code: private StorageDirView selectEvictableDirFromTier(StorageTierView tierView, long availableBytes) {
      for (StorageDirView dirView : tierView.getDirViews()) {
        if (canEvictBlocksFromDir(dirView, availableBytes)) {
          return dirView;
        }
      }
      return null;
    }
docs: Selects a dir with enough space (including space evictable) from a given tier