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help / color / mirror / Atom feedFrom: Antonin Houska <ah@cybertec.at>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Michael Paquier <michael@paquier.xyz>
Cc: Thomas Munro <thomas.munro@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: pgsql-hackers@postgresql.org <pgsql-hackers@postgresql.org>
Subject: Re: Attempt to consolidate reading of XLOG page
Date: Fri, 22 Nov 2019 16:55:20 +0100
Message-ID: <10180.1574438120@antos> (raw)
In-Reply-To: <20191122142528.GA6465@alvherre.pgsql>
References: <20191122142528.GA6465@alvherre.pgsql>
Alvaro Herrera <alvherre@2ndquadrant.com> wrote:
> On 2019-Nov-22, Michael Paquier wrote:
>
> > On Fri, Nov 22, 2019 at 10:35:51AM -0300, Alvaro Herrera wrote:
> > > FWIW I think the new code is buggy because it doesn't seem to be setting
> > > ws_off, so I suppose the optimization in ReadPageInternal to skip
> > > reading the page when it's already the page we have is not hit, except
> > > for the first page in the segment. I didn't verify this, just my
> > > impression while reading the code.
> >
> > FWIW, this matches with my impression here, third paragraph:
> > https://www.postgresql.org/message-id/20191120083802.GB47145@paquier.xyz
>
> Ah, right.
As I pointed out in
https://www.postgresql.org/message-id/88183.1574261429%40antos
seg.ws_off only replaced readOff in XLogReaderState. So we should only update
ws_off where readOff was updated before commit 709d003. This does happen in
ReadPageInternal (see HEAD) and I see no reason for the final patch to update
ws_off anywhere else.
> I was wondering if we shouldn't do away with the concept of "offset" as
> such, since the offset there is always forcibly set to the start of a
> page. Why don't we count page numbers instead? It seems like the
> interface is confusingly generic (measure in bytes) yet not offer any
> extra functionality that could not be obtained with a simpler struct
> repr (measure in pages).
Yes, I agree that page numbers would be sufficient.
> But then that's not something that we need to change in this patch.
--
Antonin Houska
Web: https://www.cybertec-postgresql.com
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To: pgsql-hackers@postgresql.org
Cc: ah@cybertec.at, alvherre@2ndquadrant.com, michael@paquier.xyz, thomas.munro@gmail.com, robertmhaas@gmail.com
Subject: Re: Attempt to consolidate reading of XLOG page
In-Reply-To: <10180.1574438120@antos>
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