Re: [HACKERS] pg_upgrade may be mortally wounded

> Bruce Momjian <maillist@candle.pha.pa.us> writes:
> > Tom, did we address this.  I forgot.
>
> No, it's still an open issue as far as I'm concerned.  I was hoping to
> hear something from Vadim about how pg_upgrade could work safely under
> MVCC...
>
>             regards, tom lane

Would a solution to this be to add instructions to pg_upgrade to require
the user to stop and restart the postmaster?  Seems like that is the
only solution unless we do that stop of postmater inside pg_upgrade, but
that seems risky.

>
>
> >> 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...
>


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