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help / color / mirror / Atom feedFrom: Robert Haas <[email protected]>
To: hlinnaka <[email protected]>
Cc: Michael Paquier <[email protected]>
Cc: Venkata Balaji N <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: Fujii Masao <[email protected]>
Cc: Borodin Vladimir <[email protected]>
Cc: PostgreSQL-development <[email protected]>
Subject: Re: Streaming replication and WAL archive interactions
Date: Wed, 22 Apr 2015 15:21:06 -0400
Message-ID: <CA+TgmoY5MJ5sDxToQZ28Jku8BFQ5T8=LMSduCZB-0zKThOx_WQ@mail.gmail.com> (raw)
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On Wed, Apr 22, 2015 at 3:01 PM, Heikki Linnakangas <[email protected]> wrote:
> On 04/22/2015 09:30 PM, Robert Haas wrote:
>> On Wed, Apr 22, 2015 at 2:17 AM, Heikki Linnakangas <[email protected]>
>> wrote:
>>>
>>> Note that it's a bit complicated to set up that scenario today. Archiving
>>> is
>>> never enabled in recovery mode, so you'll need to use a custom cron job
>>> or
>>> something to maintain the archive that C uses. The files will not
>>> automatically flow from B to the second archive. With the patch we're
>>> discussing, however, it would be easy: just set archive_mode='always' in
>>> B.
>>
>>
>> Hmm, I see. But if C never replays the last, partial segment from the
>> old timeline, how does it follow the timeline switch?
>
> At timeline switch, we copy the old segment to the new timeline, and start
> writing where we left off. So the WAL from the old timeline is found in the
> segment nominally belonging to the new timeline.
Check.
> 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?
> Note that there are two segments ending in "12". They both have the same
> point up to offset 0x37E568, corresponding to the switch point 0/1237E568.
> After that, the contents diverge: the segment on the new timeline contains a
> checkpoint/end-of-recovery record at that point, followed by new WAL
> belonging to the new timeline.
Check.
> Recovery knows about that, so that if you set recovery target to timeline 2,
> and it needs the WAL at the beginning of segment 12 (still belonging to
> timeline 1), it will try to restoring both "000000010000000000000012" and
> "000000020000000000000012".
What if you set the recovery target to timeline 3?
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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Subject: Re: Streaming replication and WAL archive interactions
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