Parse bcachefs write stats

Signed-off-by: Ananth Bhaskararaman <antsub@gmail.com>

Author: ananthb

collector/bcachefs_linux.go

@@ -115,6 +115,11 @@ func (c *bcachefsCollector) Update(ch chan<- prometheus.Metric) error {
 			c.logger.Debug("failed to parse counters", "uuid", uuid, "err", err)
 		}
 
+		// Parse btree_write_stats
+		if err := c.parseBtreeWriteStats(ch, fsPath, uuid); err != nil {
+			c.logger.Debug("failed to parse btree_write_stats", "uuid", uuid, "err", err)
+		}
+
 		// Parse device stats
 		if err := c.parseDevices(ch, fsPath, uuid); err != nil {
 			c.logger.Debug("failed to parse devices", "uuid", uuid, "err", err)
@@ -633,3 +638,80 @@ func (c *bcachefsCollector) parseDeviceIOErrors(ch chan<- prometheus.Metric, dev
 
 	return scanner.Err()
 }
+
+// parseBtreeWriteStats parses the btree_write_stats file.
+// Format:
+//
+//	                   nr        size
+//	initial:           19088     108k
+//	init_next_bset:    6647      23.5k
+//	cache_reclaim:     0         0
+//	journal_reclaim:   541080    405
+//	interior:          16788     354
+func (c *bcachefsCollector) parseBtreeWriteStats(ch chan<- prometheus.Metric, fsPath, uuid string) error {
+	const subsystem = "bcachefs"
+
+	file, err := os.Open(filepath.Join(fsPath, "btree_write_stats"))
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	scanner := bufio.NewScanner(file)
+	lineNum := 0
+	for scanner.Scan() {
+		lineNum++
+		line := scanner.Text()
+
+		// Skip header line
+		if lineNum == 1 {
+			continue
+		}
+
+		fields := strings.Fields(line)
+		if len(fields) < 3 {
+			continue
+		}
+
+		writeType := strings.TrimSuffix(fields[0], ":")
+		nrStr := fields[1]
+		sizeStr := fields[2]
+
+		nr, err := strconv.ParseFloat(nrStr, 64)
+		if err != nil {
+			c.logger.Debug("failed to parse btree_write_stats nr", "type", writeType, "err", err)
+			continue
+		}
+
+		size, err := parseHumanReadableBytes(sizeStr)
+		if err != nil {
+			c.logger.Debug("failed to parse btree_write_stats size", "type", writeType, "err", err)
+			continue
+		}
+
+		ch <- prometheus.MustNewConstMetric(
+			prometheus.NewDesc(
+				prometheus.BuildFQName(namespace, subsystem, "btree_writes_total"),
+				"Number of btree writes by type.",
+				[]string{"uuid", "type"},
+				nil,
+			),
+			prometheus.CounterValue,
+			nr,
+			uuid, writeType,
+		)
+		ch <- prometheus.MustNewConstMetric(
+			prometheus.NewDesc(
+				prometheus.BuildFQName(namespace, subsystem, "btree_write_average_size_bytes"),
+				"Average btree write size by type.",
+				[]string{"uuid", "type"},
+				nil,
+			),
+			prometheus.GaugeValue,
+			size,
+			uuid, writeType,
+		)
+	}
+
+	return scanner.Err()
+}