Re: [libvirt-users] virtual drive performance

Hi, Thank you for your input. We already tried several tweaks but without luck. For example adding io='native' did not help improve the performance. It behaved exactly the same way before and after. I've read somewhere that cache='writethrough' could also help improving the performance, but we cannot do that because we take live snapshots to backup the machine while it runs. When cache is enabled, we observed that sometimes an external live snapshot cannot be merged with blockcommit without the host being shut down. Would you please explain what <cpu mode='host-passthrough' /> should do to improve the performance? Switching from IDE to virtio basically means that the host then knows that it runs on virtualized hardware and can do things differently? But it also requires to modify the host with specialized drivers that even influence the boot process. That feels more like a hack than a solution. We're astonished why the virtualized IO is so much slower. I could understand a performance penalty of 10% or even 20%, but a drop from 120Mb/s IO read to 1.4Mb/s IO read is suspicious to everyone of us. We'd have expected at least a throughput of 50Mb/s while reading from disk which is more than half the IO that the hardware can do. Please note that we do not observe the hosting machine to peak 100% with CPU or IO (using top and iotop) when the virtualized host does some io. Is there a lock contention or something else going on? When running a virtualized host for example with virtual box we don't see such an impact. What does virtual box do differently to improve virtualized IO and could that help libvirt/qemu/kvm? On 2017-06-15 04:08, Andrea Bolognani wrote: