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Re: romfile resize

From: Jiatong Shen
Subject: Re: romfile resize
Date: Tue, 23 Feb 2021 17:57:45 +0800


  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.

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?

  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

  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.


> > thank you.
> >
> > --
> >
> > Best Regards,
> >
> > Jiatong Shen
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK


Best Regards,

Jiatong Shen

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