Re: [Qemu-devel] [PATCH V4 3/4] Add backing drive while performing backup.

[Paolo Bonzini]

Il 18/07/2013 08:39, Fam Zheng ha scritto:
> On Wed, 07/17 13:04, Ian Main wrote:
>> This patch adds the original source drive as a backing drive to our target
>> image so that the target image will appear complete during backup.  This
>> is especially useful for SYNC_MODE_NONE as it allows export via NBD to
>> have a complete point-in-time snapshot available for export.
>>
>> Signed-off-by: Ian Main <address@hidden>
>> ---
>>  block/backup.c | 13 +++++++++++++
>>  1 file changed, 13 insertions(+)
>>
>> diff --git a/block/backup.c b/block/backup.c
>> index 68abd23..e086e76 100644
>> --- a/block/backup.c
>> +++ b/block/backup.c
>> @@ -323,6 +323,10 @@ static void coroutine_fn backup_run(void *opaque)
>>  
>>      hbitmap_free(job->bitmap);
>>  
>> +    /* Set the target backing drive back to NULL before calling delete or
>> +     * it will also delete the underlying drive. */
>> +    target->backing_hd = NULL;
>> +
>>      bdrv_iostatus_disable(target);
>>      bdrv_delete(target);
>>  
>> @@ -362,6 +366,15 @@ void backup_start(BlockDriverState *bs, 
>> BlockDriverState *target,
>>          return;
>>      }
>>  
>> +    /* Manually set the backing hd to be the backup source drive so
>> +     * that all reads done while we are backing up will be passed
>> +     * on to the original source drive.  This allows reading from the
>> +     * image while the backup is in progress, or in the case of
>> +     * SYNC_MODE_NONE allows a complete image to be present for export.
>> +     * Note that we do this for all modes including SYNC_MODE_TOP as
>> +     * even then it allows on-the-fly reading. */
>> +    target->backing_hd = bs;
>> +
> 
> Also set target->backing_file and target->backing_format here? Paolo?

I don't think so, it is temporary while the job runs so that the NBD
server can already return the actual data.

Paolo