Re: Proposing pg_hibernate

On Tue, Jun 3, 2014 at 7:57 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Thu, May 29, 2014 at 12:12 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>> IMHO, all of these caveats, would affect a very small fraction of
>>> use-cases and are eclipsed by the benefits this extension provides in
>>> normal cases.
>>
>> I agree with you that there are only few corner cases where evicting
>> shared buffers by this utility would harm, but was wondering if we could
>> even save those, say if it would only use available free buffers. I think
>> currently there is no such interface and inventing a new interface for this
>> case doesn't seem to reasonable unless we see any other use case of
>> such a interface.
>
> It seems like it would be best to try to do this at cluster startup
> time, rather than once recovery has reached consistency. Of course,
> that might mean doing it with a single process, which could have its
> own share of problems. But I'm somewhat inclined to think that if
> recovery has already run for a significant period of time, the blocks
> that recovery has brought into shared_buffers are more likely to be
> useful than whatever pg_hibernate would load.

I am not absolutely sure of the order of execution between recovery
process and the BGWorker, but ...

For sizeable shared_buffers size, the restoration of the shared
buffers can take several seconds. I have a feeling the users wouldn't
like their master database take up to a few minutes to start accepting
connections. From my tests [1], " In the 'App after Hibernator' [case]
... This took 70 seconds for reading the ~4 GB database."

[1]: http://gurjeet.singh.im/blog/2014/04/30/postgres-hibernator-reduce-planned-database-down-times/

Best regards,
--
Gurjeet Singh http://gurjeet.singh.im/

EDB www.EnterpriseDB.com