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
