Re: Streaming base backups

On Fri, Jan 14, 2011 at 11:19, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> On 14.01.2011 08:45, Fujii Masao wrote:
>>
>> On Fri, Jan 14, 2011 at 4:13 AM, Magnus Hagander<magnus(at)hagander(dot)net>
>>  wrote:
>>>>
>>>> At the end of the backup by walsender, it forces a switch to a new
>>>> WAL file and waits until the last WAL file has been archived. So we
>>>> should change postmaster so that it doesn't cause the archiver to
>>>> end before walsender ends when shutdown is requested?
>>>
>>> Um. I have to admit I'm not entirely following what you mean enough to
>>> confirm it, but it *sounds* correct :-)
>>>
>>> What scenario exactly is the problematic one?
>>
>> 1. Smart shutdown is requested while walsender is sending a backup.
>> 2. Shutdown causes archiver to end.
>>      (Though shutdown sends SIGUSR2 to walsender to exit, walsender
>>       running backup doesn't respond for now)
>> 3. At the end of backup, walsender calls do_pg_stop_backup, which
>>      forces a switch to a new WAL file and waits until the last WAL file
>> has
>>      been archived.
>>      *BUT*, since archiver has already been dead, walsender waits for
>>      that forever.
>
> Not only does it wait forever, but it writes the end-of-backup WAL record
> after bgwriter has already exited and written the shutdown checkpoint
> record.
>
> I think postmaster should treat a walsender as a regular backend, until it
> has started streaming.
>
> We can achieve that by starting up the child as PM_CHILD_ACTIVE, and
> changing the state to PM_CHILD_WALSENDER later, when streaming is started.
> Looking at the postmaster.c, that should be safe, postmaster will treat a
> backend as a regular backend anyway until it has connected to shared memory.
> It is *not* safe to switch a walsender back to a regular process, but we
> have no need to do that.

Seems reasonable to me.

I've applied a patch that exits base backups when the postmaster is
shutting down - I'm happily waiting for Heikki to submit one that
changes the shutdown logic in the postmaster :-)

--
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/