Re: making relfilenodes 56 bits - Mailing list pgsql-hackers

On Wed, Jul 20, 2022 at 7:27 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> There was also an issue where the user table from the old cluster's
> relfilenode could conflict with the system table of the new cluster.
> As a solution currently for system table object (while creating
> storage first time) we are keeping the low range of relfilenumber,
> basically we are using the same relfilenumber as OID so that during
> upgrade the normal user table from the old cluster will not conflict
> with the system tables in the new cluster.  But with this solution
> Robert told me (in off list chat) a problem that in future if we want
> to make relfilenumber completely unique within a cluster by
> implementing the CREATEDB differently then we can not do that as we
> have created fixed relfilenodes for the system tables.
>
> I am not sure what exactly we can do to avoid that because even if we
> do something  to avoid that in the new cluster the old cluster might
> be already using the non-unique relfilenode so after upgrading the new
> cluster will also get those non-unique relfilenode.

I think this aspect of the patch could use some more discussion.

To recap, the problem is that pg_upgrade mustn't discover that a
relfilenode that is being migrated from the old cluster is being used
for some other table in the new cluster. Since the new cluster should
only contain system tables that we assume have never been rewritten,
they'll all have relfilenodes equal to their OIDs, and thus less than
16384. On the other hand all the user tables from the old cluster will
have relfilenodes greater than 16384, so we're fine. pg_largeobject,
which also gets migrated, is a special case. Since we don't change OID
assignments from version to version, it should have either the same
relfilenode value in the old and new clusters, if never rewritten, or
else the value in the old cluster will be greater than 16384, in which
case no conflict is possible.

But if we just assign all relfilenode values from a central counter,
then we have got trouble. If the new version has more system catalog
tables than the old version, then some value that got used for a user
table in the old version might get used for a system table in the new
version, which is a problem. One idea for fixing this is to have two
RelFileNumber ranges: a system range (small values) and a user range.
System tables get values in the system range initially, and in the
user range when first rewritten. User tables always get values in the
user range. Everything works fine in this scenario except maybe for
pg_largeobject: what if it gets one value from the system range in the
old cluster, and a different value from the system range in the new
cluster, but some other system table in the new cluster gets the value
that pg_largeobject had in the old cluster? Then we've got trouble. It
doesn't help if we assign pg_largeobject a starting relfilenode from
the user range, either: now a relfilenode that needs to end up
containing the some user table from the old cluster might find itself
blocked by pg_largeobject in the new cluster.

One solution to all this is to do as Dilip proposes here: for system
relations, keep assigning the OID as the initial relfilenumber.
Actually, we really only need to do this for pg_largeobject; all the
other relfilenumber values could be assigned from a counter, as long
as they're assigned from a range distinct from what we use for user
relations.

But I don't really like that, because I feel like the whole thing
where we start out with relfilenumber=oid is a recipe for hidden bugs.
I believe we'd be better off if we decouple those concepts more
thoroughly. So here's another idea: what if we set the
next-relfilenumber counter for the new cluster to the value from the
old cluster, and then rewrote all the (thus-far-empty) system tables?
Then every system relation in the new cluster has a relfilenode value
greater than any in use in the old cluster, so we can afterwards
migrate over every relfilenode from the old cluster with no risk of
conflicting with anything. Then all the special cases go away. We
don't need system and user ranges for relfilenodes, and
pg_largeobject's not a special case, either. We can assign relfilenode
values to system relations in exactly the same we do for user
relations: assign a value from the global counter and forget about it.
If this cluster happens to be the "new cluster" for a pg_upgrade
attempt, the procedure described at the beginning of this paragraph
will move everything that might conflict out of the way.

One thing to perhaps not like about this is that it's a little more
expensive: clustering every system table in every database on a new
cluster isn't completely free. Perhaps it's not expensive enough to be
a big problem, though.

Thoughts?

-- 
Robert Haas
EDB: http://www.enterprisedb.com