thanos-io/thanos · error

open chunk reader for block %+v

Error message

open chunk reader for block %+v

What it means

WriteSeries opens a chunk reader for each source block via b.Chunks(); failure is wrapped as "open chunk reader for block %+v". The block's chunks file could not be opened or its reader could not be created.

Solutions

1. Verify chunks/ directory exists with segment files matching meta.json (maxTime/minTime and segment list)
2. Check permissions on the block directory and chunk files
3. Remove or repair the corrupt block; if it is replicated, delete and re-sync from another replica
4. Check filesystem/disk health (dmesg, smartctl) for underlying I/O errors

for _, b := range readers {
    meta := b.Meta()
    chunksDir := filepath.Join(meta.ULID.String(), "chunks")
    if fi, err := os.Stat(chunksDir); err != nil || !fi.IsDir() {
        return fmt.Errorf("block %s chunks dir missing", meta.ULID)
    }
}

if err := comp.WriteSeries(ctx, readers, sWriter, progress); err != nil {
    if strings.Contains(err.Error(), "open chunk reader") {
        // identify block from wrapped message, validate/repair it
        logger.Error("failed to open chunk reader", "err", err)
    }
    return err
}

When it happens

Trigger: b.Chunks() fails: missing/corrupt chunks/ directory or segment files, bad permissions, segment count mismatch with meta.json, or truncated chunk segments.

Common situations: Corrupted block after unclean shutdown or incomplete copy; block directory manually edited; storage volume errors; blocks restored from backup missing chunk segments.

Understand the failure class

Background: "open() failed", "failed to open file", "cannot create file" — what a file open error means and how to fix it — this error's family across 42 libraries.

Related errors

• open index reader for block %+v
• write
• open block
• open index file
• remove any existing index at path

AI-assisted analysis of thanos-io/thanos@35b8b99117 (2026-09-07). Data as JSON: /api/errors/da39549ed12f8fbb. Report an issue: GitHub.

Appendix: source

Thrown at pkg/compactv2/compactor.go:103

	)
	defer func() {
		errs := tsdb_errors.NewMulti(err)
		if cerr := tsdb_errors.CloseAll(closers); cerr != nil {
			errs.Add(errors.Wrap(cerr, "close"))
		}
		err = errs.Err()
	}()

	for _, b := range readers {
		indexr, err := b.Index()
		if err != nil {
			return errors.Wrapf(err, "open index reader for block %+v", b.Meta())
		}
		closers = append(closers, indexr)

		chunkr, err := b.Chunks()
		if err != nil {
			return errors.Wrapf(err, "open chunk reader for block %+v", b.Meta())
		}
		closers = append(closers, chunkr)
		sReaders = append(sReaders, seriesReader{ir: indexr, cr: chunkr})
	}

	symbols, set, err := compactSeries(ctx, sReaders...)
	if err != nil {
		return errors.Wrapf(err, "compact series from %v", func() string {
			var metas []string
			for _, m := range readers {
				metas = append(metas, fmt.Sprintf("%v", m.Meta()))
			}
			return strings.Join(metas, ",")
		}())
	}

	for _, m := range modifiers {
		symbols, set = m.Modify(symbols, set, w.changeLogger, p)

View on GitHub (pinned to 35b8b99117)