Re: [Xen-users] poor IO performance with dom0 on centos7

[Mike <mike+xen@xxxxxxxxxxxxxxxxx>]

On 09/15/2015 12:01 PM, Christoph wrote:
> Im afraid it wasnt the solution, with centos7 as domU it is much 
> better/faster but I still have a IO problem on the NFS domU...
> Any other hints for me to make it faster? (except buy new hardware, it 
> was fast enough with centos6 as dom0)
I would like to add to this general thread:

I also have performance problems with xen and nfs. My dom0 uses NFS to 
mount a directory containing all of my virtual machine hard disk images, 
and there are situations in which NFS performance simply drops or stops 
completely, causing the domU kernels to all complain about task blocked 
for > 120 seconds, etc. I have only been able to address the symptom by 
moving the highest apparent virtual host to a dedicated box that only 
runs that instance, and that seems to have mostly resolved the issue. 
The problem isn't so much nfs performance itself, but some issue within 
the dom0 kernel or system where it's simply running out of some resource 
or getting IO starved for some reason. I literally have had the IO for a 
number of running VM's on one host, to come to a complete stand 
still.... effectively bringing the vm into deadlock...but killing the 
affected VMs and restarting them immediately fixes the problem (but 
results in other difficult problems like corrupt databases and such). So 
even tho my symptoms are mostly resolved by moving things around, the 
fact is, there is some underlaying performance problem in dom0 affecting 
nfs, I just don't know how to really quantify it.


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