public BlockMgr buildBlockMgr(FileSet fileSet, String ext, int blockSize)
{
FileRef ref = FileRef.create(fileSet, ext) ;
BlockMgr blockMgr = blockMgrs.get(ref) ;
if ( blockMgr == null )
throw new TDBException("No BlockMgr for "+ref) ;
blockMgr = new BlockMgrReadonly(blockMgr) ;
return blockMgr ;
}