Re: [Patch] Optimize dropping of relation buffers using dlist

On Wed, Sep 16, 2020 at 7:46 AM Kyotaro Horiguchi
<horikyota(dot)ntt(at)gmail(dot)com> wrote:
>
> At Wed, 2 Sep 2020 08:18:06 +0530, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote in
> > On Wed, Sep 2, 2020 at 7:01 AM Kyotaro Horiguchi
> > <horikyota(dot)ntt(at)gmail(dot)com> wrote:
> > > Isn't a relation always locked asscess-exclusively, at truncation
> > > time? If so, isn't even the result of lseek reliable enough?
> > >
> >
> > Even if the relation is locked, background processes like checkpointer
> > can still touch the relation which might cause problems. Consider a
> > case where we extend the relation but didn't flush the newly added
> > pages. Now during truncate operation, checkpointer can still flush
> > those pages which can cause trouble for truncate. But, I think in the
> > recovery path such cases won't cause a problem.
>
> I reconsided on this and still have a doubt.
>
> Is this means lseek(SEEK_END) doesn't count blocks that are
> write(2)'ed (by smgrextend) but not yet flushed? (I don't think so,
> for clarity.) The nblocks cache is added just to reduce the number of
> lseek()s and expected to always have the same value with what lseek()
> is expected to return.
>

See comments in ReadBuffer_common() which indicates such a possibility
("Unfortunately, we have also seen this case occurring because of
buggy Linux kernels that sometimes return an lseek(SEEK_END) result
that doesn't account for a recent write."). Also, refer my previous
email [1] on this and another email link in that email which has a
discussion on this point.

> The reason it is reliable only during recovery
> is that the cache is not shared but the startup process is the only
> process that changes the relation size during recovery.
>

Yes, that is why we are planning to do this optimization for recovery path.

> If any other process can extend the relation while smgrtruncate is
> running, the current DropRelFileNodeBuffers should have the chance
> that a new buffer for extended area is allocated at a buffer location
> where the function already have passed by, which is a disaster.
>

The relation might have extended before smgrtruncate but the newly
added pages can be flushed by checkpointer during smgrtruncate.

[1] - https://www.postgresql.org/message-id/CAA4eK1LH2uQWznwtonD%2Bnch76kqzemdTQAnfB06z_LXa6NTFtQ%40mail.gmail.com

--
With Regards,
Amit Kapila.