Message-ID: <5538A927.7000407@iki.fi>
Date: Thu, 23 Apr 2015 11:11:19 +0300
From: Heikki Linnakangas <hlinnaka@iki.fi>
Reply-To: hlinnaka@iki.fi
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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>
Subject: Re: Streaming replication and WAL archive interactions
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	<54949108.3030109@vmware.com>
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	<552FA38F.9060005@iki.fi>
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	<5535FE71.1010905@iki.fi>
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	<55373D17.5050706@iki.fi>
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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


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