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

On Tue, Sep 8, 2020 at 3:11 PM Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com>
wrote:

>
>
> On 2020/09/08 19:28, Magnus Hagander wrote:
> >
> >
> > On Tue, Sep 8, 2020 at 8:10 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com
> <mailto:amit(dot)kapila16(at)gmail(dot)com>> wrote:
> >
> > We use the timestamp of the global statfile if we are not able to
> > determine it for a particular database either because the entry for
> > that database doesn't exist or there is an error while reading the
> > specific database entry. This was not taken care of while reading
> > other entries like ArchiverStats or SLRUStats. See
> > pgstat_read_db_statsfile_timestamp. I have observed this while
> > reviewing Sawada-San's patch related to logical replication stats
> [1].
> >
> > I think this can only happen if due to some reason the statfile got
> > corrupt or we
> > have some bug in stats writing code, the chances of both are rare
> and even
> > if that happens we will use stale statistics.
> >
> > The attached patch by Sawada-San will fix this issue. As the chances
> of this
> > the problem is rare and nobody has reported any issue related to
> this,
> > so it might be okay not to backpatch this.
> >
> > Thoughts?
> >
> >
> > Why are we reading the archiver statis and and slru stats in
> "pgstat_read_db_statsfile_timestamp" in the first place?
>
> Maybe because they are written before database stats entries? That is,
> to read the target database stats entry and get the timestamp from it,
> we need to read (or lseek) all the global stats entries written before
> them.
>
>
Oh meh. Yeah, I'm reading this thing completely wrong :/ Ignore my comments
:)

> That seems well out of scope for that function.
> >
> > If nothing else the comment at the top of that function is out of touch
> with reality. We do seem to read it into local buffers and then ignore the
> contents. I guess we're doing it just to verify that it's not corrupt -- so
> perhaps the function should actually have a different name than it does
> now, since it certainly seems to do more than actually read the statsfile
> timestamp.
> >
> > Specifically in this patch it looks wrong -- in the case of say the SLRU
> stats being corrupt, we will now return the timestamp of the global stats
> file even if there is one existing for the database requested, which
> definitely breaks the contract of the function.
>
> Yes.
> We should return false when fread() for database entry fails, instead?
> That is,
>
> 1. If corrupted stats file is found, the function always returns false.
> 2. If the file is not currupted and the target database entry is found,
> its timestamp is returned.
> 3. If the file is not corrupted and the target is NOT found, the timestamp
> of global entry is returned
>

Yeah, with more coffee and re-reading it, I'm not sure how we could have
the database stats being OK if the archiver or slru stats are not.

That said, I think it still makes sense to return false if the stats file
is corrupt. How much can we trust the first block, if the block right after
it is corrupt? So yeah, I think your described order seems correct. But
that's also what the code already did before this patch, isn't it?

--
Magnus Hagander
Me: https://www.hagander.net/ <http://www.hagander.net/>
Work: https://www.redpill-linpro.com/ <http://www.redpill-linpro.com/>