Re: Attempt to consolidate reading of XLOG page

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