Added metric for symbol table size

Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in>

block.go

@@ -15,6 +15,7 @@
 package tsdb
 
 import (
+	"encoding/binary"
 	"encoding/json"
 	"io/ioutil"
 	"os"
@@ -248,6 +249,10 @@ type Block struct {
 	dir  string
 	meta BlockMeta
 
+	// Symbol Table Size in bytes.
+	// We maintain this variable to avoid recalculation everytime.
+	symbolTableSize uint64
+
 	chunkr     ChunkReader
 	indexr     IndexReader
 	tombstones TombstoneReader
@@ -275,12 +280,23 @@ func OpenBlock(dir string, pool chunkenc.Pool) (*Block, error) {
 		return nil, err
 	}
 
+	// Calculating symbol table size.
+	tmp := make([]byte, 8)
+	symTblSize := uint64(0)
+	for _, v := range ir.SymbolTable() {
+		// Size of varint length of the symbol.
+		symTblSize += uint64(binary.PutUvarint(tmp, uint64(len(v))))
+		// Size of the symbol.
+		symTblSize += uint64(len(v))
+	}
+
 	pb := &Block{
-		dir:        dir,
-		meta:       *meta,
-		chunkr:     cr,
-		indexr:     ir,
-		tombstones: tr,
+		dir:             dir,
+		meta:            *meta,
+		chunkr:          cr,
+		indexr:          ir,
+		tombstones:      tr,
+		symbolTableSize: symTblSize,
 	}
 	return pb, nil
 }
@@ -350,6 +366,11 @@ func (pb *Block) Tombstones() (TombstoneReader, error) {
 	return blockTombstoneReader{TombstoneReader: pb.tombstones, b: pb}, nil
 }
 
+// GetSymbolTableSize returns the Symbol Table Size in the index of this block.
+func (pb *Block) GetSymbolTableSize() uint64 {
+	return pb.symbolTableSize
+}
+
 func (pb *Block) setCompactionFailed() error {
 	pb.meta.Compaction.Failed = true
 	return writeMetaFile(pb.dir, &pb.meta)

db.go

@@ -119,6 +119,7 @@ type DB struct {
 
 type dbMetrics struct {
 	loadedBlocks         prometheus.GaugeFunc
+	symbolTableSize      prometheus.GaugeFunc
 	reloads              prometheus.Counter
 	reloadsFailed        prometheus.Counter
 	compactionsTriggered prometheus.Counter
@@ -138,6 +139,19 @@ func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
 		defer db.mtx.RUnlock()
 		return float64(len(db.blocks))
 	})
+	m.symbolTableSize = prometheus.NewGaugeFunc(prometheus.GaugeOpts{
+		Name: "prometheus_tsdb_symbol_table_size",
+		Help: "Size of symbol table on disk (in bytes)",
+	}, func() float64 {
+		db.mtx.RLock()
+		blocks := db.blocks[:]
+		db.mtx.RUnlock()
+		symTblSize := float64(0)
+		for _, b := range blocks {
+			symTblSize += float64(b.GetSymbolTableSize())
+		}
+		return symTblSize
+	})
 	m.reloads = prometheus.NewCounter(prometheus.CounterOpts{
 		Name: "prometheus_tsdb_reloads_total",
 		Help: "Number of times the database reloaded block data from disk.",
@@ -166,6 +180,7 @@ func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
 	if r != nil {
 		r.MustRegister(
 			m.loadedBlocks,
+			m.symbolTableSize,
 			m.reloads,
 			m.reloadsFailed,
 			m.cutoffs,