Re: [HACKERS] pg_upgrade may be mortally wounded

Bruce Momjian <maillist@candle.pha.pa.us> writes:
>> BTW, it seems to me that it is a good idea to kill and restart the
>> postmaster immediately after pg_upgrade finishes.  Otherwise there might
>> be buffers in shared memory that do not reflect the actual contents of
>> the corresponding pages of the relation files (now that pg_upgrade
>> overwrote the files with other data).

> Your issue with buffer cache is a major one.  Clearly, this would be a
> problem.   However, it is my understanding that the buffer cache after
> initdb would only contain system table info, so if they pg_upgrade after
> that, there is no way they have bad stuf in the cache, right?

Cached copies of system tables obviously are no problem, since
pg_upgrade doesn't overwrite those.  I'm concerned whether there can
be cached copies of pages from user tables or indexes.  Since we've
just done a bunch of CREATE INDEXes (and a VACUUM, if my latest hack
is right), it seems at least possible that this would happen.

Now all those user tables will be empty (zero-length files), so there is
nothing to cache.  But the user indexes are *not* zero-length --- it looks
like they are at least 2 pages long even when empty.  So there seems
to be a real risk of having a cached copy of one of the pages of a user
index while pg_upgrade is overwriting the index file with new data...
        regards, tom lane