Fix wrong value for OpenBSD memory buffer cache

Fixes #1972

Signed-off-by: ston1th <ston1th@giftfish.de>

collector/meminfo_openbsd_amd64.go

@@ -20,18 +20,54 @@ import (
 	"unsafe"
 )
 
+const (
+	CTL_VFS        = 10
+	VFS_GENERIC    = 0
+	VFS_BCACHESTAT = 3
+)
+
+type bcachestats struct {
+	Numbufs        int64
+	Numbufpages    int64
+	Numdirtypages  int64
+	Numcleanpages  int64
+	Pendingwrites  int64
+	Pendingreads   int64
+	Numwrites      int64
+	Numreads       int64
+	Cachehits      int64
+	Busymapped     int64
+	Dmapages       int64
+	Highpages      int64
+	Delwribufs     int64
+	Kvaslots       int64
+	Kvaslots_avail int64
+	Highflips      int64
+	Highflops      int64
+	Dmaflips       int64
+}
+
 func (c *meminfoCollector) getMemInfo() (map[string]float64, error) {
 	uvmexpb, err := unix.SysctlRaw("vm.uvmexp")
 	if err != nil {
 		return nil, err
 	}
+
+	mib := [3]_C_int{CTL_VFS, VFS_GENERIC, VFS_BCACHESTAT}
+	bcstatsb, err := sysctl(mib[:])
+	if err != nil {
+		return nil, err
+	}
+
 	uvmexp := *(*unix.Uvmexp)(unsafe.Pointer(&uvmexpb[0]))
 	ps := float64(uvmexp.Pagesize)
 
+	bcstats := *(*bcachestats)(unsafe.Pointer(&bcstatsb[0]))
+
 	// see uvm(9)
 	return map[string]float64{
 		"active_bytes":                  ps * float64(uvmexp.Active),
-		"cache_bytes":                   ps * float64(uvmexp.Vnodepages),
+		"cache_bytes":                   ps * float64(bcstats.Numbufpages),
 		"free_bytes":                    ps * float64(uvmexp.Free),
 		"inactive_bytes":                ps * float64(uvmexp.Inactive),
 		"size_bytes":                    ps * float64(uvmexp.Npages),