Re: Attempt to consolidate reading of XLOG page

Alvaro Herrera <alvherre(at)2ndquadrant(dot)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