kopia/kopia · error

compression not supported in index v1

Error message

compression not supported in index v1

What it means

writeEntry refuses to write entries that carry a compression header ID into an index v1 file, because the v1 on-disk format predates per-entry compression and has no field to store it. It is raised during buildV1 when any indexed entry's CompressionHeaderID is non-zero. Repos with compression enabled must use index v2 or later.

Solutions

1. Disable compression in the repository settings or use the default index format (v2+) that supports compression.
2. Remove or re-create compressed content before writing a v1 index.
3. Upgrade tooling so indexes are written in v2 format instead of v1.

Example fix

// before
repoOpts := repo.NewParameters()
repoOpts.IndexVersion = 1
repoOpts.Compressor = compression.HeaderZstd
// after
repoOpts.IndexVersion = 2 // v2 indexes support compression
repoOpts.Compressor = compression.HeaderZstd

// Go: verify no entry carries a compression header before v1 write
for _, e := range entries {
	if e.CompressionHeaderID != 0 {
		return errors.New("entry compressed; index v1 unsupported — use v2")
	}
}

func isV1CompressionSafe(e ContentEntry) bool { return e.CompressionHeaderID == 0 }

idx, err := index.WriteIndex(ctx, out, b)
if err != nil && strings.Contains(err.Error(), "compression not supported in index v1") {
	// rebuild with index v2 instead
}

When it happens

Trigger: Building an index v1 (via buildV1 -> writeEntry) while the repository/content manager has compression enabled, so entry.CompressionHeaderID != 0.

Common situations: A repository configured with a compressor (e.g. zstd) but still writing or downgrading to index v1; migration of an old repo where compression was later enabled; tooling that forces legacy index format output.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• encryption key ID not supported in index v1
• unsupported compressor
• unsupported - too many index v2 format infos
• closing index shards
• CompactEpoch

AI-assisted analysis of kopia/kopia@82495e54b5 (2026-09-07). Data as JSON: /api/errors/1cdda4d2804e7a3b. Report an issue: GitHub.

Appendix: source

Thrown at repo/content/index/index_v1.go:351

		}
	}

	b.extraDataOffset = uint32(v1HeaderSize + b.entryCount*(b.keyLength+b.entryLength)) //nolint:gosec

	return extraData
}

func (b *indexBuilderV1) writeEntry(w io.Writer, it *Info, entry []byte) error {
	var hashBuf [maxContentIDSize]byte

	k := contentIDToBytes(hashBuf[:0], it.ContentID)

	if len(k) != b.keyLength {
		return errors.Errorf("inconsistent key length: %v vs %v", len(k), b.keyLength)
	}

	if it.CompressionHeaderID != 0 {
		return errors.New("compression not supported in index v1")
	}

	if it.EncryptionKeyID != 0 {
		return errors.New("encryption key ID not supported in index v1")
	}

	if err := b.formatEntry(entry, it); err != nil {
		return errors.Wrap(err, "unable to format entry")
	}

	if _, err := w.Write(k); err != nil {
		return errors.Wrap(err, "error writing entry key")
	}

	if _, err := w.Write(entry); err != nil {
		return errors.Wrap(err, "error writing entry")
	}

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