app/vmagent: add -remoteWrite.decimalPlaces command-line flag, which may be used for reducing disk space usage on the remote storage

app/vmagent/remotewrite/remotewrite.go

@@ -6,6 +6,7 @@ import (
 	"sync"
 	"sync/atomic"
 
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/decimal"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/flagutil"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/httpserver"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
@@ -30,6 +31,9 @@ var (
 		"for each -remoteWrite.url. When buffer size reaches the configured maximum, then old data is dropped when adding new data to the buffer. "+
 		"Buffered data is stored in ~500MB chunks, so the minimum practical value for this flag is 500000000. "+
 		"Disk usage is unlimited if the value is set to 0")
+	decimalPlaces = flag.Int("remoteWrite.decimalPlaces", 0, "The number of significant decimal places to leave in metric values before writing them to remote storage. "+
+		"See https://en.wikipedia.org/wiki/Significant_figures . Zero value saves all the significant decimal places. "+
+		"This option may be used for increasing on-disk compression level for the stored metrics")
 )
 
 var rwctxs []*remoteWriteCtx
@@ -118,8 +122,19 @@ func Stop() {
 
 // Push sends wr to remote storage systems set via `-remoteWrite.url`.
 //
-// Note that wr may be modified by Push due to relabeling.
+// Note that wr may be modified by Push due to relabeling and rounding.
 func Push(wr *prompbmarshal.WriteRequest) {
+	if *decimalPlaces > 0 {
+		// Round values according to decimalPlaces
+		for i := range wr.Timeseries {
+			samples := wr.Timeseries[i].Samples
+			for j := range samples {
+				s := &samples[j]
+				s.Value = decimal.Round(s.Value, *decimalPlaces)
+			}
+		}
+	}
+
 	var rctx *relabelCtx
 	rcs := allRelabelConfigs.Load().(*relabelConfigs)
 	prcsGlobal := rcs.global

lib/decimal/decimal.go

@@ -256,6 +256,38 @@ func maxUpExponent(v int64) int16 {
 	}
 }
 
+// Round f to value with the given number of significant decimal digits.
+func Round(f float64, digits int) float64 {
+	if digits <= 0 || digits >= 18 {
+		return f
+	}
+	if math.IsNaN(f) || math.IsInf(f, 0) || f == 0 {
+		return f
+	}
+	n := int64(math.Pow10(digits))
+	isNegative := f < 0
+	if isNegative {
+		f = -f
+	}
+	v, e := positiveFloatToDecimal(f)
+	if v > vMax {
+		v = vMax
+	}
+	var rem int64
+	for v > n {
+		rem = v % 10
+		v /= 10
+		e++
+	}
+	if rem >= 5 {
+		v++
+	}
+	if isNegative {
+		v = -v
+	}
+	return ToFloat(v, e)
+}
+
 // ToFloat returns f=v*10^e.
 func ToFloat(v int64, e int16) float64 {
 	f := float64(v)

lib/decimal/decimal_test.go

@@ -7,6 +7,29 @@ import (
 	"testing"
 )
 
+func TestRoundInplace(t *testing.T) {
+	f := func(f float64, digits int, resultExpected float64) {
+		t.Helper()
+		result := Round(f, digits)
+		if math.IsNaN(result) {
+			if !math.IsNaN(resultExpected) {
+				t.Fatalf("unexpected result; got %v; want %v", result, resultExpected)
+			}
+		}
+		if result != resultExpected {
+			t.Fatalf("unexpected result; got %v; want %v", result, resultExpected)
+		}
+	}
+	f(1234, 0, 1234)
+	f(-12.34, 20, -12.34)
+	f(12, 1, 10)
+	f(25, 1, 30)
+	f(2.5, 1, 3)
+	f(-0.56, 1, -0.6)
+	f(1234567, 3, 1230000)
+	f(-1.234567, 4, -1.235)
+}
+
 func TestPositiveFloatToDecimal(t *testing.T) {
 	f := func(f float64, decimalExpected int64, exponentExpected int16) {
 		t.Helper()