Re: Running pg_dump from a slave server

On Wed, Aug 17, 2016 at 2:09 PM, Sameer Kumar <sameer(dot)kumar(at)ashnik(dot)com>
wrote:

>
>
> On Wed, Aug 17, 2016 at 12:00 PM Venkata B Nagothi <nag1010(at)gmail(dot)com>
> wrote:
>
>> On Wed, Aug 17, 2016 at 1:31 PM, Sameer Kumar <sameer(dot)kumar(at)ashnik(dot)com>
>> wrote:
>>
>>>
>>>
>>> On Wed, Aug 17, 2016 at 10:34 AM Patrick B <patrickbakerbr(at)gmail(dot)com>
>>> wrote:
>>>
>>>> Hi guys,
>>>>
>>>> I'm using PostgreSQL 9.2 and I got one master and one slave with
>>>> streaming replication.
>>>>
>>>> Currently, I got a backup script that runs daily from the master, it
>>>> generates a dump file with 30GB of data.
>>>>
>>>> I changed the script to start running from the slave instead the
>>>> master, and I'm getting this errors now:
>>>>
>>>> pg_dump: Dumping the contents of table "invoices" failed: PQgetResult()
>>>>> failed.
>>>>> pg_dump: Error message from server: ERROR: canceling statement due to
>>>>> conflict with recovery
>>>>> DETAIL: User was holding a relation lock for too long.
>>>>
>>>>
>>> Looks like while your pg_dump sessions were trying to fetch the data,
>>> someone fired a DDL or REINDEX or VACUUM FULL on the master database.
>>>
>>>>
>>>> Isn't that possible? I can't run pg_dump from a slave?
>>>>
>>>
>>> Well you can do that, but it has some limitation. If you do this quite
>>> often, it would be rather better to have a dedicated standby for taking
>>> backups/pg_dumps. Then you can set max_standby_streaming_delay and
>>> max_standby_archiving_delay to -1. But I would not recommend doing this if
>>> you use your standby for other read queries or for high availability.
>>>
>>> Another option would be avoid such queries which causes Exclusive Lock
>>> on the master database during pg_dump.
>>>
>>
>> Another work around could be to pause the recovery, execute the pg_dump
>> and then, resume the recovery process. Not sure if this work around has
>> been considered.
>>
>> You can consider executing "pg_xlog_replay_pause()" before executing
>> pg_dump and then execute "pg_xlog_replay_resume()" after the pg_dump
>> process completes.
>>
>
> Ideally I would not prefer if I had only one standby. If I am right, it
> would increase the time my standby would take to complete recovery and
> become active during a promotion (if I need it during a failure of master).
> It may impact high availability/uptime. Isn't it?
>

Yes, depending on how long pg_dump takes on standby, the lag would increase
by that much time. It does have an impact on high availability up-time
which needs to be taken into consideration and same is the case with
max_standby* parameters as you mentioned above. I am just referring to an
option for a clean pg_dump from standby, ofcourse, provided it adheres with
high-availability up-time SLA.

Regards,
Venkata B N

Fujitsu Australia