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

Noah Misch <noah@leadboat.com> writes:
> On Wed, Mar 25, 2020 at 04:42:31PM -0400, Tom Lane wrote:
>> So I think what we're actually trying to accomplish here is to
>> ensure that instead of deleting up to half of the SLRU space
>> before the cutoff, we delete up to half-less-one-segment.
>> Maybe it should be half-less-two-segments, just to provide some
>> cushion against edge cases.  Reading the first comment in
>> SetTransactionIdLimit makes one not want to trust too much in
>> arguments based on the exact value of xidWrapLimit, while for
>> the other SLRUs it was already unclear whether the edge cases
>> were exactly right.

> That could be interesting insurance.  While it would be sad for us to miss an
> edge case and print "must be vacuumed within 2 transactions" when wrap has
> already happened, reaching that message implies the DBA burned ~1M XIDs, all
> in single-user mode.  More plausible is FreezeMultiXactId() overrunning the
> limit by tens of segments.  Hence, if we do buy this insurance, let's skip far
> more segments.  For example, instead of unlinking segments representing up to
> 2^31 past XIDs, we could divide that into an upper half that we unlink and a
> lower half.  The lower half will stay in place; eventually, XID consumption
> will overwrite it.  Truncation behavior won't change until the region of CLOG
> for pre-oldestXact XIDs exceeds 256 MiB.  Beyond that threshold,
> vac_truncate_clog() will unlink the upper 256 MiB and leave the rest.  CLOG
> maximum would rise from 512 MiB to 768 MiB.  Would that be worthwhile?

Hmm.  I'm not particularly concerned about the disk-space-consumption
angle, but I do wonder about whether we'd be sacrificing the ability to
recover cleanly from a situation that the code does let you get into.
However, as long as we're sure that the system will ultimately reuse/
recycle a not-deleted old segment file without complaint, it's hard to
find much fault with your proposal.  Temporarily wasting some disk
space is a lot more palatable than corrupting data, and these code
paths are necessarily not terribly well tested.  So +1 for more
insurance.

			regards, tom lane