Horton

Re: Attempt to consolidate reading of XLOG page

Michael Paquier <michael@paquier.xyz>

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From: Michael Paquier <michael@paquier.xyz>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Antonin Houska <ah@cybertec.at>, Thomas Munro <thomas.munro@gmail.com>, Robert Haas <robertmhaas@gmail.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2019-11-18T12:29:03Z
Lists: pgsql-hackers 
On Fri, Nov 15, 2019 at 06:41:02PM -0300, Alvaro Herrera wrote:
> I don't quite understand why you backed off from switching to pread.  It
> seemed a good change to me.
>
> [...]
>
> 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.

This has been quickly mentioned on the thread which has introduced
pread():
https://www.postgresql.org/message-id/c2f56d0a-cadd-3df1-ae48-b84dc8128c37@redhat.com

Now, read() > pread() > read()+lseek(), and we don't actually need to
seek into the file for all the cases where we read a WAL page.  And on
a platform which uses the fallback implementation, this increases the
number of lseek() calls.  I can see as you say that using it directly
in the refactoring can simplify the code.
--
Michael

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.