ODC Appreciation Day : DMU – Oracle Database Migration Assistant for Unicode


(Picture from Oracle.com)

It was very difficult for me to choose a topic for #ThanksODC this year 🙂 I finally decided to talk about something very useful, but maybe not so widespread.

For once, I am going to praise a GUI 🙂 : The Oracle Database Migration Assistant for Unicode (DMU). Migrating a database’s character set is not trivial. This is why DMU comes in handy.

It is a free tool designed to migrate a database’s character set from legacy to Unicode, like AL32UTF8, which is the character set recommended by Oracle.
Edit, thanks to Łukasz Bednarek : DMU 2.2 is supported to migrate databases in version and later. You can use DMU 2.1 with version And for versions older than, you can use DMU 2.1 if the database has patch 9825461 applied. Please check the compatibility matrix to make sure DMU 2.1 is supported for your version.
DMU now replaces CSSCAN and CSALTER utilities starting from Oracle 12.1, and they are therefore desupported.

I used DMU several times to migrate small and big databases in place. It is also very useful when you cannot rely on Data Pump to perform a migration, for example because of the volume of data or the allowed downtime window.

DMU is very easy to install and to use with its nice GUI, and in particular, it handles all the migration steps : scanning, cleansing, converting and validating.

Here are a few benefits provided by DMU :

  • Monitor the progression of the whole process, with DMU GUI or even in Enterprise Manager
  • Check what the data look like after scanning
  • Launch the scanner with the command-line
  • Choose wether you want DMU to use CTAS operation to modify a table, instead of using a time-consuming update in some cases
  • Automate data cleansing by
    • altering the length semantics of CHAR and VARCHAR2 table columns from byte semantics to character semantics
    • replacing the wrong characters with a set of valid equivalent characters
  • Allow conversion of data with issues
  • Export and import migration profile to launch a character set migration on another database but with the same settings

I sometimes had to use a few tricks to make the migration go as I want or to make it more performant, and I found a lot of helpful tips in MOS note 2018250.1, which is a gold mine about DMU.

As I started my DBA career 3 years ago, when Oracle 12.1 was already widely used, I used CSSCAN and CSALTER only a very few times, but I can compare and say that DMU is a lot smoother 😉

Edit : I am really happy to participate to the #ThanksODC event this year, as it marks my blog’s first birthday 🙂

1 thought on “ODC Appreciation Day : DMU – Oracle Database Migration Assistant for Unicode

  1. Pingback: DMU reports “Need conversion” on table WRI$_SQLSET_DEFINITIONS in Data dictionary – Floo Bar

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