defer cancel()

	results := make(chan ObjectInfo, workerSize)
	if err := api.Walk(ctx, r.Bucket, r.Prefix, results, WalkOptions{
		Marker:       lastObject,
		LatestOnly:   false, // we need to visit all versions of the object to implement purge: retainVersions
		VersionsSort: WalkVersionsSortDesc,
	}); err != nil {
		// Do not need to retry if we can't list objects on source.
		return err
	}

	}
	if meta.Format != formatBackendErasure && meta.Format != formatBackendErasureSingle {
		return "", fmt.Errorf(`found backend type %s, expected %s or %s - to migrate to a supported backend visit https://min.io/docs/minio/linux/operations/install-deploy-manage/migrate-fs-gateway.html`, meta.Format, formatBackendErasure, formatBackendErasureSingle)
	}
	// Erasure backend found, proceed to detect version.

	uatomic "go.uber.org/atomic"
)

const (
	dataScannerSleepPerFolder        = time.Millisecond                 // Time to wait between folders.
	dataUsageUpdateDirCycles         = 16                               // Visit all folders every n cycles.
	dataScannerCompactLeastObject    = 500                              // Compact when there is less than this many objects in a branch.