kopia/kopia · error

error writing pack

Error message

error writing pack

What it means

Returned by processWritePackResultLocked when an asynchronously written pack fails. The pending pack is moved back to failedPacks so a later Flush retries it; this error reports the writeErr from the background pack writer to the caller of writePackAndAddToIndexUnlocked/Locked.

Solutions

1. Read writeErr in the chain — it is the original storage/preparation error.
2. Call Flush() again; the pack was re-queued in failedPacks and will be retried.
3. Check network stability and storage provider throttling (S3 503 SlowDown, etc.); consider retry-able blob storage options.
4. If persistent, verify the storage backend is writable and not read-only (e.g. wrong bucket permissions, repository opened read-only by mistake).

Example fix

// before: assuming a failed pack write is fatal
if err := bm.writePackAndAddToIndexUnlocked(ctx, pp); err != nil {
    return err
}
// after: failed packs are retried by Flush; surface but allow recovery
if err := bm.writePackAndAddToIndexUnlocked(ctx, pp); err != nil {
    logger.Warnf("pack write failed (will retry on Flush): %v", err)
    if retryErr := bm.Flush(ctx); retryErr != nil {
        return retryErr
    }
}

// Go: failed packs are auto-requeued; retry via Flush
if err := bm.Flush(ctx); err != nil {
    return errors.Wrap(err, "pack write failed; failed packs will be retried")
}

When it happens

Trigger: writePackAndAddToIndexUnlocked detects pp.err set by the async writePackAndAddToIndexUnlocked goroutine (or the synchronous path fails); any error preparing or uploading the pack data blob surfaces here wrapped as 'error writing pack'.

Common situations: Background pack writer hit a transient storage failure (connection reset, throttling by S3); the context was canceled while the pack write was in flight; the pack blob failed integrity checks after upload.

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

• error writing previously failed pack
• can't save pack data blob
• error writing pack content
• error writing pending content
• unable to create pack

AI-assisted analysis of kopia/kopia@82495e54b5 (2026-09-07). Data as JSON: /api/errors/41b5a4f742737410. Report an issue: GitHub.

Appendix: source

Thrown at repo/content/content_manager.go:562

	// after finishing writing, remove from both writingPacks and failedPacks
	bm.writingPacks = removePendingPack(bm.writingPacks, pp)
	bm.failedPacks = removePendingPack(bm.failedPacks, pp)

	if writeErr == nil {
		// success, add pack index builder entries to index.
		for _, info := range packFileIndex {
			bm.packIndexBuilder.Add(info)
		}

		pp.currentPackData.Close()

		return nil
	}

	// failure - add to failedPacks slice again
	bm.failedPacks = append(bm.failedPacks, pp)

	return errors.Wrap(writeErr, "error writing pack")
}

func (sm *SharedManager) prepareAndWritePackInternal(ctx context.Context, pp *pendingPackInfo, onUpload func(int64)) (index.Builder, error) {
	ctx = contentlog.WithParams(ctx,
		logparam.String("span:writePack", string(pp.packBlobID)))

	mp, mperr := sm.format.GetMutableParameters(ctx)
	if mperr != nil {
		return nil, errors.Wrap(mperr, "mutable parameters")
	}

	packFileIndex, err := sm.preparePackDataContent(ctx, mp, pp)
	if err != nil {
		return nil, errors.Wrap(err, "error preparing data content")
	}

	if pp.currentPackData.Length() > 0 {
		if err := sm.writePackFileNotLocked(ctx, pp.packBlobID, pp.currentPackData.Bytes(), onUpload); err != nil {

View on GitHub (pinned to 82495e54b5)