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Re: [vfio-users] Speed tips requested

From what I've read Red Hat recommends virtio with raw on no cache with io thread due to the reasons listed above. Not sure about LVM but they did also say (or someone did) do not use BTRFS for keeping the image. 

The only optimization I would immediately recommended is to do host-passthrough as the CPU option in your guests xml. It's noticeably different assuming you haven't already done it

On Mar 26, 2017 11:13 AM, "Bronek Kozicki" <brok spamcop net> wrote:
On 26/03/2017 15:31, Alex Williamson wrote:
On Sun, Mar 26, 2017 at 4:33 AM, Patrick O'Callaghan <poc usb ve
<mailto:poc usb ve>> wrote:

    On Sun, 2017-03-26 at 10:58 +0100, Bronek Kozicki wrote:
    > Assuming you use libvirt, make sure to use vCPU pinning. For disk access, try cache='writeback' io='threads'. If you switch to scsio-vfio, this will give you the ability to define queue length which might additionally improve IO. Also, try QCOW2 format for guest disk, it might enable some additional optimizations. However given you host seem to have little spare capacity, YMMV

    Thanks. I'm already using CPU pinning as I said. The disk options are
    both set to "hypervisor default" so I'll try changing them. I'd
    configured the guest disk as 'raw' assuming that would be faster than
    QCOW2 but I'll look into it.

Generally the recommendation is to use raw (not sparse), LVM, or a block
device for the best performance.  QCOW is never going to be as fast as
these at writing unused blocks since it needs to go out and allocate new
blocks from the underlying file system when this happens.

I am not going to argue with your experience here, only wanted to note that QCOW2 can be created with preallcation=falloc (or full, which is not very useful) which means there will no extra allocations in the rutime. Everything will be allocated at the moment of disk creation with qemu-img create -f qcow2 -o preallcation=falloc ....


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