@Override
  public Set<StoreKey> findMissingKeys(List<StoreKey> keys) throws StoreException {
    checkStarted();
    final Timer.Context context = metrics.findMissingKeysResponse.time();
    try {
      Set<StoreKey> missingKeys = index.findMissingKeys(keys);
      return missingKeys;
    } catch (StoreException e) {
      if (e.getErrorCode() == StoreErrorCodes.IOError) {
        onError();
      }
      throw e;
    } finally {
      context.stop();
    }
  }
Checks the used capacity of the store against the configured percentage thresholds to see if the store
should be read-only or read-write
ambry-store/src/main/java/com/github/ambry/store/BlobStore.java