Re: Updated backup APIs for non-exclusive backups

On 11/26/18 12:31 AM, Laurenz Albe wrote:
> On Sun, 2018-11-25 at 16:04 -0500, Stephen Frost wrote:
>>>> There isn’t any need to write the backup label before you restore the database,
>>>> just as you write recovery.conf then.
>>>
>>> Granted.
>>> But it is pretty convenient, and writing it to the data directory right away
>>> is a good thing on top, because it reduces the danger of inadvertedly
>>> starting the backup without recovery.
>>
>> Writing it into the data directory is *not* a good thing because as soon as you do
>> that you risk there being an issue if there’s a crash.  Writing into the backup
>> isn’t a bad idea but if you’re managing your backups then writing it somewhere else
>> (such as where you write your WAL) and associating it with the backup (presumably
>> it has a label) should make it easy to pull back when you restore.
> 
> If there is a crash during the backup procedure, the backup is bad.
> Doesn't matter during which part of the backup procedure it happens.

Yes, but in this case with exclusive backups your cluster also will not 
start.  That's a lot different than a bad backup because you should have 
many backups and a way to mark when they are complete.  If your cluster 
requires manual intervention to start because a backup was in progress 
that's not a good thing.

I'm of the opinion that we should remove the exclusive option for PG12. 
The recovery changes recently committed will completely break any 
automated recovery written for PG <= 11 so we might as go all the way.

Requiring non-exclusive backup will be an easy change for tools that 
already understand non-exclusive backup, which has been available in the 
last three versions of Postgres.

The exclusive backup method tries to make a hard problem look simple but 
does not succeed and causes issues besides.  Performing backups properly 
*is* hard and we don't do anyone a service by making it look like 
something can be done with a trivial bash script.

Regards,

-- 
-David
david@pgmasters.net