Re: [Qemu-devel] RFC: incremental backups: qmp-block-dirty-bitmap-diff

[John Snow <jsnow@redhat.com>]

On 02/14/2016 03:02 AM, Fam Zheng wrote:
> On Tue, 02/09 19:48, John Snow wrote:
>> - Reading an entire drive to populate a bitmap with the understanding
>> that an incremental backup is soon to follow is inefficient if the drive
>> is more than just a little dirty: it may have been quicker to just
>> create a new full backup and bitmap.
> 
> Above all I think this is a good idea. Just as an alternative, can we add
> another sync mode for drive-backup?
> 
>     (QMP) drive-backup device=d0 target=target.qcow2 format=qcow2 sync=diff \
>                        base=full-backup.qcow2
> 
> (the data of d0 will be compared against full-backup.qcow2 and only different
> clusters will be copied to target.qcow2)
> 
> Fam
> 

I like this idea too, since it has the chance to do a lot of things for
us all at once. If we allow a user to pass a bitmap that we can
synchronize to this backup, it will be one command that can do a few
things all at once.

Using the "bitmap-diff" approach [re-]establishes a bitmap, but still
requires you to make the next backup. sync=diff just combines the two
logical steps into one.


There are two phases here:
[Perform the diff] [Create a backup from that diff]


backup mode=diff as suggested above performs both,
block-dirty-bitmap-diff as suggested previously performs just the first
action, while we already have commands to perform the second.

I think arguably we'd only /need/ the bitmap-diff command, but I
wouldn't personally mind having both for flexibility reasons (which
historically don't fly too far on the qemu mailing list...)


I think I will move ahead with prototyping the QMP command and see how
far I get. If it's not too bad I'll send some code out for further debate.

--js