Re: Proposing pg_hibernate

On Thu, Jun 5, 2014 at 11:32 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> On Thu, Jun 5, 2014 at 5:39 PM, Gurjeet Singh <gurjeet(at)singh(dot)im> wrote:
>>
>> > On Tue, Jun 3, 2014 at 5:43 PM, Gurjeet Singh <gurjeet(at)singh(dot)im> wrote:
>> Case 2 also won't cause any buffer restores because the save-files are
>> created only on clean shutdowons; not on a crash or immediate
>> shutdown.
>
> How do you ensure that buffers are saved only on clean shutdown?

Postmaster sends SIGTERM only in "smart" or "fast" shutdown requests.

> Buffer saver process itself can crash while saving or restoring
> buffers.

True. That may lead to partial list of buffers being saved. And the
code in Reader process tries hard to read only valid data, and punts
at the first sight of data that doesn't make sense or on ERROR raised
from Postgres API call.

> IIUC on shutdown request, postmaster will send signal to BG Saver
> and BG Saver will save the buffers and then postmaster will send
> signal to checkpointer to shutdown. So before writing Checkpoint
> record, BG Saver can crash (it might have saved half the buffers)

Case handled as described above.

> or may BG saver saves buffers, but checkpointer crashes (due to
> power outage or any such thing).

Checkpointer process' crash seems to be irrelevant to Postgres
Hibernator's workings.

I think you are trying to argue the wording in my claim "save-files
are created only on clean shutdowons; not on a crash or immediate
shutdown", by implying that a crash may occur at any time during and
after the BufferSaver processing. I agree the wording can be improved.
How about

... save-files are created only when Postgres is requested to shutdown
in normal (smart or fast) modes.

Note that I am leaving out the mention of crash.

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

EDB www.EnterpriseDB.com