memory usage of pg_upgrade

pg_upgrade reserves 5 times MAXPGPATH, or 5120 characters, for the
tablespace name of every object (table, toast table, index) in the
database being upgraded. This adds up pretty quickly when there is a
very large number of objects. It could be changed to char* to a
separately allocated name that takes only as much space it needs. But
maybe it would be better to point into os_info.old_tablespaces or
something like that, as surely there are not going to be one
independent file space per object.

typedef struct
{
...
char tablespace[MAXPGPATH];
} RelInfo;

The struct FileNameMap has 4 more .

Since there seems to be some interest in improving the scalability of
pg_upgrade, this is one of the things to consider fixing. What is the
best way to do it?

Cheers,

Jeff