Re: Add a GUC check hook to ensure summarize_wal cannot be enabled when wal_level is minimal

On Wed, Jul 3, 2024 at 10:09 AM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
> The documentation states that "WAL summarization cannot be enabled when wal_level is set to minimal." Therefore, at
startup,the postmaster checks these settings and exits with an error if they are not configured properly. 
>
> However, I found that summarize_wal can still be enabled while the server is running with wal_level=minimal. Please
seethe following example to cause this situation. I think this is a bug. 

Well, that's unfortunate. I suppose I got confused about whether
summarize_wal could be changed without a server restart.

I think the fix is probably not to cross-check the GUC values, but to
put something in the summarizer that prevents it from generating a
summary file if wal_level==minimal. Because an incremental backup
based on such summaries would be no good. I won't be working the next
couple of days due to the US holiday tomorrow, but I've made a note to
look into this more next week.

--
Robert Haas
EDB: http://www.enterprisedb.com