Feedback-ID: i0fe9450f:Fastmail
Date: Thu, 12 Oct 2023 07:22:09 +0900
From: Michael Paquier <michael@paquier.xyz>
To: David Steele <david@pgmasters.net>
Cc: Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: The danger of deleting backup_label
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On Tue, Oct 10, 2023 at 05:06:45PM -0400, David Steele wrote:
> That fails because there is a check to make sure the checkpoint is valid
> when pg_control is loaded. Another possibility is to use a special LSN li=
ke
> we use for unlogged tables. Anything >=3D 24 and < WAL segment size will =
work
> fine.

Do we have any reason to do that in the presence of a backup_label
file anyway?  We'll know the LSN of the checkpoint based on what the
base backup wants us to use.  Using a fake-still-rather-valid value
for the LSN in the control file to bypass this check does not address
the issue you are pointing at: it is just avoiding this check.  A
reasonable answer would be, IMO, to just not do this check at all
based on the control file in this case.

>> If the contents of the control file are tweaked before sending it
>> through a BASE_BACKUP, it would cover more than just pg_basebackup.
>> Switching the way the control file is sent with new contents in
>> sendFileWithContent() rather than sendFile() would be one way, for
>> instance..
>=20
> Good point, and that makes this even more compelling. If we include
> pg_control into backup_label then there is no need to modify pg_control (=
as
> above) -- we can just exclude it from the backup entirely. That will
> certainly require some rejigging in recovery but seems worth it for backup
> solutions that can't easily modify pg_control. The C-based solutions can =
do
> this pretty easily but it is a pretty high bar for anyone else.

I have little idea about that, but I guess that you are referring to
backrest here.
--
Michael

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