Horton

Re: Attempt to consolidate reading of XLOG page

Alvaro Herrera <alvherre@2ndquadrant.com>

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From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Antonin Houska <ah@cybertec.at>
Cc: Michael Paquier <michael@paquier.xyz>, Thomas Munro <thomas.munro@gmail.com>, Robert Haas <robertmhaas@gmail.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2019-11-22T13:35:51Z
Lists: pgsql-hackers 
On 2019-Nov-22, Antonin Houska wrote:

> I thought that in [1] you try discourage me from using pg_pread(), but now it
> seems to be the opposite. Ideally I'd like to see no overhead added by my
> patch at all, but the code simplicity should matter too.

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.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Commits

  1. Remove logical_read_local_xlog_page

  2. Refactor WAL file-reading code into WALRead()

  3. Rework WAL-reading supporting structs

  4. Make XLogReaderInvalReadState static

  5. Use pg_pread() and pg_pwrite() for data files and WAL.