Re: Running pg_dump from a slave server

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?

> Regards,
> Venkata B N
>
> Fujitsu Australia
>
--
--
Best Regards
Sameer Kumar | DB Solution Architect
*ASHNIK PTE. LTD.*

101 Cecil Street, #11-11 Tong Eng Building, Singapore 069 533

T: +65 6438 3504 | M: +65 8110 0350

Skype: sameer.ashnik | www.ashnik.com