add WriteToFlag to Tx

For in memory workload, it does not make sense to use
o_direct to copy the file. Adding a option to clear out
o_direct and for other future cases.

bolt_linux.go

@@ -4,8 +4,6 @@ import (
 	"syscall"
 )
 
-var odirect = syscall.O_DIRECT
-
 // fdatasync flushes written data to a file descriptor.
 func fdatasync(db *DB) error {
 	return syscall.Fdatasync(int(db.file.Fd()))

bolt_openbsd.go

@@ -11,8 +11,6 @@ const (
 	msInvalidate             // invalidate cached data
 )
 
-var odirect int
-
 func msync(db *DB) error {
 	_, _, errno := syscall.Syscall(syscall.SYS_MSYNC, uintptr(unsafe.Pointer(db.data)), uintptr(db.datasz), msInvalidate)
 	if errno != 0 {

bolt_windows.go

@@ -40,8 +40,6 @@ func unlockFileEx(h syscall.Handle, reserved, locklow, lockhigh uint32, ol *sysc
 	return nil
 }
 
-var odirect int
-
 // fdatasync flushes written data to a file descriptor.
 func fdatasync(db *DB) error {
 	return db.file.Sync()

tx.go

@@ -29,6 +29,16 @@ type Tx struct {
 	pages          map[pgid]*page
 	stats          TxStats
 	commitHandlers []func()
+
+	// WriteFlag specifies the flag for write related methods
+	// like WriteTo.
+	// Tx opens the database file with the specified
+	// flag to copy the data.
+	//
+	// By default, the flag is set to empty for in-memory workload.
+	// To avoid cache trashing for large on disk workload, set this
+	// flag with o_direct.
+	WriteFlag int
 }
 
 // init initializes the transaction.
@@ -272,14 +282,11 @@ func (tx *Tx) Copy(w io.Writer) error {
 // WriteTo writes the entire database to a writer.
 // If err == nil then exactly tx.Size() bytes will be written into the writer.
 func (tx *Tx) WriteTo(w io.Writer) (n int64, err error) {
-	// Attempt to open reader directly.
+	// Attempt to open reader with WriteFlag
 	var f *os.File
-	if f, err = os.OpenFile(tx.db.path, os.O_RDONLY|odirect, 0); err != nil {
+	if f, err = os.OpenFile(tx.db.path, os.O_RDONLY|tx.WriteFlag, 0); err != nil {
-		// Fallback to a regular open if that doesn't work.
-		if f, err = os.OpenFile(tx.db.path, os.O_RDONLY, 0); err != nil {
 		return 0, err
 	}
-	}
 
 	// Copy the meta pages.
 	tx.db.metalock.Lock()