare Hardware, around 30 Minutes in a KVM-VM and 
more than 1 hour(!) in a xen VM - See attached graphs and focus on the front of 
the diagram (red and blue "foot" of the xen graph) 

What
 I'd like to know is, if this is be a glitch in a device driver, an 
error in our configuration or might be eliminated in any other way.

Or is this a issue of hypervisor design or just nobody noticed it so far and it 
should be 
looked at by the developers. 

Without these two significant problems Xen 
would outperform kvm in almost any possible manner .... 

Best regards 
Andreas



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Attachment: RAM-Bandwidth.png
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