{
// Find from file ref.
FileRef ref = FileRef.create(fileSet, ext) ;
BlockMgr baseMgr = blockMgrs.get(ref) ;
if ( baseMgr == null )
throw new TDBException("No BlockMgr for "+ref) ;
BlockMgrJournal blkMgr = new BlockMgrJournal(txn, ref, baseMgr) ;
txn.addComponent(blkMgr) ;
return blkMgr ;
}