Message-ID: <38078.192.168.207.54.1427912995.squirrel@webmail.etecsa.cu>
In-Reply-To: <551C295D.6060602@evaristesys.com>
References: <551C295D.6060602@evaristesys.com>
Date: Wed, 1 Apr 2015 13:29:55 -0500 (GMT+5)
Subject: Re: Restoring normal master-slave roles after replication failure
From: "Gilberto Castillo" <gilberto.castillo@etecsa.cu>
To: "Alex Balashov" <abalashov@evaristesys.com>
Cc: pgsql-admin@postgresql.org
Reply-To: gilberto.castillo@etecsa.cu
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Hello Alex,

Have you seen pgpool or rpmgr-3?


>
> Hello all,
>
> I'm terribly sorry if this question has been asked before in a number of
> different forms; I wasn't able to find quite the nuance I was looking
> for in the archives.
>
> We're running PostgreSQL 9.4 with streaming replication + hot_standby +
>   the newfangled physical replication slots. On the master, the
> wal_archive level is 'hot_standby', and on the slave, it's 'archive'[1].
>
> As I have understood the conventional wisdom from scouring the
> documentation and list archives, if the master fails and we
> trigger-promote the slave to master and then, after fixing the master
> server, we need to restore it to the master role and restore the slave
> server back down to the hot_standby role, we need to reinitialise
> replication. That is, we need to do a brand new base backup of the slave
> back to the master.
>
> My question is:
>
> Isn't there a better way that lends itself to some degree of automation?
> We have a ~500 GB database and while we can do a base backup, it's
> neither pretty nor quick. Can we not configure $OLD_SLAVE as a master
> (with WAL senders, replication role and all), put $OLD_MASTER in
> recovery mode and have it play out the accumulated WALs in reverse from
> $NEW_MASTER, then shut down $NEW_MASTER, reconfigure both replicas to
> their appropriate roles, and bring them back up like before?
>
> If this is possible, does it depend on setting wal_archive to
> 'hot_standby' level on the slave during the course of normal operation,
> too? Shouldn't 'archive' be sufficient to restore from? What exactly
> does the 'hot_standby' WAL level contain that 'archive' doesn't?
>
> If I'm wrong, is there no other way to do this except to re-do a whole
> base backup back to the master? How do people with very large databases
> deal with this? Just grin and bear it?
>
> Many thanks in advance, and I apologise again if this has previously
> been answered in some canonical way.
>
> -- Alex
>
> [1] This is as it should be, right? We want to have backup WALs in case
> we experience a total physical meltdown of the master and need to do
> PITR, and/or to archive them to WAL-E from the slave if the slave were
> promoted to master.
>
> --
> Alex Balashov | Principal | Evariste Systems LLC
> 303 Perimeter Center North, Suite 300
> Atlanta, GA 30346
> United States
>
> Tel: +1-800-250-5920 (toll-free) / +1-678-954-0671 (direct)
> Web: http://www.evaristesys.com/, http://www.csrpswitch.com/
>
>
> --
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Saludos,
Gilberto Castillo
ETECSA, La Habana, Cuba

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