Horton

Re: Streaming replication and WAL archive interactions

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Michael Paquier <michael.paquier@gmail.com>, Venkata Balaji N <nag1010@gmail.com>, Andres Freund <andres@anarazel.de>, Fujii Masao <masao.fujii@gmail.com>, Borodin Vladimir <root@simply.name>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2015-04-23T08:11:19Z
Lists: pgsql-hackers 
On 04/22/2015 11:58 PM, Robert Haas wrote:
> On Wed, Apr 22, 2015 at 3:34 PM, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>> On 04/22/2015 10:21 PM, Robert Haas wrote:
>>> On Wed, Apr 22, 2015 at 3:01 PM, Heikki Linnakangas <hlinnaka@iki.fi>
>>> wrote:
>>>> For example, imagine that perform point-in-time recovery to WAL position
>>>> 0/1237E568, on timeline 1. That falls within segment
>>>> 000000010000000000000012. Then we end recovery, and switch to timeline 2.
>>>> After the switch, and some more WAL-logged actions, we'll have these
>>>> files
>>>> in pg_xlog:
>>>>
>>>> 000000010000000000000011
>>>> 000000010000000000000012
>>>> 000000020000000000000012
>>>> 000000020000000000000013
>>>> 000000020000000000000014
>>>
>>>
>>> Is the 000000010000000000000012 file a "partial" segment of the sort
>>> you're proposing to no longer achive?
>>
>> If you did pure archive recovery, with no streaming replication involved,
>> then no. If it was created by streaming replication, and the replication had
>> not filled the whole segment yet, then yes, it would be a partial segment.
>
> Why the difference?

Because we don't archive partial segments, except for the last one at a 
timeline switch, and there was no timeline switch to timeline 1 within 
that segment.

It doesn't really matter, though. The behaviour at the switch from 
timeline 1 to 2 works the same, whether the 000000010000000000000012 
segment is complete or not.

- Heikki