I have been in a shop about 5 months ago and they were using AS400 transparent gateway connecting oracle to db2 and indeed it could only read and update DB2 tables not vice-versa the pain was that to change the version of AS400 they were forced to change transparenet gateway version as well consequently I think Oracle suggested them to migrate the database server version as well (and all this just because IBM told them that their AS400 version was going to be desupported)