For the DDS implementations that support the persistence profile, you can set the durability QoS policy to PERSISTENT. This makes sure the data kept on permanent storage to outlive even a system session. However, this does not implicate a 'human readable' format on disk.
To write the data to a file in your own format you can consider: 1. Some DDS implementations provide DBMS integration, which allows you to store the data in database tables. 2. Generating type specific code from your IDL definition(s) to store data for that specific type. 3. When reflection is available (for instance in the Java language) resolving the attributes of your types in a generic manner is also an option.