Re: new heapcheck contrib module

On Thu, Jul 30, 2020 at 4:18 PM Mark Dilger
<mark(dot)dilger(at)enterprisedb(dot)com> wrote:
> > Maybe I'm just being dense here -- exactly what problem are you worried about?
>
> Per tuple, tuple_is_visible() potentially checks whether the xmin or xmax committed via TransactionIdDidCommit. I am worried about concurrent truncation of clog entries causing I/O errors on SLRU lookup when performing that check. The three strategies I had for dealing with that were taking the XactTruncationLock (formerly known as CLogTruncationLock, for those reading this thread from the beginning), locking out vacuum, and the idea upthread from Andres about setting PROC_IN_VACUUM and such. Maybe I'm being dense and don't need to worry about this. But I haven't convinced myself of that, yet.

I don't get it. If you've already checked that the XIDs are >=
relfrozenxid and <= ReadNewFullTransactionId(), then this shouldn't be
a problem. It could be, if CLOG is hosed, which is possible, because
if the table is corrupted, why shouldn't CLOG also be corrupted? But
I'm not sure that's what your concern is here.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company