Thread: BUG #11518: full_page_writes is off after executing pg_start_backup()

The following bug has been logged on the website:

Bug reference:      11518
Logged by:          sudalai
Email address:      sudalait2@gmail.com
PostgreSQL version: 9.2.2
Operating system:   ubuntu
Description:

Hi,
   The documentation says, full_page_writes is forcibly ON between
pg_start_backup() and pg_stop_backup().
    I started the server with full_page_writes=off. And i executed
pg_start_up(), the full_page_writes is not changed. I executed "show
full_page_writes" it shows OFF.
But it should be On.

On Tue, Sep 30, 2014 at 1:18 AM, <sudalait2@gmail.com> wrote:

>    The documentation says, full_page_writes is forcibly ON between
> pg_start_backup() and pg_stop_backup().
>     I started the server with full_page_writes=off. And i executed
> pg_start_up(), the full_page_writes is not changed. I executed "show
> full_page_writes" it shows OFF.
> But it should be On.
>
There are two things controlling full_page_writes in the backend:
- The GUC parameter, which is the output result of what you can see with
pg_settings or SHOW
- A flag in the structure called XLogCtl, defined at the top of xlog.c in
shared memory that is used to control the state of XLOGs, and defined
depending on the value of the GUC full_page_writes or the state of system
like a server in backup state. It is actually this shared memory flag that
is updated when pg_start_backup is used. Only the flag in shared memory is
enforced to true when pg_start_backup is kicked, meaning that the value
update is actually not visible for the user, but for the system it is in
shared memory.
This is not a bug.
--
Michael