Re: [Qemu-devel] [PATCH 2/5] migration: Create block capability

[Eric Blake <eblake@redhat.com>]

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On 05/16/2017 11:00 AM, Markus Armbruster wrote:

>>>>  #          periodic mode. (Since 2.8)
>>>>  #
>>>> +# @block-incremental: enable block incremental migration (Since 2.10)
>>>> +#
>>>
>>> What's "block incremental migration" and why should I care?
>>
>> This is good, I will try.
>>
>> "block incremental migration assumes that we have a base image in both
>> sides, and then we continue writting in one of the sides.  This way we
>> need to only migrate the changes since the previous state where it was
>> the same in both sides".
>>
>> I am not sure what to put there, really.
> 
> Well, to suggest something, I'd first have to figure out WTF incremental
> block migration does.  Your text helps me some, but not enough.  What
> exactly is being migrated, and what exactly is assumed to be shared
> between source and destination?
> 
> Block migration is scandalously underdocumented.

If I have:

base <- active

on the source, then:

block migration without incremental creates:

active

on the destination (the entire disk contents are migrated).  Conversely,
block migration WITH incremental assumes that I have pre-created 'base'
on the destination (easy to do, since base is read-only so it can be
copied prior to starting the migration), and the migration results in:

base <- active

on the destination, where only the contents of active were transferred
by qemu.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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