apache/cassandra · error · IOException

could not read required number of bytes from file to be…

Error message

could not read required number of bytes from file to be streamed: read %d bytes, wanted %d bytes

What it means

AsyncStreamingOutputPlus.writeFileToChannel streams a file to the network channel by reading slices via FileChannel.read into a buffer. If a read returns fewer bytes than requested (toWrite), the file could not supply the expected bytes and an IOException is thrown, since a short read would corrupt the streamed data.

Solutions

Ensure the file being streamed is not deleted/truncated concurrently (hold a reference/retry logic that re-checks file length)
Re-check the file length before each chunk and clamp toWrite to the remaining bytes
Retry the streaming operation; the source file state was inconsistent
Verify filesystem health if short reads occur without concurrent modification

Example fix

// before
long toWrite = Math.min(chunkSize, fileLength - position);
long read = fc.read(outBuffer, position);
if (read != toWrite) throw new IOException(...);

// after
long fileLength = fc.size();
long toWrite = Math.min(chunkSize, fileLength - position);
if (toWrite <= 0) break; // file shrank; stop instead of short-read
long read = fc.read(outBuffer, position);
if (read != toWrite) throw new IOException(...);

Defensive patterns

Strategy: try-catch

Validate before calling

long remaining = fileChannel.size() - position;
if (remaining < toWrite) throw new IOException("File shorter than expected: " + remaining + " < " + toWrite);

Try / catch

try { output.writeFileToChannel(fileChannel, length, limits); }
catch (IOException e) {
  if (e.getMessage().contains("could not read required number of bytes")) {
    logger.warn("Source file changed during stream, retrying", e);
    retryStream();
  } else throw e;
}

Prevention

Do not delete/compact files concurrently with streaming them
Verify file size immediately before streaming and clamp chunk sizes
Design streaming consumers to tolerate and retry short-read failures

When it happens

Trigger: During writeFileToChannel, FileChannel.read(buffer, position) returns a value less than the requested byte count — e.g. the file was truncated or modified concurrently so fewer bytes remain at `position` than the planned `toWrite` size.

Common situations: A file being streamed is truncated or compacted/deleted concurrently by another process (e.g. compaction removing an SSTable mid-stream); filesystem errors; incorrect byte counts computed from a stale file length.

Understand the failure class

Background: "failed to read file", EACCES, ENOENT and "could not read <path>" errors: when a program can't read a file from disk — this error's family across 49 libraries.

Related errors

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/4a2fba8d781db3ec. Report an issue: GitHub.

Appendix: source

Thrown at src/java/org/apache/cassandra/net/AsyncStreamingOutputPlus.java:187

    @VisibleForTesting
    long writeFileToChannel(FileChannel fc, RateLimiter limiter, int batchSize) throws IOException
    {
        final long length = fc.size();
        long bytesTransferred = 0;

        try
        {
            while (bytesTransferred < length)
            {
                int toWrite = (int) min(batchSize, length - bytesTransferred);
                final long position = bytesTransferred;

                writeToChannel(bufferSupplier -> {
                    ByteBuffer outBuffer = bufferSupplier.get(toWrite);
                    long read = fc.read(outBuffer, position);
                    if (read != toWrite)
                        throw new IOException(String.format("could not read required number of bytes from " +
                                                            "file to be streamed: read %d bytes, wanted %d bytes",
                                                            read, toWrite));
                    outBuffer.flip();
                }, limiter);

                if (logger.isTraceEnabled())
                    logger.trace("Writing {} bytes at position {} of {}", toWrite, bytesTransferred, length);
                bytesTransferred += toWrite;
            }
        }
        finally
        {
            // we don't need to wait until byte buffer is flushed by netty
            fc.close();
        }

        return bytesTransferred;
    }

View on GitHub (pinned to 88fd0f6a0e)