[vfio-users] Poor disk (SSD) performance inside Windows VM

[Billington, Mark]

Hi vfio-users.

Bit of background... Im quite new to the world of libvirt\kvm\qemu etc and have a background in vmware.
Ive used Unraid for quite a while and they've recently implemented VM support via libvirt, which led me down the path of virtualising my office PC and my media centre.

My Host machine specs are: 2xXeon E5520, 24GB DDR3, 2x240GB SSD (Both on an LSI SAS 2008 controller) , 8x2TB HDD (onboard mobo controller)
Im able to set up VMs, pass through a variety of devices (Sound cards\USB controllers\Graphics cards) fine, but my windows VM (which I use as my main office PC) seems to suffer when it comes to disk performance.

On the host, the SSD performs as expected. Inside VMs I get the following (BARE METAL\VM):
Sequential Read: 510MB/s \ 460MB/s                     -10%
Sequential Write: 471MB/s \ 197MB/s                    -58%
4k Read: 28.10MB/s \ 7.55MB/s                                 -73%
4k Write: 74.90MB/s \ 5.73MB/s                                -92%
4k-64threads Read: 320.30MB/s \ 215.27MB\s    -33%
4k-64threads Write: 273.94MB/s \ 96.00MB\s     -64%
Access time Read: 0.052ms \ 0.589ms                     -lots%
Access time Write: 0.051ms \ 0.899ms                    -lots%

While performance is acceptable (just!) on the VM, id like to get a little closer to 'bare metal' speeds. I appreciate that there will be some overhead, but it shouldn't be this much!

My VM XML is here: http://pastebin.com/52BwffmC

So far I've experimented with:


*         IOTHREADS - Assigned designated IOTHREADS to CPUs and to the disk controller (still in my XML).

*         Cache and IO settings changed to: 'cache='none' io='native' ' - no performance change.

*         Virtio and virtio-scsi controllers make no difference to performance.

*         Controller pass through - I tried to pass through the entire LSI2008 controller, however windows refused to detect any drives (despite installing drivers during windows installation). Linux detected the drives and installation completed, however I wasn't able to boot from the disk after the installation. Not sure how to set the LSI controller as a bootable device.

*         CPU and disk schedulers changed on the host - no performance change.

Im running out of ideas on what else to tweak at this point! Would the best way for me to achieve bare metal speeds would be to have a dedicated PCIe controller passed through (2 or 4 port SATA3 controller?)? Do any exist that play nice with libvirt and can be used as a boot device?




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