Re: [Qemu-devel] [PATCH v2 12/18] block/backup: add 'always' bitmap sync policy

[John Snow]

On 7/4/19 2:05 PM, Max Reitz wrote:
> On 04.07.19 19:43, Max Reitz wrote:
>> On 03.07.19 23:55, John Snow wrote:
>>> This adds an "always" policy for bitmap synchronization. Regardless of if
>>> the job succeeds or fails, the bitmap is *always* synchronized. This means
>>> that for backups that fail part-way through, the bitmap retains a record of
>>> which sectors need to be copied out to accomplish a new backup using the
>>> old, partial result.
>>>
>>> In effect, this allows us to "resume" a failed backup; however the new 
>>> backup
>>> will be from the new point in time, so it isn't a "resume" as much as it is
>>> an "incremental retry." This can be useful in the case of extremely large
>>> backups that fail considerably through the operation and we'd like to not 
>>> waste
>>> the work that was already performed.
>>>
>>> Signed-off-by: John Snow <address@hidden>
>>> ---
>>>  block/backup.c       | 25 +++++++++++++++++--------
>>>  qapi/block-core.json |  5 ++++-
>>>  2 files changed, 21 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/block/backup.c b/block/backup.c
>>> index 9cc5a7f6ca..495d8f71aa 100644
>>> --- a/block/backup.c
>>> +++ b/block/backup.c
>>> @@ -266,16 +266,25 @@ static void backup_cleanup_sync_bitmap(BackupBlockJob 
>>> *job, int ret)
> 
> [...]
> 
>>> +        /* If we failed and synced, merge in the bits we didn't copy: */
>>> +        bdrv_dirty_bitmap_merge_internal(bm, job->copy_bitmap,
>>> +                                         NULL, true);
>>
>> I presume this is for sync=full?
> 

Well, we don't allow bitmaps for sync=full at this point in the series.

> Ah, no.  This is necessary because the original sync bitmap was
> discarded by bdrv_dirty_bitmap_abdicate().  So yep, these bits need to
> go back into the sync bitmap then.
> 
> Max
> 

Yuh -- we actually want to clear the original bitmap for the 'always'
case, which the "abdicate" handles for us. This ought to trigger only
for the always case, so I think the conditional here is correct and as
simple as it can be.

Does your R-B stand?

--js