Document swap input plugin and move to separate file (#4342)

(cherry picked from commit b7a02c73b3)

plugins/inputs/system/SWAP_README.md

@@ -0,0 +1,30 @@
+# Swap Input Plugin
+
+The swap plugin collects system swap metrics.
+
+For a more information on what swap memory is, read [All about Linux swap space](https://www.linux.com/news/all-about-linux-swap-space).
+
+### Configuration:
+
+```toml
+# Read metrics about swap memory usage
+[[inputs.swap]]
+  # no configuration
+```
+
+### Metrics:
+
+- swap
+  - fields:
+  	- free (int)
+  	- total (int)
+  	- used (int)
+  	- used_percent (float)
+    - sin (int)
+    - sout (int)
+
+### Example Output:
+
+```
+swap total=20855394304i,used_percent=45.43883523785713,used=9476448256i,free=1715331072i 1511894782000000000
+```

plugins/inputs/system/memory.go

@@ -42,45 +42,9 @@ func (s *MemStats) Gather(acc telegraf.Accumulator) error {
 	return nil
 }
 
-type SwapStats struct {
-	ps PS
-}
-
-func (_ *SwapStats) Description() string {
-	return "Read metrics about swap memory usage"
-}
-
-func (_ *SwapStats) SampleConfig() string { return "" }
-
-func (s *SwapStats) Gather(acc telegraf.Accumulator) error {
-	swap, err := s.ps.SwapStat()
-	if err != nil {
-		return fmt.Errorf("error getting swap memory info: %s", err)
-	}
-
-	fieldsG := map[string]interface{}{
-		"total":        swap.Total,
-		"used":         swap.Used,
-		"free":         swap.Free,
-		"used_percent": swap.UsedPercent,
-	}
-	fieldsC := map[string]interface{}{
-		"in":  swap.Sin,
-		"out": swap.Sout,
-	}
-	acc.AddGauge("swap", fieldsG, nil)
-	acc.AddCounter("swap", fieldsC, nil)
-
-	return nil
-}
-
 func init() {
 	ps := newSystemPS()
 	inputs.Add("mem", func() telegraf.Input {
 		return &MemStats{ps: ps}
 	})
-
-	inputs.Add("swap", func() telegraf.Input {
-		return &SwapStats{ps: ps}
-	})
 }

plugins/inputs/system/memory_test.go

@@ -30,17 +30,6 @@ func TestMemStats(t *testing.T) {
 
 	mps.On("VMStat").Return(vms, nil)
 
-	sms := &mem.SwapMemoryStat{
-		Total:       8123,
-		Used:        1232,
-		Free:        6412,
-		UsedPercent: 12.2,
-		Sin:         7,
-		Sout:        830,
-	}
-
-	mps.On("SwapStat").Return(sms, nil)
-
 	err = (&MemStats{&mps}).Gather(&acc)
 	require.NoError(t, err)
 
@@ -61,15 +50,4 @@ func TestMemStats(t *testing.T) {
 	acc.AssertContainsTaggedFields(t, "mem", memfields, make(map[string]string))
 
 	acc.Metrics = nil
-
-	err = (&SwapStats{&mps}).Gather(&acc)
-	require.NoError(t, err)
-
-	swapfields := map[string]interface{}{
-		"total":        uint64(8123),
-		"used":         uint64(1232),
-		"used_percent": float64(12.2),
-		"free":         uint64(6412),
-	}
-	acc.AssertContainsTaggedFields(t, "swap", swapfields, make(map[string]string))
 }

plugins/inputs/system/swap.go

@@ -0,0 +1,47 @@
+package system
+
+import (
+	"fmt"
+
+	"github.com/influxdata/telegraf"
+	"github.com/influxdata/telegraf/plugins/inputs"
+)
+
+type SwapStats struct {
+	ps PS
+}
+
+func (_ *SwapStats) Description() string {
+	return "Read metrics about swap memory usage"
+}
+
+func (_ *SwapStats) SampleConfig() string { return "" }
+
+func (s *SwapStats) Gather(acc telegraf.Accumulator) error {
+	swap, err := s.ps.SwapStat()
+	if err != nil {
+		return fmt.Errorf("error getting swap memory info: %s", err)
+	}
+
+	fieldsG := map[string]interface{}{
+		"total":        swap.Total,
+		"used":         swap.Used,
+		"free":         swap.Free,
+		"used_percent": swap.UsedPercent,
+	}
+	fieldsC := map[string]interface{}{
+		"in":  swap.Sin,
+		"out": swap.Sout,
+	}
+	acc.AddGauge("swap", fieldsG, nil)
+	acc.AddCounter("swap", fieldsC, nil)
+
+	return nil
+}
+
+func init() {
+	ps := newSystemPS()
+	inputs.Add("swap", func() telegraf.Input {
+		return &SwapStats{ps: ps}
+	})
+}

plugins/inputs/system/swap_test.go

@@ -0,0 +1,38 @@
+package system
+
+import (
+	"testing"
+
+	"github.com/influxdata/telegraf/testutil"
+	"github.com/shirou/gopsutil/mem"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSwapStats(t *testing.T) {
+	var mps MockPS
+	var err error
+	defer mps.AssertExpectations(t)
+	var acc testutil.Accumulator
+
+	sms := &mem.SwapMemoryStat{
+		Total:       8123,
+		Used:        1232,
+		Free:        6412,
+		UsedPercent: 12.2,
+		Sin:         7,
+		Sout:        830,
+	}
+
+	mps.On("SwapStat").Return(sms, nil)
+
+	err = (&SwapStats{&mps}).Gather(&acc)
+	require.NoError(t, err)
+
+	swapfields := map[string]interface{}{
+		"total":        uint64(8123),
+		"used":         uint64(1232),
+		"used_percent": float64(12.2),
+		"free":         uint64(6412),
+	}
+	acc.AssertContainsTaggedFields(t, "swap", swapfields, make(map[string]string))
+}