Re: [Qemu-devel] [PATCH RFC 3/3] migration: use checkpoint during migration

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Bohdan Trach (bohdan.trach@mailbox.tu-dresden.de) wrote:
> 
> On 11/17/2015 05:05 PM, Dr. David Alan Gilbert wrote:
> > Why is the hash needed on the destination; if it's a page which the source
> > has decided isn't in a matching page, what does the destination use the
> > hash for?
> >
> 
> After the migration has finished, the hashes are still stored in RAM
> for the next migration, when the current destination becomes the new
> migration source. This way there is no need to recompute the checksums
> on the next migration -- they are already in RAM.
> 
> >>> I think there's a problem here that given the source is still running it's CPU and changing
> >>> memory; it can be writing to the page at the same time, so the page you send might not
> >>> match the hash you send;  we're guaranteed to resend the page again if it was written
> >>> to, but that still doesn't make these two things match; although as I say above
> >>> I'm not sure why SAVE_FLAG_PAGE_HASH exists.
> >>
> >> This is true. In this case, we will just delete the SAVE_FLAG_PAGE_HASH flag.
> > 
> > But how do you know to delete the SAVE_FLAG_PAGE_HASH flag?
> > 
> 
> Sorry for not stating this clear enough. We will remove this flag from
> the code, and send pages with SAVE_FLAG_PAGE instead. In this case the
> destination will compute the hash.

OK, that's fine.

Dave

> 
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> > 
> 
> -- 
> With best regards,
> Bohdan Trach
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK