Re: pg_basebackup blocking all queries with horrible performance

On Sun, Jun 10, 2012 at 10:34 PM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
> 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.

We should not do this because Magnus is proposing the patch
(http://archives.postgresql.org/pgsql-hackers/2012-06/msg00348.php)
which breaks the above assumption at all. So we should introduce
something like NOSYNC option.

Regards,

--
Fujii Masao