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From: Andres Freund <[email protected]>
To: David G. Johnston <[email protected]>
Cc: Tom Lane <[email protected]>
Cc: Peter Geoghegan <[email protected]>
Cc: [email protected]
Cc: Robert Haas <[email protected]>
Cc: Melanie Plageman <[email protected]>
Cc: David Rowley <[email protected]>
Subject: Re: Option to not use ringbuffer in VACUUM, using it in failsafe mode
Date: Wed, 11 Jan 2023 16:36:38 -0800
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAKFQuwau7X3J5qB-J8WdTwDCa_jkWyVOETLjGnu468_iOXnxFA@mail.gmail.com>
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Hi,

On 2023-01-11 17:26:19 -0700, David G. Johnston wrote:
> Should we just add "ring_buffers" to the existing "shared_buffers" and
> "temp_buffers" settings?

The different types of ring buffers have different sizes, for good reasons. So
I don't see that working well. I also think it'd be more often useful to
control this on a statement basis - if you have a parallel import tool that
starts NCPU COPYs you'd want a smaller buffer than a single threaded COPY. Of
course each session can change the ring buffer settings, but still.


> Then give VACUUM a (BUFFER_POOL=ring*|shared) option?

That seems likely to mislead, because it'd still use shared buffers when the
blocks are already present. The ring buffers aren't a separate buffer pool,
they're a subset of the normal bufferpool. Lookup is done normally, only when
a page isn't found, the search for a victim buffer first tries to use a buffer
from the ring.


> I think making DBAs aware of this dynamic and making the ring buffer usage
> user-facing is beneficial in its own right (at least, the concept that
> changes done by vacuum don't impact shared_buffers, regardless of how that
> non-impact manifests).

VACUUM can end up dirtying all of shared buffers, even with the ring buffer in
use...

Greetings,

Andres Freund

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