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Re: [Qemu-devel] Effect of qemu-img convert -m and -W options

From: Richard W.M. Jones
Subject: Re: [Qemu-devel] Effect of qemu-img convert -m and -W options
Date: Thu, 16 Nov 2017 15:10:32 +0000
User-agent: Mutt/1.5.20 (2009-12-10)

On Thu, Nov 16, 2017 at 02:47:46PM +0000, Stefan Hajnoczi wrote:
> The threads you observed are the thread pool that performs
> preadv(2)/pwritev(2) syscalls.  The Linux AIO API could be used instead
> and does not use threads for read and write operations.

I guess if I used AIO then I wouldn't get any parallelism at all since
Linux doesn't block on local file access (at least, it never used to)?

> Interesting.  Did you perform multiple runs of each setting to verify
> that the benchmark results are stable with little volatility?

I retested the -m 8 no-W/-W ones because those were so unexpected and
those are repeatable.

> Which command-line did you use to create the preallocated qcow2 file?

What I actually did was qemu-img convert -n into the existing qcow2
file, so there was no separate command for that.

> Are the source and target files on the same file system and host block
> device?  The benefit of using multiple coroutines depends on the
> performance characteristics of the source and target files.

Both local filesystems, but on different SATA devices.


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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