Re: romfile resize

[Dr. David Alan Gilbert]

* Jiatong Shen (yshxxsjt715@gmail.com) wrote:
> Hi,
> 
>   Thank you very much for the answer. so if romfile on destination got a
> larger size than source, why romfile check still does not
> pass? because dest got enough space to hold romfile.

Right.

Dave

> thank you.
> 
> Jiatong Shen
> 
> On Tue, Feb 23, 2021 at 5:46 PM Dr. David Alan Gilbert <dgilbert@redhat.com>
> wrote:
> 
> > * Philippe Mathieu-Daudé (philmd@redhat.com) wrote:
> > > Cc'ing qemu-devel@
> > >
> > > On 2/23/21 1:45 AM, Jiatong Shen wrote:
> > > > Hello,
> > > >
> > > >   we are faced with an issue where a changed sized romfile
> > > > (efi-virtio.rom) fails live migration. Do qemu load this rom from its
> > > > current host only? If yes, why cannot sync this from source vm?
> >
> > Hi,
> >   For migration to work the ROM has to be the same size on the source
> > and destination.
> >
> >   The problem is that whne the destination starts up it allocates the
> > size of the ROM based on the size of the file;  but then the migration
> > comes along and tries to copy the data from the source machine into that
> > allocation; and isn't sure what should happen when it doesn't quite fit.
> >
> >   There is some variation allowed (I think the allocated size gets
> > rounded up, maybe to the next power of 2); but you still hit problems
> > wehn the ROM size crosses certain thresholds.
> >
> >   In the latest qemu, a 'romsize' property was added (see git commit
> > 08b1df8ff463e72b0875538fb991d5393047606c ); that lets you specifiy a
> > size that's big enough to hold some space for future expansion - e.g.
> > lets say your ROM is currently 300k, you might specify romsize=512k
> > and then it doesn't matter what size the actual file is, we'll always
> > allocate 512k, and as long as the file is less than 512k migration will
> > work.
> >
> >   The more manual way to do that, is to arrange for your files to be
> > padded to a larger boundary so that you leave room for growth.
> > Some distros have done that for a while.
> >
> > Dave
> >
> > > > thank you.
> > > >
> > > > --
> > > >
> > > > Best Regards,
> > > >
> > > > Jiatong Shen
> > >
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> >
> >
> 
> -- 
> 
> Best Regards,
> 
> Jiatong Shen
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK