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Re: Promoting a standby during base backup (was Re: Switching timeline over streaming replication)

From: Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>
To: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Cc: Amit Kapila <amit(dot)kapila(at)huawei(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Simon Riggs <simon(at)2ndquadrant(dot)com>
Subject: Re: Promoting a standby during base backup (was Re: Switching timeline over streaming replication)
Date: 2012-10-08 11:58:15
Message-ID: 5072BFD7.9010501@vmware.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On 04.10.2012 20:07, Fujii Masao wrote:
> On Thu, Oct 4, 2012 at 4:59 PM, Heikki Linnakangas
>> But I wonder why promoting a standby renders the backup invalid in the first
>> place? Fujii, Simon, can you explain that?
>
> Simon had the same question and I answered it before.
>
> http://archives.postgresql.org/message-id/CAHGQGwFU04oO8YL5SUcdjVq3BRNi7WtfzTy9wA2kXtZNHicTeA@mail.gmail.com
> ---------------------------------------
>> You say
>> "If the standby is promoted to the master during online backup, the
>> backup fails."
>> but no explanation of why?
>>
>> I could work those things out, but I don't want to have to, plus we
>> may disagree if I did.
>
> If the backup succeeds in that case, when we start an archive recovery from that
> backup, the recovery needs to cross between two timelines. Which means that
> we need to set recovery_target_timeline before starting recovery. Whether
> recovery_target_timeline needs to be set or not depends on whether the standby
> was promoted during taking the backup. Leaving such a decision to a user seems
> fragile.

pg_control is backed up last, it would contain the new timeline. No need 
to set recovery_target_timeline.

> pg_basebackup -x ensures that all required files are included in the backup and
> we can start recovery without restoring any file from the archive. But
> if the standby is promoted during the backup, the timeline history
> file would become
> an essential file for recovery, but it's not included in the backup.

That is true. We could teach it to include the timeline history file, 
though.

> The situation may change if your switching-timeline patch has been committed.
> It's useful if we can continue the backup even if the standby is promoted.

It wouldn't help with pg_basebackup -x, although it would allow 
streaming replication to fetch the timeline history file.

I guess it's best to keep that restriction for now. But I'll add a TODO 
item for this.

- Heikki


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