BD-J includes support for Local Storage. Two types of Local Storage are used – mandatory Application Data Area and optional Binding Unit Data Area. All Local Storage is accessed using methods from the Java IO package.

Application Data Area is Local Storage that will be present in all BD Players for BD-J mode. The required minimum size of this Application Data Area (256 MB) will permit storage of application data like settings, game saves etc. Typically the Application Data Area will be implemented using the player's non-removable flash storage.

Additionally, the optional Binding Unit Data Area (BUDA) is available. The BUDA will be implemented on a HDD and/or a user expandable/removable storage such as a USB flash drive or SD card. There is a Java class called StorageManager in org.bluray.storage that lets you find out how much space and free space there is in the BUDA using the  getBindingunitDataAreaInfo(). 

Because storage is shared among all discs played on the BD Player, Java access control is a key part of BD-J. BD-J applications are only able to access the storage space specific to the disc and cannot access the storage belonging to other discs.

NOTE: You don't have to worry about local storage if your application is using HDMV mode, since it's a basic mode and requires smaller files that are stored directly into the player's memory from the disc.

Virtual File System

BD-J features the Virtual File System (VFS). It's a feature that allows the Java Xlet to read, write, download, and install content in the Blu-ray player's Local Storage.

The files and assets stored locally are integrated with the current optical disc's file system. Additionally, the player's read/write file system can overlay files and assets on the optical disc's file system to create a unified virtual file system. The integrated virtual file system is referred to by the system property  org.bluray.vfs.root. The VFS is useful for storing assets without needing the disc. For example, loading large assets from local storage while the A/V is playing from the optical disc at the same time or saving game progress such as high scores. It is also useful for quicker load times than loading content from the disc.

 

Blu-ray players like the PlayStation 4, let's view or remove BD saved data such as preferences, bookmarks, game progress, etc.

Storage is mandatory for all Blu-ray players after Profile 1.1. Bonus View (Profile 1.1) players have a minimum storage capacity of 256 MB, while BD-Live (Profile 2.0 or later) players can store up to 1 GB. Most players allow additional storage via HDD, SSD, Flash, USB, etc. You can also transfer saved files (game progress, bookmarks, updates, etc.) to another player via USB (a la PS2 and Xbox 360).

If a disc that needs local storage is inserted into a player that does not have any storage, like a Grace Period (Profile 1.0.) player, most discs will often display a message telling the user it requires local storage. It will also present another if the player's storage isn't enough.

Important Information

Not all Blu-ray players let you manage and delete individual saved data files in your local storage like the PS4 seen above. Most players only have a "delete storage" option, and that's it. Most players allow users to use USB flash drives to store their saved data, and you can manage it manually using a computer. However, if you're creating a video game, it's a good idea to include a built-in save manager (a separate BD-J application) to manage BD storage similar to the Nintendo 64's Controller Pak Menu.

Persistent Storage
Persistent Storage is similar to Local Storage and is required to be present on all players; it can read/write small data storage with a limited capacity of 68 KB. It's very small, so what's it for?

It's usually best for storing user preferences, bookmarks, or resume playback. It is recommended that a disc should not use more than 4 KB of data. Naturally, it may be necessary for players to delete old data that is unused. The mount point for this filesystem directory is found in dvb.persistent.root. An Xlet can control the persistent storage using the application and organization identifier, application_id, and organisation_id (English spelling). They are used to construct a hierarchical file system under this directory: <dvb.persistent.root.>/<organization_id>/<application_id>. 

 

Local Storage is faster

For complex BD-J applications, local is storage is not only used for user settings, bookmarks, game saves, but it can be used as a cache to load or stream data quicker to the memory. Data can be installed into the local storage into the VFS to be loaded later into the memory for faster load times. 

How to have fast load times for a BD-J application:

• Keep frequently used files on the outer edge of the BD
• Group together files that are loaded together
• Minimize files count
• Minimize file size
• Install necessary data ahead of time to the local storage

Sources

• Programming HD-DVD and Blu-ray Disc: The HD Cookbook

Author(s) : Æ Firestone