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For the case of simply using local disk mounted for /var/lib/nova and 
raw disk image type, you could try adding to nova.conf:

    preallocate_images = space

This implicitly changes the I/O method in libvirt from "threads" to 
"native", which in my case improved performance a lot (10 times) and 
generally is the best performance I could get.

Best Regards
Daniel

On 8/2/2019 10:53, Budai Laszlo wrote:
> Hello all,
>
> we have a problem with the performance of the disk IO in a KVM instance.
> We are trying to provision VMs with high performance SSDs. we have investigated different possibilities with different results ...
>
> 1. configure Nova to use local LVM storage (images_types = lvm) - provided the best performance, but we could not migrate our instances (seems to be a bug).
> 2. use cinder with lvm backend  and instance locality, we could migrate the instances, but the performance is less than half of the previous case
> 3. mount the ssd on /var/lib/nova/instances and use the images_type = raw in nova. We could migrate, but the write performance dropped to ~20% of the images_types = lvm performance and read performance is ~65% of the lvm case.
>
> do you have any idea to improve the performance for any of the cases 2 or 3 which allows migration.
>
> Kind regards,
> Laszlo
>
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