Re: Remove Deprecated Exclusive Backup Mode

On 7/1/20 4:32 PM, Bruce Momjian wrote:
> On Wed, Jul  1, 2020 at 04:27:58PM -0400, David Steele wrote:
>> On 7/1/20 3:58 PM, Robert Haas wrote:
>>> If we could jigger things so that you don't need to stop the backup at
>>> all, you only start it, and whether you ever finish copying everything
>>> is something about which the system need not know or care, that would
>>> be a lot nicer. I'm not sure I see how to do that, though.
>>
>> Well, the only thing pg_stop_backup() *really* needs to know is the starting
>> WAL position. pg_start_backup() gets that info so if it passes it back to
>> pg_stop_backup() that could be enough. Or as was proposed above, it just
> 
> Doesn't pg_start_backup already return this?
> 
>     SELECT pg_start_backup('test');
>      pg_start_backup
>     -----------------
> -->     0/2000028

It does, though after looking at the code I think we need the entire 
backup_label. For any of this to be useful, pg_start_backup() *must* 
return backup_label anyway so it can be stored somewhere the backup 
software can find it.

>> Here's a thought. What if we just stored the oldest starting LSN and a count
>> of how many backups have been requested. When the backup ends it checks that
>> backup count is > 0 and starting LSN is <= its starting LSN. If not, it
>> throws an error. When backups go to 0 FPWs are turned off if they were off
>> before the first backup.
> 
> Can't we just error out of an exclusive-style backup is requested to
> start and a previous exclusive-style backup has not been stopped?

That means only allowing one exclusive backup but it's not like we don't 
already have that limitation. But -- I'd rather not even need to specify 
exclusive/non-exclusive when the backup is started. Basically, all 
backups would be non-exclusive but we'd allow pg_stop_backup() to be 
called in a new session (or the same session).

Regards,
-- 
-David
david@pgmasters.net