boolean isDiskAvailable(DiskId disk) {
    DiskManager diskManager = diskToDiskManager.get(disk);
    return diskManager != null && !diskManager.areAllStoresDown();
  }
Check if a certain disk is available.
@param disk the {@link DiskId} to check.
@return {@code true} if the disk is available. {@code false} if not.
ambry-store/src/main/java/com/github/ambry/store/StorageManager.java