This probably more appropriate for the qemu users mailing list, but that list 
doesn’t get much traffic and most posts go unanswered…

As I’ve mentioned in the past, I’m migrating my environment from ESXi to oVirt 
AIO.

Under ESXi I was pretty happy with the disk performance, and noticed very 
little difference from bare metal to HV.

Under oVirt/QEMU/KVM, not so much….

Running hdparm on the disk from the HV and from the guest yields the same 
number, about 180MB/sec (SATA III disks, 7200RPM).  The problem is, during disk 
activity, and it doesn’t matter if it’s Windows 7 guests or Fedora 20 (both 
using virtio-scsi) the qemu-system-x86 process starts consuming 100% of the 
hypervisor CPU.  Hypervisor is a Core i7 950 with 24GB of RAM.  There’s 2 
Fedora 20 guests and 2 Windows 7 guests.  Each configured with 4 GB of 
guaranteed RAM.

Load averages can go up over 40 during sustained disk IO.  Performance 
obviously suffers greatly.

I have tried all combinations of having the guests on EXT 4, BTRFS and using 
EXT 4 and BTRFS inside the guests, as well as direct LUN.  Doesn’t make any 
difference.  Disk IO sends qemu-system-x86 to high CPU percentages.

This can’t be normal, so I’m wondering what I’ve done wrong.  Is there some 
magic setting I’m missing?  


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