Re: [Qemu-devel] [PATCH v12 07/17] qmp: Add support of "dirty-bitmap" sync mode for drive-backup

[Max Reitz <mreitz@redhat.com>]

On 2015-02-11 at 12:54, John Snow wrote:
>
> On 02/11/2015 12:47 PM, Max Reitz wrote:
>> Looks good to me in general, now I need to find out what the successor
>> bitmap is used for; but I guess I'll find that out by reviewing the rest
>> of this series.
>>
>> Max
>
> They don't really come up again, actually.
>
> The basic idea is this: While the backup is going on, reads and writes 
> may occur (albeit delayed) and we want to track those writes in a 
> separate bitmap for the duration of the backup operation.

Yes, I thought as much; but where are writes to the named bitmap being 
redirected to its successor? bdrv_set_dirty() doesn't do that, as far as 
I can see.

> If the backup operation fails, we use the dirty sector tracking info 
> in the successor to know what has changed since we started the backup, 
> and we merge this bitmap back into the originating bitmap; then if an 
> incremental backup is tried again, it includes all of the original 
> data plus any data changed while we failed to do a backup.
>
> If the backup operation succeeds, the originating bitmap is deleted 
> and the successor is installed in its place.
>
> It's a namespace trick: by having an anonymous bitmap as a child of 
> the "real" bitmap, the real bitmap can be frozen and prohibited from 
> being moved, renamed, deleted, etc. This prevents the user from adding 
> a new bitmap with the same name or similar while the backup is in 
> progress.

Hm, if it's just for that, wouldn't disabling the bitmap suffice?

Max

> A previous approach was to immediately take the bitmap off of the BDS, 
> but in the error case here, the logic becomes more complicated when we 
> need to re-install the bitmap but the user has already installed a new 
> bitmap with the same name, etc.
>
> So the general lifetime is this:
>
> (1) A backup is started. the block/backup routine calls create_successor.
> (2) If the backup fails to start, the block/backup routine will call 
> the "reclaim" method, which will merge the (empty) successor back into 
> the original bitmap, unfreezing it.
> (3) If the backup starts, and then fails, the bitmap is "reclaim"ed 
> (merged back into one bitmap.)
> (4) If the backup succeeds, the bitmap "abdicates" to the successor. 
> (The parent bitmap is erased and the successor is installed in its 
> place.)

Yes, see the graph at the whiteboard behind me. :-)

Max