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Re: [Qemu-devel] [PATCH RFC 3/3] migration: use checkpoint during migrat
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH RFC 3/3] migration: use checkpoint during migration |
Date: |
Tue, 17 Nov 2015 16:39:59 +0000 |
User-agent: |
Mutt/1.5.24 (2015-08-30) |
* Bohdan Trach (address@hidden) 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 / address@hidden / Manchester, UK
> >
>
> --
> With best regards,
> Bohdan Trach
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK