Re: [Qemu-devel] [PATCH 4/5] block/qcow2-bitmap: Allow resizes with persistent bitmaps

[Vladimir Sementsov-Ogievskiy]

06.03.2019 18:36, Eric Blake wrote:
> On 3/6/19 9:33 AM, Vladimir Sementsov-Ogievskiy wrote:
> 
>>> +    /* Flush bitmaps */
>>> +    if (s->nb_bitmaps) {
>>> +        Error *local_err = NULL;
>>> +
>>> +        /* Usually, bitmaps get resized after this call.
>>> +         * Force it earlier so we can make the metadata consistent. */
>>> +        bdrv_dirty_bitmap_truncate(bs, offset);
>>> +        qcow2_flush_persistent_dirty_bitmaps(bs, &local_err);
>>> +        if (local_err) {
>>> +            ret = -EINVAL;
>>> +            goto fail;
>>> +        }
>>> +    }
>>
>> Why to flush after resize? Bitmaps will be IN_USE in the image anyway...
>>
>> Could we implement resize without flush first,  it would be one patch + 
>> test? And then consider
>> flushing in separate?
> 
> What happens with migration if we don't flush the new size to disk, so
> the on-disk format has a different size than the in-memory version?
> 

migration works fine, as we store bitmap not on close but on inactivate.

Bitmaps may be migrated in two ways:

1. dirty-bitmaps migration capability enabled: they migrate through migration 
channel, RAM to RAM,
    BdrvDirtyBitmap to BdrvDirtyBitmap, stored bitmaps are unrelated and they 
are marked IN_USE anyway

2. dirty-bitmaps migration capability disabled: they migrate through storage, 
so they are stored in
    inactivation on source and then loaded in invalidation on target.

A good chunk of information is also in a huge comment inside qcow2_do_open() 
function.


Note: we use the following hack for resize, I never tried to send it to the 
list:

in qcow2_co_truncated, instead of "Can't resize an image which has bitmaps":

     if (s->nb_bitmaps) {
         /* FIXME: not loaded bitmaps will be lost */
         Error *local_err = NULL;
         qcow2_remove_all_persistent_dirty_bitmaps(bs, &local_err);
         if (local_err) {
             error_propagate(errp, local_err);
             return -EINVAL;
         }
     }

-- 
Best regards,
Vladimir