Re: pg_basebackup blocking all queries with horrible performance

On Sun, Jun 10, 2012 at 9:25 PM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
> On Sun, Jun 10, 2012 at 7:43 PM, Magnus Hagander <magnus(at)hagander(dot)net> wrote:
>> On Sat, Jun 9, 2012 at 2:51 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> Fujii Masao <masao(dot)fujii(at)gmail(dot)com> writes:
>>>> This seems a bug. I think we should prevent pg_basebackup from
>>>> becoming synchronous standby. Thought?
>>>
>>> Absolutely.  If we have replication clients that are not actually
>>> capable of being standbys, there *must* be a way for the master
>>> to know that.
>>
>> I thought we fixed this already by sending InvalidXlogRecPtr as flush
>> location? And that this only applied in 9.2?
>>
>> Are you saying we picked pg_basebackup *in backup mode* (not log
>> streaming) as synchronous standby?
>
> Yes.
>
>> If so then yes, that is
>> *definitely* a bug that should be fixed. We should never select a
>> connection that's not even streaming log as standby!
>
> Agreed. Attached patch prevents pg_basebackup from becoming sync
> standby. Also this patch fixes another problem: currently only walsender
> which reaches STREAMING state can become sync walsender. OTOH,
> sync walsender thinks that walsender with higher priority will be sync one
> whether its state is STREAMING, and switches to potential sync walsender.
> So when the standby with higher priority connects to the master, we
> might have no sync standby until it reaches the STREAMING state.
> To fix this problem, the patch switches walsender's state from sync to
> potential *after* walsender with higher priority has reached the
> STREAMING state.
>
> We also should not select (1) background stream process forked from
> pg_basebackup and (2) pg_receivexlog as sync standby because they
> don't send back replication progress. To address this, I'm thinking to
> introduce new option "NOSYNC" in "START_REPLICATION" command
> as follows, and to change (1) and (2) so that they specify NOSYNC.
>
>    START_REPLICATION XXX/XXX [NOSYNC]
>
> If the standby specifies NOSYNC option, it's never assigned as sync
> standby even if its name is in synchronous_standby_names. Thought?

The standby which always sends InvalidXLogRecPtr back should not
become sync one. So instead of NOSYNC option, by checking whether
InvalidXLogRecPtr is sent, we can avoid problematic sync standby.

Regards,

--
Fujii Masao