Re: [Qemu-devel] Very poor IO performance which looks like some design problem.

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/04/2015 22:38, ein wrote:
> 
> Qemu creates more than 70 threads and everyone of them tries to write to
> disk, which results in:
> 1. High I/O time.
> 2. Large latency.
> 2. Poor sequential read/write speeds.
> 
> When I limited number of cores, I guess I limited number of threads as
> well. That's why I got better numbers.
> 
> I've tried to combine AIO native and thread setting with deadline
> scheduler. Native AIO was much more worse.
> 
> The final question, is there any way to prevent Qemu for making so large
> number of processes when VM does only one sequential R/W operation?

Use "aio=native,cache=none".  If that's not enough, you'll need to use
XFS or a block device; ext4 suffers from spinlock contention on O_DIRECT
I/O.

Paolo