Re: [PATCH] Teach pg_waldump to extract FPIs from the WAL

On Mon, Apr 25, 2022 at 10:24:52AM -0500, David Christensen wrote:
> On Mon, Apr 25, 2022 at 6:03 AM Bharath Rupireddy
> <bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
>> Thanks for working on this. I'm just thinking if we can use these FPIs
>> to repair the corrupted pages? I would like to understand more
>> detailed usages of the FPIs other than inspecting with pageinspect.
>
> My main use case was for being able to look at potential corruption,
> either in the WAL stream, on heap, or in tools associated with the WAL
> stream. I suppose you could use the page images to replace corrupted
> on-disk pages (and in fact I think I've heard of a tool or two that
> try to do that), though don't know that I consider this the primary
> purpose (and having toast tables and the list, as well as clog would
> make it potentially hard to just drop-in a page version without
> issues). Might help in extreme situations though.

You could do a bunch of things with those images, even make things
worse if you are not careful enough.

>> 10) Along with pg_pwrite(), can we also fsync the files (of course
>> users can choose it optionally) so that the writes will be durable for
>> the OS crashes?
>
> Can add; you thinking a separate flag to disable this with default true?

We expect data generated by tools like pg_dump, pg_receivewal
(depending on the use --synchronous) or pg_basebackup to be consistent
when we exit from the call. FWIW, flushing this data does not seem
like a strong requirement for something aimed at being used page-level
chirurgy or lookups, because the WAL segments should still be around
even if the host holding the archives is unplugged.
--
Michael