| From: | Nathan Bossart <nathandbossart(at)gmail(dot)com> |
|---|---|
| To: | Stephen Frost <sfrost(at)snowman(dot)net> |
| Cc: | John Morris <john(dot)morris(at)crunchydata(dot)com>, Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Michael Paquier <michael(at)paquier(dot)xyz>, Robert Haas <robertmhaas(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Atomic ops for unlogged LSN |
| Date: | 2023-11-07 17:02:49 |
| Message-ID: | 20231107170249.GA782414@nathanxps13 |
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| Lists: | pgsql-hackers |
On Tue, Nov 07, 2023 at 11:47:46AM -0500, Stephen Frost wrote:
> We only care about the value of the unlogged LSN being correct during
> normal shutdown when we're writing out the shutdown checkpoint, but by
> that time everything else has been shut down and the value absolutely
> should not be changing.
I agree that's all true. I'm trying to connect how this scenario ensures
we see the most up-to-date value in light of this comment above
pg_atomic_read_u32():
* The read is guaranteed to return a value as it has been written by this or
* another process at some point in the past. There's however no cache
* coherency interaction guaranteeing the value hasn't since been written to
* again.
Is there something special about all other backends being shut down that
ensures this returns the most up-to-date value and not something from "some
point in the past" as the stated contract for this function seems to
suggest?
--
Nathan Bossart
Amazon Web Services: https://aws.amazon.com
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