[vfio-users] vm lags/pauses when IO overloaded on host

[Marcin Falkiewicz]

ZFS's zvol is really bad for virtual disk backend when you have only few HDDs (or even SSDs) - I had a lot problems with latency and throughput on my 2-disk mirrored testbench. Hard to tell if it's ZFS itself, ZoL or just not enough disks and slow controller.

On the other hand, LVM runs great on single SSD (directsync+native) or even RAID1 HDDs (mdadm; none+native), achieving close to bare metal performance with virtio-scsi.

On 07/19/2016 04:12 PM, Jiri 'Ghormoon' Novak wrote:
> Hi,
>
> for quite long time, I've got an issue with my drives configuration on
> my VMs that do vga passthrough (but I think it's unrelated to
> passthrough as they would lag without passthrough too).
> the config is
>
> -drive
> file=/dev/something,if=none,id=drive-virtio-disk0,format=raw,cache=none,aio=native
> -device
> virtio-blk-pci,scsi=off,bus=pci.2,addr=0x1,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1
>
> what parameter am I missing so the guest would be waiting for IO and not
> the host pausing it until it can provide the IO?
>
> for some time I thought it was windows problem ,but it also shows on my
> linux guest when the IO lacks.
>
> if it matters, the backend is zfs zvol. that's why especially on HDDs it
> hurts a lot. on SSD it's not that bad but still if I load the IO a lot
> from multiple sources, it has problem.
>
> Thanks,
> Ghor
>
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