findBlock(BufferTag pageID) returns (Vfd, off_t)
{
   offset = pageID.blockNum * BLOCKSIZE
   fileName = relpath(pageID.rnode)
   if (pageID.forkNum > 0)
      fileName = fileName+"."+pageID.forkNum
   if (fileName is not in Vfd pool)
      fd = allocate new Vfd for fileName
   else
      fd = use Vfd from pool
   if (offset > fd.fileSize) {
      fd = allocate new Vfd for next fork
      offset = offset - fd.fileSize
   }
   return (fd, offset)
}