Tuesday, 27 August 2013

Migrating Oracle to PostgreSQL

I have an oracle dump from a database that i want to migrate to postgres for a project. I setup a 100gb VM, I setup oracle 11gr2 and i compiled postgres 9.2.3 on the same VM. i restored the dump to oracle and now i had to make a migration strategy.
I had some ideas like getting the schema from the dump with expdp (sqlfile=) and manually translate it to postgres but i remembered a project that i saw some time ago called ora2pg. This cool thing is basically a perl program that needs except of perl (duh)  DBI and DBD for oracle and postgres. i set it up and i started experimenting first on the DDL's. the output was actually very good ! the only things that i had to change was some function based indexes and it was parse-able from postgres without problems, next was data. No problems there , it created a 20gb output file that all i had to do was to throw it to postgres. Just because i wasn't very proactive with my VM's disk space i faced some problems... 20Gb from the export plus the oracle data
(36664764 /opt/oradata/) + oracle software + other things ?
that was about 65% of my diskspace, and i had to also maintain a second live copy of these data+indexes on postgres.. So i gzipped the whole output file and i used "zcat |psql". worked like charm ,actually its still running as i write this because ...
And here are somethings to take under consideration before starting!
the dumpfile is 20gb, what happens if something goes wrong ?
You drop the database fix the ddl script and reapply the data
Do it one object at the time, not like i did or at least separate the small from the big tables in groups and do the big tables one at the time, or something like :
ERROR: invalid input syntax for integer: "26.19"
CONTEXT: COPY twitter_tmp, line 1, column network_score: "26.19"

might end up fucking you up !
ora2pg translated a number column as integer , something that came back to bite me in the ass 2 hours after import run...
i changed the ddl script and i rerun it, if it fails again i have to start over.. SO, don't be like me , act smart, do it one part at the time :)
in all fairness i still have no idea what tables are in the database and my job has been made easy mode with ora2pg. So.. i cant blame it, i can only blame me for not MAKING A MIGRATION STRATEGY afterall.
i will also show you the parameters that i have changed in order to make the import faster, the difference is HUGE actually..

shared_buffers = 2GB
synchronous_commit = off
wal_buffers = 16MB
wal_writer_delay = 1000ms
checkpoint_segments = 64  

Anyway so far i think i'm good with the table definitions (including constraints) and the data (hopefully) but there is a lot more that have to be done, views , code etc...

thanks for reading
to be continued...

1 comment:

  1. Hello sir,

    I am using Ora2pg tool to migrate from oracle to postgresql.

    I want to know how much time does it take approximately to do so.