code: private StorageDirView selectEvictableDirFromAnyTier(BlockMetadataManagerView view,
        long availableBytes) {
      for (StorageTierView tierView : view.getTierViews()) {
        for (StorageDirView dirView : tierView.getDirViews()) {
          if (canEvictBlocksFromDir(dirView, availableBytes)) {
            return dirView;
          }
        }
      }
      return null;
    }
docs: Selects a dir with enough space (including space evictable) from all tiers