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Re: [vfio-users] Speed tips requested



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 ....



B.


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