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

[Bohdan Trach <bohdan.trach@mailbox.tu-dresden.de>]

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.

> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> 

-- 
With best regards,
Bohdan Trach