From: Nathan Bossart <nathandbossart@gmail.com>
To: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: cary.huang@highgo.ca
Cc: pgsql-hackers@lists.postgresql.org
Cc: satyanarlapuram@gmail.com
Subject: Re: Switching XLog source from archive to streaming when primary available
Date: Sat, 8 Oct 2022 14:52:21 -0700
Message-ID: <20221008215221.GA894639@nathanxps13> (raw)
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References: <CALj2ACUBuc574n8g-QUQGha=J8C-cWJ4KwW1dOGe5HG0NH4s2w@mail.gmail.com>
<20220915.172207.1940822794018442836.horikyota.ntt@gmail.com>
<CALj2ACUpprK2nMBAr4VMoOGL4PrSj7V6upJQFhPRirUM4a0fQg@mail.gmail.com>
<20220916.153629.1624554489607517175.horikyota.ntt@gmail.com>
<CALj2ACUkABNTs5kiXSM-2qW_NBpP6gC-T76=Mwy-GqQd7UE3uw@mail.gmail.com>
<CALj2ACUg_MpcOwHZQm-9M3ScyLK-NLy9=yBhU9TfTfC0U3w1YQ@mail.gmail.com>
On Mon, Sep 19, 2022 at 07:49:21PM +0530, Bharath Rupireddy wrote:
> SwitchFromArchiveToStreamEnabled() seemed better at this point. I'm
> attaching the v7 patch with that change. Please review it further.
As I mentioned upthread [0], I'm still a little concerned that this patch
will cause the state machine to go straight from archive recovery to
streaming replication, skipping recovery from pg_wal. I wonder if this
could be resolved by moving the standby to the pg_wal phase instead.
Concretely, this line
+ if (switchSource)
+ break;
would instead change currentSource from XLOG_FROM_ARCHIVE to
XLOG_FROM_PG_WAL before the call to XLogFileReadAnyTLI(). I suspect the
behavior would be basically the same, but it would maintain the existing
ordering.
However, I do see the following note elsewhere in xlogrecovery.c:
* The segment can be fetched via restore_command, or via walreceiver having
* streamed the record, or it can already be present in pg_wal. Checking
* pg_wal is mainly for crash recovery, but it will be polled in standby mode
* too, in case someone copies a new segment directly to pg_wal. That is not
* documented or recommended, though.
Given this information, the present behavior might not be too important,
but I don't see a point in changing it without good reason.
[0] https://postgr.es/m/20220906215704.GA2084086%40nathanxps13
--
Nathan Bossart
Amazon Web Services: https://aws.amazon.com
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Cc: nathandbossart@gmail.com, bharath.rupireddyforpostgres@gmail.com, horikyota.ntt@gmail.com, cary.huang@highgo.ca, pgsql-hackers@lists.postgresql.org, satyanarlapuram@gmail.com
Subject: Re: Switching XLog source from archive to streaming when primary available
In-Reply-To: <20221008215221.GA894639@nathanxps13>
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