Re: pg_basebackups and slots

On Thu, Dec 15, 2016 at 10:26 AM, Magnus Hagander <magnus(at)hagander(dot)net>
wrote:

>
>
> On Thu, Dec 15, 2016 at 10:04 AM, Magnus Hagander <magnus(at)hagander(dot)net>
> wrote:
>
>> I've started work on a patch to make pg_basebackup use the temporary
>> slots feature that has been committed (thanks Petr!!). The reason for this
>> is to avoid the cases where a burst of traffic on the master during the
>> backup can cause the receive log part of the basebackup to fall far enough
>> behind that it fails.
>>
>> I have a few considerations at this point, about interaction with
>> existing options.
>>
>> Right now, we have -S/--slot which specifies a slot name. If you want to
>> use that, you have to create the slot ahead of time, and it will be a
>> permanent slot (of course). This is primarily documented as a feature to
>> use for replication (to make sure xlog is kept around until the standby is
>> started up), but it does also solve the same problem. But to use it for
>> base backups today you have to manually create the slot, then base backup,
>> then drop the slot, which is error prone.
>>
>> My thought is that if -S/--slot is not specified, but -X stream is, then
>> we use a temporary slot always. This obviously requires the server to be
>> configured with enough slots (I still think we should change the default
>> here, but that's a different topic), but I think that's acceptable. Then we
>> should add a "--no-slot" to make it revert to previous behaviour.
>>
>> Does that seem reasonable? Or would people prefer it to default to off?
>>
>>
So here's a patch that does this, for discussion. It implements the
following behavior for -X:

* When used with <10.0 servers, behave just like before.
* When -S <name> is specified, behave just like before (use an existing
replication slot, fail if it does not exist)
* When used on 10.0 with no -S, create and use a temporary replication slot
while running, with name pg_basebackup_<pid>.
* When used with 10.0 with no -S but --no-slot specified, run without a
slot like before.

--
Magnus Hagander
Me: http://www.hagander.net/
Work: http://www.redpill-linpro.com/