What causes "sync before first pass of downsampling" in thanos-io/thanos?

Check the wrapped inner error for the failing object store operation

sync before first pass of downsampling

Error message

sync before first pass of downsampling

What it means

Wrapper error in runCompact's downsampling first pass (cmd/thanos/compact.go:454) around sy.SyncMetas(ctx). The bucket metadata syncer failed to refresh block metas from the object store before downsampling began, so the process cannot build a reliable filtered block list.

Solutions

Check the wrapped inner error for the failing object store operation
Retry thanos compact; SyncMetas is idempotent and retried on the next run
Verify object store credentials and network path
Enable request logging/metrics on the bucket client to identify throttling
Increase request timeouts/retries on the object store client config

Defensive patterns

Strategy: retry

Try / catch

if err := sy.SyncMetas(ctx); err != nil {
	if errors.Is(err, context.Canceled) { return err }
	return errors.Wrap(err, "sync before first pass of downsampling")
}

Prevention

Configure bucket client retries/backoff for 5xx and 429 responses
Use valid, long-lived credentials for long compaction cycles
Alert on object-store error rate metrics during compaction windows

When it happens

Trigger: sy.SyncMetas returning an error: object store list/get failures, network interruption, throttling (429), or malformed meta.json objects during sync.

Common situations: S3/GCS outages or rate limiting during large buckets, expired credentials, proxy/firewall interruptions between Thanos and the store.

Understand the failure class

Background: 'Something went wrong' / 'Request failed (500)' / 'HTTP error! status: 404' — what failed HTTP requests actually mean and how to find the real cause — this error's family across 28 libraries.

Related errors

AI-assisted analysis of thanos-io/thanos@35b8b99117 (2026-09-07). Data as JSON: /api/errors/a83e400425c09bf2. Report an issue: GitHub.

Appendix: source

Thrown at cmd/thanos/compact.go:454

		compact.BestEffortCleanAbortedPartialUploads(ctx, logger, sy.Partial(), insBkt, compactMetrics.partialUploadDeleteAttempts, compactMetrics.blocksCleaned, compactMetrics.blockCleanupFailures, ignoreDeletionMarkFilter.DeletionMarkBlocks())
		compactMetrics.cleanups.Inc()

		return nil
	}

	compactMainFn := func() error {
		if err := compactor.Compact(ctx); err != nil {
			return errors.Wrap(err, "compaction")
		}

		if !conf.disableDownsampling {
			// After all compactions are done, work down the downsampling backlog.
			// We run two passes of this to ensure that the 1h downsampling is generated
			// for 5m downsamplings created in the first run.
			level.Info(logger).Log("msg", "start first pass of downsampling")
			if err := sy.SyncMetas(ctx); err != nil {
				return errors.Wrap(err, "sync before first pass of downsampling")
			}

			filteredMetas := sy.Metas()
			noDownsampleBlocks := noDownsampleMarkerFilter.NoDownsampleMarkedBlocks()
			for ul := range noDownsampleBlocks {
				delete(filteredMetas, ul)
			}

			for _, meta := range filteredMetas {
				resolutionLabel := meta.Thanos.ResolutionString()
				downsampleMetrics.downsamples.WithLabelValues(resolutionLabel)
				downsampleMetrics.downsampleFailures.WithLabelValues(resolutionLabel)
			}

			if err := downsampleBucket(
				ctx,
				logger,
				downsampleMetrics,

View on GitHub (pinned to 35b8b99117)