File system level backup

Hi,

I have two virtual machines with two different versions of Postgresql. One
machine contains Postgres 7.4.19 and another has Postgres 8.4.3. I also
have other instances of these two virtual machines. I need to transfer the
database from one machine to other. Both machines contain a database with
the same name, for example: testdb, but with different data/values, but the
structure is exactly same.

I want to do the following:

. Take file system level backup from from first machine for
Postgres database 'testdb' and restore it in another instance that has SAME
version of Postgres. i.e. backup the database 'testdb' from Postgres 7.4.19
and restore it on another virtual machine with same Postgres version 7.4.19.

. Take file system level backup from from first machine for
Postgres database 'testdb' and restore it in another instance that has
DIFFERENT version of Postgres. i.e. backup the database testdb from
Postgres 7.4.19 and restore it to another virtual machine with different
Postgres version 8.4.3.

I can achieve it with pg_dump and pg_restore, but not with file level
backups. The data directory is /var/lib/pgsql/data that contains base
directory amongst others, that contains directories for each database
referred to by their OIDs. If I replace the complete data directory of one
machine from the instance of another machine (with same or different
Postgres version), It is failing to identify OIDs for that database.

It should be possible in Postgres to swap two data directories in two
different virtual machines without requiring pg_dump and pg_restore. With
me, it doesn't work in both the cases mentioned above. In first case, it
gives an error of missing OID for the database. In second case, it is
giving version incompatibility issue.

Is there a way in Postgres to do file system level backup? The objective is
to push /var/lib/pgsql/data directory across different virtual machines,
without the need to backup and restore Postgres database with 'pg_dump' and
'pg_restore' commands. Any help will be highly appreciated.

Kind Regards,

Manoj Agarwal