Re: [PATCH 2/9] migration: Fix file migration with fdset

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, May 03, 2024 at 12:23:51PM -0400, Peter Xu wrote:
> On Fri, Apr 26, 2024 at 11:20:35AM -0300, Fabiano Rosas wrote:
> > When the migration using the "file:" URI was implemented, I don't
> > think any of us noticed that if you pass in a file name with the
> > format "/dev/fdset/N", this allows a file descriptor to be passed in
> > to QEMU and that behaves just like the "fd:" URI. So the "file:"
> > support has been added without regard for the fdset part and we got
> > some things wrong.
> > 
> > The first issue is that we should not truncate the migration file if
> > we're allowing an fd + offset. We need to leave the file contents
> > untouched.
> 
> I'm wondering whether we can use fallocate() instead on the ranges so that
> we always don't open() with O_TRUNC.  Before that..  could you remind me
> why do we need to truncate in the first place?  I definitely missed
> something else here too.

You're mixing distinct concepts here. fallocate makes a file region
non-sparse, while O_TRUNC removes all existing allocation, making it
sparse if we write at non-contiguous offsets. I don't think we would
want to call fallocate, since we /want/ a sparse file so that we
don't needlessly store large regions of all-zeros as RAM maps.


With regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|