| From: | Michael Paquier <michael(at)paquier(dot)xyz> |
|---|---|
| To: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> |
| Cc: | Antonin Houska <ah(at)cybertec(dot)at>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Attempt to consolidate reading of XLOG page |
| Date: | 2019-11-18 12:29:03 |
| Message-ID: | 20191118122903.GH1543@paquier.xyz |
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| 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
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