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help / color / mirror / Atom feedFrom: Michael Paquier <[email protected]>
To: David Christensen <[email protected]>
Cc: Bharath Rupireddy <[email protected]>
Cc: PostgreSQL-development <[email protected]>
Subject: Re: [PATCH] Teach pg_waldump to extract FPIs from the WAL
Date: Tue, 26 Apr 2022 11:42:49 +0900
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAOxo6XKDO_q2dbUaz-hcLs1-d5d=OdMecm=A3tRYrb=yngVsQw@mail.gmail.com>
References: <CAOxo6XKjQb2bMSBRpePf3ZpzfNTwjQUc4Tafh21=jzjX6bX8CA@mail.gmail.com>
<CALj2ACU-_HixN-rz0tKsab6b3w-+3w733ceUW6oKSJ7Pgp2R+Q@mail.gmail.com>
<CAOxo6XKDO_q2dbUaz-hcLs1-d5d=OdMecm=A3tRYrb=yngVsQw@mail.gmail.com>
On Mon, Apr 25, 2022 at 10:24:52AM -0500, David Christensen wrote:
> On Mon, Apr 25, 2022 at 6:03 AM Bharath Rupireddy
> <[email protected]> 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
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Subject: Re: [PATCH] Teach pg_waldump to extract FPIs from the WAL
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