add function to retreive index's section memory size and use this to get symbol table size

block.go

@@ -84,6 +84,9 @@ type IndexReader interface {
 	// LabelIndices returns a list of string tuples for which a label value index exists.
 	LabelIndices() ([][]string, error)
 
+	// GetSymbolTableSize returns the size occupied by the symbol table of Reader object.
+	GetSymbolTableSize() uint32
+
 	// Close releases the underlying resources of the reader.
 	Close() error
 }
@@ -382,6 +385,11 @@ func (r blockIndexReader) LabelIndices() ([][]string, error) {
 	return ss, errors.Wrapf(err, "block: %s", r.b.Meta().ULID)
 }
 
+func (r blockIndexReader) GetSymbolTableSize() uint32 {
+	ss := r.ir.GetSymbolTableSize()
+	return ss
+}
+
 func (r blockIndexReader) Close() error {
 	r.b.pendingReaders.Done()
 	return nil
@@ -474,7 +482,7 @@ func (pb *Block) CleanTombstones(dest string, c Compactor) (bool, error) {
 	numStones := 0
 
 	pb.tombstones.Iter(func(id uint64, ivs Intervals) error {
-		for _ = range ivs {
+		for range ivs {
 			numStones++
 		}
 

db.go

@@ -125,6 +125,7 @@ type dbMetrics struct {
 	cutoffs              prometheus.Counter
 	cutoffsFailed        prometheus.Counter
 	tombCleanTimer       prometheus.Histogram
+	symbolTableSize      prometheus.GaugeFunc
 }
 
 func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
@@ -162,7 +163,23 @@ func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
 		Name: "prometheus_tsdb_tombstone_cleanup_seconds",
 		Help: "The time taken to recompact blocks to remove tombstones.",
 	})
-
+	m.symbolTableSize = prometheus.NewGaugeFunc(prometheus.GaugeOpts{
+		Name: "prometheus_tsdb_symbol_table_size",
+		Help: "Size occupied by symbol table",
+	}, func() float64 {
+		db.mtx.RLock()
+		defer db.mtx.RUnlock()
+		var size uint64
+		for _, block := range db.blocks {
+			ind, err := block.Index()
+			if err != nil {
+				size = 0
+				break
+			}
+			size += uint64(ind.GetSymbolTableSize())
+		}
+		return float64(size)
+	})
 	if r != nil {
 		r.MustRegister(
 			m.loadedBlocks,
@@ -172,6 +189,7 @@ func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
 			m.cutoffsFailed,
 			m.compactionsTriggered,
 			m.tombCleanTimer,
+			m.symbolTableSize,
 		)
 	}
 	return m

index/index.go

@@ -749,6 +749,23 @@ func (r *Reader) decbufUvarintAt(off int) decbuf {
 	return dec
 }
 
+// getSectionSize returns the first 4 bytes in a section starting at offset off
+// to get the content length bytes.
+func (r *Reader) getSectionSize(off int) uint32 {
+	b := r.b.Range(off, off+4)
+	l := uint32(binary.BigEndian.Uint32(b))
+
+	if l <= 0 {
+		l = 0
+	}
+	return l
+}
+
+// getSymbolTableSize returns the bytes taken by the symbol table of Reader object.
+func (r *Reader) GetSymbolTableSize() uint32 {
+	return r.getSectionSize(int(r.toc.symbols))
+}
+
 // readSymbols reads the symbol table fully into memory and allocates proper strings for them.
 // Strings backed by the mmap'd memory would cause memory faults if applications keep using them
 // after the reader is closed.