Add option to adjust fill percentage.

This commit adds the ability to adjust the fill percentage for splitting nodes. This
works by setting a threshold that is a percentage of a total page size. When that
threshold is crossed during a split then a new node is created.

This is primarily beneficial for append-only workloads.

Fixes #163.

cmd/bolt/bench.go

@@ -46,6 +46,7 @@ func Bench(options *BenchOptions) {
 		fatal(err)
 		return
 	}
+	db.FillPercent = options.FillPercent
 	defer db.Close()
 
 	// Enable streaming stats.
@@ -280,6 +281,7 @@ type BenchOptions struct {
 	MemProfile    string
 	BlockProfile  string
 	StatsInterval time.Duration
+	FillPercent   float64
 	Clean         bool
 }
 

cmd/bolt/main.go

@@ -8,6 +8,7 @@ import (
 	"os"
 	"time"
 
+	"github.com/boltdb/bolt"
 	"github.com/codegangsta/cli"
 )
 
@@ -114,6 +115,7 @@ func NewApp() *cli.App {
 				&cli.StringFlag{Name: "memprofile", Usage: "Memory profile output path"},
 				&cli.StringFlag{Name: "blockprofile", Usage: "Block profile output path"},
 				&cli.StringFlag{Name: "stats-interval", Value: "0s", Usage: "Continuous stats interval"},
+				&cli.Float64Flag{Name: "fill-percent", Value: bolt.DefaultFillPercent, Usage: "Fill percentage"},
 				&cli.BoolFlag{Name: "work", Usage: "Print the temp db and do not delete on exit"},
 			},
 			Action: func(c *cli.Context) {
@@ -134,6 +136,7 @@ func NewApp() *cli.App {
 					MemProfile:    c.String("memprofile"),
 					BlockProfile:  c.String("blockprofile"),
 					StatsInterval: statsInterval,
+					FillPercent:   c.Float64("fill-percent"),
 					Clean:         !c.Bool("work"),
 				})
 			},

db.go

@@ -24,6 +24,15 @@ const version = 2
 // Represents a marker value to indicate that a file is a Bolt DB.
 const magic uint32 = 0xED0CDAED
 
+const (
+	minFillPercent = 0.1
+	maxFillPercent = 1.0
+)
+
+// DefaultFillPercent is the percentage that split pages are filled.
+// This value can be changed by setting DB.FillPercent.
+const DefaultFillPercent = 0.5
+
 var (
 	// ErrDatabaseNotOpen is returned when a DB instance is accessed before it
 	// is opened or after it is closed.
@@ -54,6 +63,11 @@ type DB struct {
 	// debugging purposes.
 	StrictMode bool
 
+	// Sets the threshold for filling pages when they split. By default,
+	// the database will fill to 50% but it can be useful to increase this
+	// amount if you know that your write workloads are typically append-only.
+	FillPercent float64
+
 	path     string
 	file     *os.File
 	data     []byte
@@ -94,7 +108,7 @@ func (db *DB) String() string {
 // Open creates and opens a database at the given path.
 // If the file does not exist then it will be created automatically.
 func Open(path string, mode os.FileMode) (*DB, error) {
-	var db = &DB{opened: true}
+	var db = &DB{opened: true, FillPercent: DefaultFillPercent}
 
 	// Open data file and separate sync handler for metadata writes.
 	db.path = path

node.go

@@ -215,8 +215,14 @@ func (n *node) split(pageSize int) []*node {
 		return nodes
 	}
 
-	// Set fill threshold to 50%.
-	threshold := pageSize / 2
+	// Determine the threshold before starting a new node.
+	var fillPercent = n.bucket.tx.db.FillPercent
+	if fillPercent < minFillPercent {
+		fillPercent = minFillPercent
+	} else if fillPercent > maxFillPercent {
+		fillPercent = maxFillPercent
+	}
+	threshold := int(float64(pageSize) * fillPercent)
 
 	// Group into smaller pages and target a given fill size.
 	size := pageHeaderSize