@Override
  public PartitionFileStore getFileStore(PartitionId id) throws Exception {
    Store store = getStore(id, true);
    if (store == null) {
      return null;
    }
    return store.getFileStore();
  }
Maybe restore disk's availability and update instance capacity.
@return A set of {@link DiskId}s that are still unavailable.
ambry-store/src/main/java/com/github/ambry/store/StorageManager.java