apache/hadoop · error · UnsupportedOperationException

hsync not supported by {out}

Error message

hsync not supported by {out}

What it means

BufferedIOStatisticsOutputStream.hsync() mirrors hflush(): the inner stream must implement org.apache.hadoop.fs.Syncable for the sync to be forwarded after the buffer flush. If the inner stream is not Syncable and downgradeSyncable=false, UnsupportedOperationException('hsync not supported by <out>') is thrown.

Source

Thrown at hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/statistics/BufferedIOStatisticsOutputStream.java:150

  /**
   * If the inner stream is Syncable, flush the buffer and then
   * invoke the inner stream's hsync() operation.
   *
   * Otherwise: throw an exception, unless the stream was constructed with
   * {@link #downgradeSyncable} set to true, in which case the stream
   * is just flushed.
   * @throws IOException IO Problem
   * @throws UnsupportedOperationException if the inner class is not syncable
   */
  @Override
  public void hsync() throws IOException {
    if (out instanceof Syncable) {
      flush();
      ((Syncable) out).hsync();
    } else {
      if (!downgradeSyncable) {
        throw new UnsupportedOperationException("hsync not supported by "
            + out);
      } else {
        flush();
      }
    }
  }
}

View on GitHub (pinned to 2add963021)

Solutions

1. Construct with downgradeSyncable=true if a plain flush is acceptable
2. Ensure the inner stream implements Syncable (wrap it in FSDataOutputStream)
3. Check 'out instanceof Syncable' before calling hsync()

Example fix

// before
BufferedIOStatisticsOutputStream out =
    new BufferedIOStatisticsOutputStream(inner, false);
out.hsync(); // UnsupportedOperationException when inner is not Syncable

// after
BufferedIOStatisticsOutputStream out =
    new BufferedIOStatisticsOutputStream(inner, true);
out.hsync(); // degrades to flush()

if (out instanceof Syncable) {
  statsStream.hsync();
} else {
  statsStream.flush(); // or construct the wrapper with downgradeSyncable=true
}

static boolean supportsHsync(OutputStream out) {
  return out instanceof org.apache.hadoop.fs.Syncable;
}

Catch UnsupportedOperationException from hsync() and fall back to flush() only as a last resort; the constructor flag downgradeSyncable=true makes the downgrade explicit and non-throwing.

When it happens

Trigger: Constructing the wrapper over a non-Syncable stream (compression, crypto, in-memory, mock streams) with downgradeSyncable=false, then calling hsync() to force durability.

Common situations: Durability flushes ('hsync' for data + metadata) attempted on instrumented pipelines whose bottom layer is not Syncable; tests with mock inner streams.

Related errors

• hflush not supported by {out}
• write (b[{b.length}], {off}, {len})
• catch exception when allocating BosBlockBuffer:
• Stream closed
• %s: Stream is closed!

AI-assisted analysis of apache/hadoop@2add963021 (2026-08-22). Data as JSON: /api/errors/e865fcc1bd489ff7. Report an issue: GitHub.