fix(agent): resolve device symlinks so LVM volumes map to disk I/O (#2486)

agent/disk.go

@@ -97,21 +97,41 @@ func isDockerSpecialMountpoint(mountpoint string) bool {
 	return false
 }
 
+// evalSymlinks resolves device symlinks; it is a seam so tests can fake the
+// /dev topology (e.g. /dev/vg/lv -> /dev/dm-N created by udev for LVM).
+var evalSymlinks = filepath.EvalSymlinks
+
 // registerFilesystemStats resolves the tracked key and stats payload for a
 // filesystem before it is inserted into fsStats.
 func registerFilesystemStats(existing map[string]*system.FsStats, device, mountpoint string, root bool, customName string, ctx fsRegistrationContext) (string, *system.FsStats, bool) {
 	key := device
+	resolvedKey := ""
 	if !ctx.isWindows {
 		key = filepath.Base(device)
+		// Device-mapper mounts appear as symlinked paths like /dev/vg/lv whose
+		// base name matches neither the diskstats name (dm-N) nor the dm label
+		// (vg-lv); the resolved target's base is one of those existing names.
+		// Bare names (folder devices, ZFS datasets) are skipped because they
+		// would resolve relative to the agent's working directory.
+		if filepath.IsAbs(device) {
+			if resolved, err := evalSymlinks(device); err == nil {
+				if base := filepath.Base(resolved); base != key {
+					resolvedKey = base
+				}
+			}
+		}
 	}
 
 	if root {
 		// Try to map root device to a diskIoCounters entry. First checks for an
 		// exact key match, then uses findIoDevice for normalized / prefix-based
-		// matching (e.g. nda0p2 -> nda0), and finally falls back to FILESYSTEM.
+		// matching (e.g. nda0p2 -> nda0) and the symlink-resolved device name,
+		// and finally falls back to FILESYSTEM.
 		if _, ioMatch := ctx.diskIoCounters[key]; !ioMatch {
 			if matchedKey, match := findIoDevice(key, ctx.diskIoCounters); match {
 				key = matchedKey
+			} else if matchedKey, match := findIoDevice(resolvedKey, ctx.diskIoCounters); match {
+				key = matchedKey
 			} else if ctx.filesystem != "" {
 				if matchedKey, match := findIoDevice(ctx.filesystem, ctx.diskIoCounters); match {
 					key = matchedKey
@@ -137,6 +157,8 @@ func registerFilesystemStats(existing map[string]*system.FsStats, device, mountp
 			if _, ioMatch = ctx.diskIoCounters[key]; !ioMatch {
 				if matchedKey, match := findIoDevice(key, ctx.diskIoCounters); match {
 					key = matchedKey
+				} else if matchedKey, match := findIoDevice(resolvedKey, ctx.diskIoCounters); match {
+					key = matchedKey
 				}
 			}
 		}

agent/disk_test.go

@@ -273,6 +273,175 @@ func TestBuildFsStatRegistration(t *testing.T) {
 		assert.Empty(t, key)
 		assert.Nil(t, stats)
 	})
+
+	t.Run("maps lvm symlinked device to io device through resolved name", func(t *testing.T) {
+		setEvalSymlinks(t, func(path string) (string, error) {
+			if path == "/dev/vg1/volume_1" {
+				return "/dev/dm-1", nil
+			}
+			return path, nil
+		})
+
+		key, stats, ok := registerFilesystemStats(
+			map[string]*system.FsStats{},
+			"/dev/vg1/volume_1",
+			"/volume1",
+			false,
+			"",
+			fsRegistrationContext{
+				isWindows: false,
+				efPath:    "/extra-filesystems",
+				diskIoCounters: map[string]disk.IOCountersStat{
+					"dm-0": {Name: "dm-0", Label: "vg1-syno_vg_reserved_area"},
+					"dm-1": {Name: "dm-1", Label: "vg1-volume_1"},
+				},
+			},
+		)
+
+		assert.True(t, ok)
+		assert.Equal(t, "dm-1", key)
+		assert.Equal(t, "/volume1", stats.Mountpoint)
+	})
+
+	t.Run("maps lvm device through resolved mapper label", func(t *testing.T) {
+		setEvalSymlinks(t, func(path string) (string, error) {
+			if path == "/dev/vg1/volume_1" {
+				return "/dev/mapper/vg1-volume_1", nil
+			}
+			return path, nil
+		})
+
+		key, _, ok := registerFilesystemStats(
+			map[string]*system.FsStats{},
+			"/dev/vg1/volume_1",
+			"/volume1",
+			false,
+			"",
+			fsRegistrationContext{
+				isWindows: false,
+				efPath:    "/extra-filesystems",
+				diskIoCounters: map[string]disk.IOCountersStat{
+					"dm-1": {Name: "dm-1", Label: "vg1-volume_1"},
+				},
+			},
+		)
+
+		assert.True(t, ok)
+		assert.Equal(t, "dm-1", key)
+	})
+
+	t.Run("maps lvm root device through resolved name", func(t *testing.T) {
+		setEvalSymlinks(t, func(path string) (string, error) {
+			if path == "/dev/vg1/volume_1" {
+				return "/dev/dm-1", nil
+			}
+			return path, nil
+		})
+
+		key, stats, ok := registerFilesystemStats(
+			map[string]*system.FsStats{},
+			"/dev/vg1/volume_1",
+			"/",
+			true,
+			"",
+			fsRegistrationContext{
+				isWindows: false,
+				efPath:    "/extra-filesystems",
+				diskIoCounters: map[string]disk.IOCountersStat{
+					"dm-1": {Name: "dm-1", Label: "vg1-volume_1"},
+				},
+			},
+		)
+
+		assert.True(t, ok)
+		assert.Equal(t, "dm-1", key)
+		assert.True(t, stats.Root)
+	})
+
+	t.Run("resolved device wins over filesystem fallback", func(t *testing.T) {
+		setEvalSymlinks(t, func(path string) (string, error) {
+			if path == "/dev/vg1/volume_1" {
+				return "/dev/dm-1", nil
+			}
+			return path, nil
+		})
+
+		key, _, ok := registerFilesystemStats(
+			map[string]*system.FsStats{},
+			"/dev/vg1/volume_1",
+			"/",
+			true,
+			"",
+			fsRegistrationContext{
+				filesystem: "sda",
+				isWindows:  false,
+				efPath:     "/extra-filesystems",
+				diskIoCounters: map[string]disk.IOCountersStat{
+					"dm-1": {Name: "dm-1", Label: "vg1-volume_1"},
+					"sda":  {Name: "sda"},
+				},
+			},
+		)
+
+		assert.True(t, ok)
+		assert.Equal(t, "dm-1", key)
+	})
+
+	t.Run("keeps base name when symlink resolution fails", func(t *testing.T) {
+		setEvalSymlinks(t, func(path string) (string, error) {
+			return "", os.ErrNotExist
+		})
+
+		key, _, ok := registerFilesystemStats(
+			map[string]*system.FsStats{},
+			"/dev/vg1/volume_1",
+			"/volume1",
+			false,
+			"",
+			fsRegistrationContext{
+				isWindows: false,
+				efPath:    "/extra-filesystems",
+				diskIoCounters: map[string]disk.IOCountersStat{
+					"dm-1": {Name: "dm-1", Label: "vg1-volume_1"},
+				},
+			},
+		)
+
+		assert.True(t, ok)
+		assert.Equal(t, "volume_1", key)
+	})
+
+	t.Run("does not resolve symlinks for relative device names", func(t *testing.T) {
+		setEvalSymlinks(t, func(path string) (string, error) {
+			return "/dev/sdb1", nil
+		})
+
+		key, _, ok := registerFilesystemStats(
+			map[string]*system.FsStats{},
+			"data",
+			"/mnt/data",
+			false,
+			"",
+			fsRegistrationContext{
+				isWindows: false,
+				efPath:    "/extra-filesystems",
+				diskIoCounters: map[string]disk.IOCountersStat{
+					"sdb1": {Name: "sdb1"},
+				},
+			},
+		)
+
+		assert.True(t, ok)
+		assert.Equal(t, "data", key)
+	})
+}
+
+// setEvalSymlinks swaps the device-symlink resolver for the duration of a test.
+func setEvalSymlinks(t *testing.T, fn func(string) (string, error)) {
+	t.Helper()
+	old := evalSymlinks
+	evalSymlinks = fn
+	t.Cleanup(func() { evalSymlinks = old })
 }
 
 func TestAddConfiguredRootFs(t *testing.T) {