nats-io/nats-server · critical

short write to temporary file (%d != %d)

Error message

short write to temporary file (%d != %d)

What it means

Returned when a write to the temporary block file completed without error but wrote fewer bytes than the buffer length (short write). The file store treats this as a failed update and aborts via errorCleanup, leaving the original block file intact.

Source

Thrown at server/filestore.go:8064

		// writing metadata.
		meta := &CompressionInfo{
			Algorithm:    alg,
			OriginalSize: uint64(originalSize),
		}
		buf = append(meta.MarshalMetadata(), buf...)
	}

	// Re-encrypt the block if necessary.
	if err = mb.encryptOrDecryptIfNeeded(buf); err != nil {
		return errorCleanup(err)
	}

	// Write the new block data (which might be compressed or encrypted) to the
	// temporary file.
	if n, err := tmpFD.Write(buf); err != nil {
		return errorCleanup(fmt.Errorf("failed to write to temporary file: %w", err))
	} else if n != len(buf) {
		return errorCleanup(fmt.Errorf("short write to temporary file (%d != %d)", n, len(buf)))
	}
	if err := tmpFD.Sync(); err != nil {
		return errorCleanup(fmt.Errorf("failed to sync temporary file: %w", err))
	}
	if err := tmpFD.Close(); err != nil {
		return errorCleanup(fmt.Errorf("failed to close temporary file: %w", err))
	}

	// Now replace the original file with the newly updated temp file.
	if err := os.Rename(tmpFN, origFN); err != nil {
		return fmt.Errorf("failed to move temporary file into place: %w", err)
	}

	// Since the message block might be retained in memory, make sure the
	// compression algorithm is up-to-date, since this will be needed when
	// compacting or truncating.
	mb.cmp = alg

View on GitHub (pinned to 3a66a489d2)

Solutions

1. Move the JetStream store onto a reliable local filesystem (ext4/xfs), not network mounts.
2. Check disk quotas (quota -s) on the storage volume.
3. Inspect storage subsystem logs for partial-write/IO errors.
4. Retry the operation; the temp file is cleaned up automatically.

Example fix

// before: store on NFS mount
StoreDir: /mnt/nfs/jetstream
// after: local disk
StoreDir: /var/lib/nats/jetstream

if err != nil && strings.Contains(err.Error(), "short write to temporary file") {
    // treat storage as unreliable; migrate store to local disk, alert ops
}

When it happens

Trigger: tmpFD.Write(buf) returns n != len(buf) during a file-store message block update — a partial write of the compressed/encrypted block buffer.

Common situations: Underlying storage returning partial writes (NFS/CIFS mounts, iSCSI issues), disk quotas silently truncating writes, or extremely large block buffers on unreliable storage.

Related errors

• failed to write to temporary file: %w
• failed to close temporary file: %w
• failed to sync temporary file: %w
• failed to move temporary file into place: %w
• fileStore requires file storage type in config

AI-assisted analysis of nats-io/nats-server@3a66a489d2 (2026-09-02). Data as JSON: /api/errors/a217caf3f1c130a4. Report an issue: GitHub.