Re: What does qemu-img convert -W documentation mean?

[Richard W.M. Jones]

On Wed, May 26, 2021 at 04:01:01PM +0200, Jakob Bohm wrote:
> On 2021-05-26 15:40, Richard W.M. Jones wrote:
> >Firstly there's a real interesting thread about nbdcopy vs qemu-img
> >convert performance:
> >https://listman.redhat.com/archives/libguestfs/2021-May/thread.html#00119
> >
> >I have a question about qemu-img convert -W flag.  The documentation
> >says:
> >
> >  -W  Allow out-of-order writes to the destination.  This  option  im‐
> >      proves performance, but is only recommended for preallocated de‐
> >      vices like host devices or other raw block devices.
> >
> >I don't understand why out of order writes are bad for things like raw
> >files and qcow2 files, or for non-preallocated block devices.
> >Wouldn't they always be a win?
> >
> >Rich.
> >
> Out of order writes to things that allocate one block at a time from the
> lower layer would cause the mapping from virtual clusters to clusters at
> the lower layer to befragmented, literally out of order, which hurts
> performanceof future access.

Thanks.  If I'm understanding this correctly, then it would be more
accurate to say that -W is only recommended for preallocated targets
[instead of "devices"], and listing preallocated raw format files as
an additional possibility.  If that's accurate then I'll come up with
a patch.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
Fedora Windows cross-compiler. Compile Windows programs, test, and
build Windows installers. Over 100 libraries supported.
http://fedoraproject.org/wiki/MinGW