Re: avoid multiple hard links to same WAL file after a crash

From: Nathan Bossart <nathandbossart(at)gmail(dot)com>
To: Michael Paquier <michael(at)paquier(dot)xyz>
Cc: Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>, tgl(at)sss(dot)pgh(dot)pa(dot)us, robertmhaas(at)gmail(dot)com, pgsql-hackers(at)postgresql(dot)org
Subject: Re: avoid multiple hard links to same WAL file after a crash
Date: 2022-05-02 23:06:13
Message-ID: 20220502230613.GA3398932@nathanxps13
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On Mon, May 02, 2022 at 10:39:07AM -0700, Nathan Bossart wrote:
> On Mon, May 02, 2022 at 07:48:18PM +0900, Michael Paquier wrote:
>> The WAL receiver upgrades the ERROR to a FATAL, and restarts
>> streaming shortly after. Using durable_rename() would not be an issue
>> here.
> Thanks for investigating this one. I think I agree that we should simply
> switch to durable_rename() (without a file existence check beforehand).

Here is a new patch set. For now, I've only removed the file existence
check in writeTimeLineHistoryFile(). I don't know if I'm totally convinced
that there isn't a problem here (e.g., due to concurrent .ready file
creation), but since some platforms have been using rename() for some time,
I don't know how worried we should be. I thought about adding some kind of
locking between the WAL receiver and startup processes, but that seems
excessive. Alternatively, we could just fix xlog.c as proposed earlier
[0]. AFAICT that is the only caller that can experience problems due to
the multiple-hard-link issue. All other callers are simply renaming a
temporary file into place, and the temporary file can be discarded if left
behind after a crash.


Nathan Bossart
Amazon Web Services:

Attachment Content-Type Size
v5-0001-Replace-calls-to-durable_rename_excl-with-durable.patch text/x-diff 4.9 KB
v5-0002-Remove-durable_rename_excl.patch text/x-diff 4.0 KB

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