Re: Replication slot stats misgivings

On Sat, Apr 3, 2021 at 11:07 PM vignesh C <vignesh21@gmail.com> wrote:
>
> On Fri, Apr 2, 2021 at 9:29 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > On Fri, Apr 2, 2021 at 1:55 AM vignesh C <vignesh21@gmail.com> wrote:
> > >
> > > On Thu, Apr 1, 2021 at 5:58 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > > >
> > > > On Thu, Apr 1, 2021 at 3:43 PM vignesh C <vignesh21@gmail.com> wrote:
> > > > >
> > > > > On Wed, Mar 31, 2021 at 11:32 AM vignesh C <vignesh21@gmail.com> wrote:
> > > > > >
> > > > > > On Tue, Mar 30, 2021 at 11:00 AM Andres Freund <andres@anarazel.de> wrote:
> > > > > > >
> > > > > > > Hi,
> > > > > > >
> > > > > > > On 2021-03-30 10:13:29 +0530, vignesh C wrote:
> > > > > > > > On Tue, Mar 30, 2021 at 6:28 AM Andres Freund <andres@anarazel.de> wrote:
> > > > > > > > > Any chance you could write a tap test exercising a few of these cases?
> > > > > > > >
> > > > > > > > I can try to write a patch for this if nobody objects.
> > > > > > >
> > > > > > > Cool!
> > > > > > >
> > > > > >
> > > > > > Attached a patch which has the test for the first scenario.
> > > > > >
> > > > > > > > > E.g. things like:
> > > > > > > > >
> > > > > > > > > - create a few slots, drop one of them, shut down, start up, verify
> > > > > > > > >   stats are still sane
> > > > > > > > > - create a few slots, shut down, manually remove a slot, lower
> > > > > > > > >   max_replication_slots, start up
> > > > > > > >
> > > > > > > > Here by "manually remove a slot", do you mean to remove the slot
> > > > > > > > manually from the pg_replslot folder?
> > > > > > >
> > > > > > > Yep - thereby allowing max_replication_slots after the shutdown/start to
> > > > > > > be lower than the number of slots-stats objects.
> > > > > >
> > > > > > I have not included the 2nd test in the patch as the test fails with
> > > > > > following warnings and also displays the statistics of the removed
> > > > > > slot:
> > > > > > WARNING:  problem in alloc set Statistics snapshot: detected write
> > > > > > past chunk end in block 0x55d038b8e410, chunk 0x55d038b8e438
> > > > > > WARNING:  problem in alloc set Statistics snapshot: detected write
> > > > > > past chunk end in block 0x55d038b8e410, chunk 0x55d038b8e438
> > > > > >
> > > > > > This happens because the statistics file has an additional slot
> > > > > > present even though the replication slot was removed.  I felt this
> > > > > > issue should be fixed. I will try to fix this issue and send the
> > > > > > second test along with the fix.
> > > > >
> > > > > I felt from the statistics collector process, there is no way in which
> > > > > we can identify if the replication slot is present or not because the
> > > > > statistic collector process does not have access to shared memory.
> > > > > Anything that the statistic collector process does independently by
> > > > > traversing and removing the statistics of the replication slot
> > > > > exceeding the max_replication_slot has its drawback of removing some
> > > > > valid replication slot's statistics data.
> > > > > Any thoughts on how we can identify the replication slot which has been dropped?
> > > > > Can someone point me to the shared stats patch link with which message
> > > > > loss can be avoided. I wanted to see a scenario where something like
> > > > > the slot is dropped but the statistics are not updated because of an
> > > > > immediate shutdown or server going down abruptly can occur or not with
> > > > > the shared stats patch.
> > > > >
> > > >
> > > > I don't think it is easy to simulate a scenario where the 'drop'
> > > > message is dropped and I think that is why the test contains the step
> > > > to manually remove the slot. At this stage, you can probably provide a
> > > > test patch and a code-fix patch where it just drops the extra slots
> > > > from the stats file. That will allow us to test it with a shared
> > > > memory stats patch on which Andres and Horiguchi-San are working. If
> > > > we still continue to pursue with current approach then as Andres
> > > > suggested we might send additional information from
> > > > RestoreSlotFromDisk to keep it in sync.
> > >
> > > Thanks for your comments, Attached patch has the fix for the same.
> > > Also attached a couple of more patches which addresses the comments
> > > which Andres had listed i.e changing char to NameData type and also to
> > > display the unspilled/unstreamed transaction information in the
> > > replication statistics.
> > > Thoughts?
> >
> > Thank you for the patches!
> >
> > I've looked at those patches and here are some comments on 0001, 0002,
> > and 0003 patch:
>
> Thanks for the comments.
>
> > 0001 patch:
> >
> > -       values[0] = PointerGetDatum(cstring_to_text(s->slotname));
> > +       values[0] = PointerGetDatum(cstring_to_text(s->slotname.data));
> >
> > We can use NameGetDatum() instead.
>
> I felt we will not be able to use NameGetDatum because this function
> will not have access to the value throughout the loop and NameGetDatum
> must ensure the pointed-to value has adequate lifetime.
>
> > ---
> > 0002 patch:
> >
> > The patch uses logical replication to test replication slots
> > statistics but I think it's necessarily necessary. It would be more
> > simple to use logical decoding. Maybe we can add TAP tests to
> > contrib/test_decoding.
> >
>
> I will try to change it to test_decoding if feasible and post in the
> next version.
>

I have modified the patch to include tap tests in contrib/test_decoding.
Attached v3 patch has the changes for the same.
Thoughts?

Regards,
Vignesh