Re: Spurious "apparent wraparound" via SimpleLruTruncate() rounding

On Sun, Jan 10, 2021 at 11:44:14AM +0500, Andrey Borodin wrote:
> > 10 янв. 2021 г., в 03:15, Noah Misch <noah(at)leadboat(dot)com> написал(а):
> >
> > No; it deletes the most recent ~1B and leaves the older segments. An
> > exception is multixact, as described in the commit message and the patch's
> > change to a comment in TruncateMultiXact().
>
> Thanks for clarification.
> One more thing: retention point at 3/4 of overall space (half of wraparound) seems more or less random to me. Why not 5/8 or 9/16?

No reason for that exact value. The purpose of that patch is to mitigate bugs
that cause the server to write data into a region of the SLRU that we permit
truncation to unlink. If the patch instead tested "diff > INT_MIN * .99", the
new behavior would get little testing, because xidWarnLimit would start first.
Also, the new behavior wouldn't mitigate bugs that trespass >~20M XIDs into
unlink-eligible space. If the patch tested "diff > INT_MIN * .01", more sites
would see disk consumption grow. I think reasonable multipliers range from
0.5 (in the patch today) to 0.9, but it's a judgment call.

> Can you please send revised patches with fixes?

Attached.