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

[Markus Armbruster]

Eric Blake <address@hidden> writes:

> 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.

Can you draft a documentation comment for @block-incremental?