henrygd/beszel · warning

size field not found in arcstats

Error message

size field not found in arcstats

What it means

ARCSize scans arcstats for a line prefixed with `size`; if the whole file is scanned without finding one, it returns this error. It means the ZFS ARC statistics file exists but exposes no `size` kstat entry.

Source

Thrown at agent/zfs/zfs_linux.go:33

	file, err := os.Open("/proc/spl/kstat/zfs/arcstats")
	if err != nil {
		return 0, err
	}
	defer file.Close()

	scanner := bufio.NewScanner(file)
	for scanner.Scan() {
		line := scanner.Text()
		if strings.HasPrefix(line, "size") {
			fields := strings.Fields(line)
			if len(fields) < 3 {
				return 0, fmt.Errorf("unexpected arcstats size format: %s", line)
			}
			return strconv.ParseUint(fields[2], 10, 64)
		}
	}

	return 0, fmt.Errorf("size field not found in arcstats")
}

View on GitHub (pinned to b38fb7dafa)

Solutions

1. Confirm `grep size /proc/spl/kstat/zfs/arcstats` returns a line; if not, check `lsmod | grep zfs` and load the ZFS kernel module.
2. On newer OpenZFS, check for renamed/moved stats files and point the reader at the correct path.
3. Inside a container, ensure /proc/spl is mounted/visible from the host.
4. Gracefully degrade: treat missing ARC stats as metric-unavailable if ZFS is not installed.

Example fix

// before
return 0, fmt.Errorf("size field not found in arcstats")
// after
if _, err := os.Stat("/proc/spl/kstat/zfs/arcstats"); err != nil {
	return 0, nil // ZFS not present — metric unavailable
}
return 0, fmt.Errorf("size field not found in arcstats")

if data, err := os.ReadFile("/proc/spl/kstat/zfs/arcstats"); err != nil || !strings.Contains(string(data), "\nsize ") {
	// ZFS stats unavailable — skip ARC metric entirely
}

func arcstatsHasSize(data []byte) bool {
	for _, line := range strings.Split(string(data), "\n") {
		if strings.HasPrefix(line, "size") { return true }
	}
	return false
}

size, err := zfs.ARCSize()
if err != nil {
	log.Printf("ARC size unavailable, reporting 0: %v", err)
	size = 0
}

When it happens

Trigger: The opened path exists but is not a populated arcstats file (empty or filtered procfs), or a ZFS build where the `size` kstat was renamed/removed, or /proc/spl not properly mounted.

Common situations: ZFS userspace tools present but kernel module not loaded (empty stats); OpenZFS variants that moved stats to a different kstat file; containers where procfs is restricted.

Related errors

• unexpected arcstats size format: %s
• system missing required fields
• system exists
• system not found
• ${resp.Error}

AI-assisted analysis of henrygd/beszel@b38fb7dafa (2026-08-31). Data as JSON: /api/errors/b3b7b337a96c8b33. Report an issue: GitHub.