Hooks for a 'blobcontainer' type plug-in have been added to Drizzle that allow a plug-in to catch insert. update, or delete operations on BLOB columns in any table and handle the BLOB storage itself. The plug-in gets called above the storage engine level so it is independent of the storage engines. This is very similar to the way that PBMS works in MySQL 5.1 with engines other than PBXT where it uses triggers to perform the same function. But the way it is done with Drizzle is a lot more efficient.
So in Drizzle the PBMS engine is both a 'blobcontainer' plug-in as well as a storage engine.
Surprisingly little needed to be done to the PBMS engine to get it to build with Drizzle. It was a big help having the PBXT engine already ported to Drizzle so for a large part all I needed to do was look to see what Paul had done and do the same. The same source code is used for both the Drizzle and MySQL builds and the build procedure is the same. When you configure the engine different build flags will automatically be set depending if the target is a MySQL source tree or a Drizzle source tree. But before you try doing a build yourself you should know that the 'blobcontainer' plug-in support has not been merged into the main Drizzle build branch yet.
The one drawback to the way the 'blobcontainer' plug-in works currently is that it take a shotgun approach to BLOB management in that either all blobs in all tables are stored using the 'blobcontainer' plug-in or none are. This is because there is no way of telling the plug-in which BLOB columns are to be stored using the plug-in and which are not. I am in hopes that some way will be found so that BLOB columns can be selectively handled by the 'blobcontainer' plug-in. Possible ways of doing this would be to have a special BLOB column type for BLOBs to be stored with the 'blobcontainer' plug-in or provide a column attribute to signal how the BLOB data is to be stored. In MySQL 5.1 only longblob columns are stored using the PBMS engine. I have no doubt though that a good solution for this problem will be found before long.