Re: Enforce "max_wal_size/ min_wal_size must be at least twice wal_segment_size" limit while setting GUCs - Mailing list pgsql-hackers

From Kyotaro Horiguchi
Subject Re: Enforce "max_wal_size/ min_wal_size must be at least twice wal_segment_size" limit while setting GUCs
Date
Msg-id 20220524.104620.432425450504076645.horikyota.ntt@gmail.com
Whole thread Raw
In response to Re: Enforce "max_wal_size/ min_wal_size must be at least twice wal_segment_size" limit while setting GUCs  (Kyotaro Horiguchi <horikyota.ntt@gmail.com>)
List pgsql-hackers
At Tue, 24 May 2022 10:19:53 +0900 (JST), Kyotaro Horiguchi <horikyota.ntt@gmail.com> wrote in 
> At Mon, 23 May 2022 10:08:54 -0400, Tom Lane <tgl@sss.pgh.pa.us> wrote in 
> > Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com> writes:
> > > Thanks. Having check hooks for min_wal_size and max_wal_size that
> > > throw errors if they aren't at least twice the wal_segment_size has a
> > > "BIG" problem - ./initdb -D data --wal-segsize=1 (or a value < 16)
> > > fails.
> > 
> > In general, you can't do that (i.e. try to enforce constraints between
> > GUC values via check hooks).  It's been tried in the past and failed
> > miserably, because the hooks can't know whether the other value is
> > about to be changed.
> 
> I thought that wal_segment_size is a semi-constant for a server life.
> But looking at the startup sequence closely, postmaster tries
> changing max_wal_size before reading control file.
> 
> Couldn't we use PGC_S_TEST for this purpose?  AlterSystemSetConfigFile
> is calling parse_and_validate_value() with PGC_S_FILE, but it is
> actually a "value to be used later"(@guc.h:93). So it can be thought
> that PG_S_TEST is the right choice there.  If it is still not work
> perfectly, we could have a new source value, say PGC_S_ALTER_SYSTEM,
> exactly for this use. (but I don't see a following users comes in
> future..)

This duscussion is based on the assumption that "wal_segment_size can
be assumed to be a constant when a check function is called with
PGC_S_FILE".

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center



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