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From: Stephen Frost <sfrost@snowman.net>
To: Andres Freund <andres@anarazel.de>
Cc: Thomas Munro <thomas.munro@gmail.com>
Cc: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Dmitry Dolgov <9erthalion6@gmail.com>
Cc: David Steele <david@pgmasters.net>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: WIP: WAL prefetch (another approach)
Date: Fri, 4 Dec 2020 14:01:45 -0500
Message-ID: <20201204190145.GW16415@tamriel.snowman.net> (raw)
In-Reply-To: <20201204185157.w3qtqyym6fixmnrg@alap3.anarazel.de>
References: <CA+hUKGJyuSTC5sXTv-2FmtjfFPYFU8t8JPnhV4jdMKmuKJ4tHw@mail.gmail.com>
	<20201010112935.qrhm572v4djrjljo@development>
	<CA+hUKGJwuzYCLxCP7QpK54uHBxEDGNcnDF4CJuJnuvFDpn4h5A@mail.gmail.com>
	<5437ce5b-d305-bd85-23ec-ddb15735d93f@enterprisedb.com>
	<20201113151344.GU16415@tamriel.snowman.net>
	<CA+hUKGJ1=pOiNjSgXYJnjE3OyRtp8tjMRcON256e5EFpzPpgtA@mail.gmail.com>
	<20201118210039.GP16415@tamriel.snowman.net>
	<CA+hUKG+BEjLHiAxJEBw1QSckFJFL8Uq6EhAuqdK6KnavcHjefQ@mail.gmail.com>
	<20201204182738.GU16415@tamriel.snowman.net>
	<20201204185157.w3qtqyym6fixmnrg@alap3.anarazel.de>

Greetings,

* Andres Freund (andres@anarazel.de) wrote:
> On 2020-12-04 13:27:38 -0500, Stephen Frost wrote:
> > If I follow correctly, this patch will scan ahead in the WAL and let
> > the kernel know that certain blocks will be needed soon.  Ideally,
> > though I don't think it does yet, we'd only do that for blocks that
> > aren't already in shared buffers, and only for non-FPIs (even better if
> > we can skip past pages for which we already, recently, passed an FPI).
> 
> The patch uses PrefetchSharedBuffer(), which only initiates a prefetch
> if the page isn't already in s_b.

Great, glad that's already been addressed in this, that's certainly
good.  I think I knew that and forgot it while composing that response
over the past rather busy week. :)

> And once we have AIO, it can actually initiate IO into s_b at that
> point, rather than fetching it just into the kernel page cache.

Sure.

Thanks,

Stephen

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  Cc: sfrost@snowman.net, andres@anarazel.de, thomas.munro@gmail.com, tomas.vondra@enterprisedb.com, tomas.vondra@2ndquadrant.com, 9erthalion6@gmail.com, david@pgmasters.net, alvherre@2ndquadrant.com
  Subject: Re: WIP: WAL prefetch (another approach)
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