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

[Max Reitz]

On 05.07.19 18:59, John Snow wrote:
> 
> 
> 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?

Sure.

Max

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