From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Antonin Houska <ah@cybertec.at>
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, 15 Nov 2019 18:41:02 -0300
Message-ID: <20191115214102.GA15616@alvherre.pgsql> (raw)
In-Reply-To: <44237.1573556793@antos>
On 2019-Nov-12, Antonin Houska wrote:
> ok, the next version uses explicit lseek(). Maybe the fact that XLOG is mostly
> read sequentially (i.e. without frequent seeks) is the reason pread() has't
> been adopted so far.
I don't quite understand why you backed off from switching to pread. It
seemed a good change to me.
Here's a few edits on top of your latest.
The new routine WALRead() is not at all the same as the previous
XLogRead, so I don't see why we would keep the name. Hence renamed.
I see no reason for the openSegment callback to return the FD in an out
param instead of straight return value. Changed that way.
Having seek/open be a boolean "xlr_seek" seems a bit weird. Changed to
an "operation" enum. (Maybe if we go back to pg_pread we can get rid of
this.) Accordingly, change WALReadRaiseError and WALDumpReadPage.
Change xlr_seg to be a struct rather than pointer to struct. It seems a
bit dangerous to me to return a pointer that we don't know is going to
be valid at raise-error time. Struct assignment works fine for the
purpose.
Renamed XLogDumpReadPage to WALDumpReadPage, because what the heck is
XLogDump anyway? That doesn't exist.
I would only like to switch this back to pg_pread() (from seek/read) and
I'd be happy to commit this.
What is logical_read_local_xlog_page all about? Seems useless. Let's
get rid of it.
--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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To: pgsql-hackers@postgresql.org
Cc: alvherre@2ndquadrant.com, ah@cybertec.at, michael@paquier.xyz, thomas.munro@gmail.com, robertmhaas@gmail.com
Subject: Re: Attempt to consolidate reading of XLOG page
In-Reply-To: <20191115214102.GA15616@alvherre.pgsql>
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