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From Andrei Mikhailovsky <>
Subject Disk offering QOS issues / question
Date Thu, 17 Jul 2014 12:04:51 GMT
Hello guys, 

I've been playing around with disk QOS on ACS 4.2.1 and KVM and noticed several issues. 

The first issue that I've discovered is that if you attach a QOS enabled disk to a _running_
vm the QOS settings on the disk are not passed on to the hypervisor. Example of the xml file
using virsh dumpxml: 

<disk type='network' device='disk'> 

<driver name='qemu' type='raw' cache='none'/> 

<auth username='admin'> 

<secret type='ceph' uuid='cf771bc7-8998-354d-8e10-5564585a3c20'/> 


<source protocol='rbd' name='Primary-ubuntu-1/ee1551bc-50ce-42b1-85a3-9441ac43e1b0'>

<host name='ceph-mon' port='6789'/> 


<target dev='vde' bus='virtio'/> 

<alias name='virtio-disk4'/> 

<address type='pci' domain='0x0000' bus='0x00' slot='0x08' function='0x0'/> 


where as it should have an <iotune> block like this: 







However, if you stop and start the guest vm, the <iotune> blocked is passed on like
it should. Has anyone noticed this behaviour? 

Second issue that i've discovered (and it is likely an issue with KVM itself) is that the
QOS limitation for the read throughput seems to be 4 times lower in benchmarks than what is
shown in iotune block. For example, i've limited the read and write throughput to 100,000,000

The benchmarks are showing me correct throughput for the write benchmarks, but the read benchmarks
are giving me about 25-26MB/s instead of around 100MB/s. Has anyone seen this behaviour? 


Andrei Mikhailovsky 
Arhont Information Security 

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