thanos-io/thanos · error

write chunks

Error message

write chunks

What it means

sWriter.WriteChunks failed while bulk-writing the collected chunks of one series into the destination block's chunk file and index. Compaction uses bulk writes for efficiency; any failure here aborts writing the whole output block.

Solutions

1. Check free disk space and inode availability on the compactor's data directory; clean up partial compaction output.
2. Raise open-file limits (ulimit -n) if errors indicate fd exhaustion from many chunk segments.
3. Retry compaction from scratch — discard partial output block; writers are not resumable.
4. Check filesystem permissions on the output block directory.

if err := ensureWritableDir(outputBlockDir); err != nil {
    return fmt.Errorf("output dir not writable: %w", err)
}
if free := freeDiskBytes(dataDir); free < estimatedChunkBytes {
    return fmt.Errorf("not enough disk for compaction output: need ~%d, have %d", estimatedChunkBytes, free)
}

if err := WriteSeries(ctx, set, symbols); err != nil {
    os.RemoveAll(partialOutputDir) // discard poisoned partial block
    return err
}

When it happens

Trigger: write() calls sWriter.WriteChunks(chks...) after collecting a series' chunks; failure when writing chunk bytes to the destination block (segment file IO error, disk full, writer already closed/errored).

Common situations: Disk full or quota exceeded on the scratch volume, too many open files (ulimit), destination directory removed, an earlier silent write error poisoning the writer.

Understand the failure class

Background: "failed to write file", "Could not save figure", "Error saving remote file" — file write failed: causes and fixes across languages and libraries — this error's family across 38 libraries.

Related errors

• cannot populate chunk
• chunk iter
• write
• get series
• add symbol

AI-assisted analysis of thanos-io/thanos@35b8b99117 (2026-09-07). Data as JSON: /api/errors/05d4e6690b8c1fed. Report an issue: GitHub.

Appendix: source

Thrown at pkg/compactv2/chunk_series_set.go:220

		for chksIter.Next() {
			// We are not iterating in streaming way over chunk as it's more efficient to do bulk write for index and
			// chunk file purposes.
			chks = append(chks, chksIter.At())
		}

		if chksIter.Err() != nil {
			return errors.Wrap(chksIter.Err(), "chunk iter")
		}

		// Skip the series with all deleted chunks.
		if len(chks) == 0 {
			// All series will be ignored.
			p.SeriesProcessed()
			continue
		}

		if err := sWriter.WriteChunks(chks...); err != nil {
			return errors.Wrap(err, "write chunks")
		}
		if err := sWriter.AddSeries(ref, s.Labels(), chks...); err != nil {
			return errors.Wrap(err, "add series")
		}
		for _, chk := range chks {
			// ChunkPool is used by tsdb.OpenBlock BlockReader.
			if err := w.chunkPool.Put(chk.Chunk); err != nil {
				return errors.Wrap(err, "put chunk")
			}
		}
		ref++
		p.SeriesProcessed()
	}
	if populatedSet.Err() != nil {
		return errors.Wrap(populatedSet.Err(), "iterate populated chunk series set")
	}

	return nil

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