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



On Thu, Nov 16, 2017 at 06:00:54PM +0000, Richard W.M. Jones wrote:
> On Thu, Nov 16, 2017 at 05:30:48PM +0000, Stefan Hajnoczi wrote:
> > On Thu, Nov 16, 2017 at 3:10 PM, Richard W.M. Jones <rjones redhat com> wrote:
> > > Both local filesystems, but on different SATA devices.
> > 
> > Okay.  I'm curious what the strace -f output looks like (only the
> > preadv(2)/pwritev(2) syscalls are interesting at the moment).
> 
> Here's the full strace from one run:
> 
>   http://oirase.annexia.org/tmp/strace.xz
> 
> The command is (with the output.qcow2 file created in a previous run):
> 
>   $ strace -o /tmp/strace -f -s 1024 qemu-img convert -f raw fedora-27.img -O qcow2 /var/tmp/output.qcow2 -m 8 -W -n
> 
> The version of qemu is: qemu-2.10.0-7.fc28.x86_64

Requests are being handled in parallel and the exact order is random due
to I/O completion times and thread scheduling.

I didn't see an obvious pattern to optimize but only spent a little time
studying the strace.

Stefan

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