Re: Inconsistency in determining the timestamp of the db statfile.

On Wed, Sep 9, 2020 at 3:17 PM Magnus Hagander <magnus(at)hagander(dot)net> wrote:
>
> On Wed, Sep 9, 2020 at 9:11 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>
>> On Wed, Sep 9, 2020 at 10:54 AM Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com> wrote:
>> >
>> > On 2020/09/09 12:04, Amit Kapila wrote:
>> > >
>> > > No, before patch as well, if we can't read the DB entry say because
>> > > the file is corrupt, we return true and use timestamp of global stats
>> > > file and this is what is established by the original commit 187492b6.
>> > > So, if we consider that as correct then the functionality is something
>> > > like once we have read the timestamp of the global statfile, we use
>> > > that if we can't read the actual db entry for whatever reason. It
>> > > seems if we return false, the caller will call this function again in
>> > > the loop. Now, I see the point that if we can't read some part of the
>> > > file we should return false instead but not sure if that is helpful
>> > > from the perspective of the caller of
>> > > pgstat_read_db_statsfile_timestamp.
>> >
>> > When false is returned, the caller backend_read_statsfile() seems to
>> > request the stats collector to write a fresh stats data into the file,
>> > and then pgstat_read_db_statsfile_timestamp() can try to read the fresh
>> > file that may not be corrupted. So returning false in that case seems ok
>> > to me...
>> >
>>
>> Hmm, the request to get fresh data is sent irrespective of true or
>> false. We send it to get the latest data if it is not present and it
>
>
> No we don't. Just before we request it, there is:
> /* Normal acceptance case: file is not older than cutoff time */
> if (ok && file_ts >= min_ts)
> break;
>
> So it only requests a new file in the case that it returned false.
>

What if the second part of the above 'if' statement is false, then
basically even when pgstat_read_db_statsfile_timestamp() has returned
true, we can send a request. IIUC, here the basic idea is if the stats
in the file is updated before cut_off time, then we do send the
request and wait for updated stats.

--
With Regards,
Amit Kapila.