求指教 boltdb 中 关于文件 grow 和 remmap 的逻辑

https://github.com/etcd-io/bbolt/blob/master/tx.go#L141 tx.Commit()方法中, 当重新写 freelist 时 tx.meta.pgid 有变动,就调用 grow,这个优化怎么理解呢? 完全 get 不到它的点,

commitFreelist 里调用 grow 方法时, 非 windows 系统 需要先调用 ftruncate, 再用 fsync.

func (db *DB) grow(sz int) error {
	// Ignore if the new size is less than available file size.
	if sz <= db.filesz {
		return nil
	}

	// If the data is smaller than the alloc size then only allocate what's needed.
	// Once it goes over the allocation size then allocate in chunks.
	if db.datasz < db.AllocSize {
		sz = db.datasz
	} else {
		sz += db.AllocSize
	}

	// Truncate and fsync to ensure file size metadata is flushed.
	// https://github.com/boltdb/bolt/issues/284
	if !db.NoGrowSync && !db.readOnly {
		if runtime.GOOS != "windows" {
			if err := db.file.Truncate(int64(sz)); err != nil {
				return fmt.Errorf("file resize error: %s", err)
			}
		}
		if err := db.file.Sync(); err != nil {
			return fmt.Errorf("file sync error: %s", err)
		}
		if db.Mlock {
			// unlock old file and lock new one
			if err := db.mrelock(db.filesz, sz); err != nil {
				return fmt.Errorf("mlock/munlock error: %s", err)
			}
		}
	}

	db.filesz = sz
	return nil
}

但在 mmap()方法里, windows 先调用了 ftruncate, 再调用 fsync, unix 这里只有 mmap. 这又是什么操作?

// windows
func mmap(db *DB, sz int) error {
	if !db.readOnly {
		// Truncate the database to the size of the mmap.
		if err := db.file.Truncate(int64(sz)); err != nil {
			return fmt.Errorf("truncate: %s", err)
		}
	}

	// Open a file mapping handle.
	sizelo := uint32(sz >> 32)
	sizehi := uint32(sz) & 0xffffffff
	h, errno := syscall.CreateFileMapping(syscall.Handle(db.file.Fd()), nil, syscall.PAGE_READONLY, sizelo, sizehi, nil)
	if h == 0 {
		return os.NewSyscallError("CreateFileMapping", errno)
	}

	// Create the memory map.
	addr, errno := syscall.MapViewOfFile(h, syscall.FILE_MAP_READ, 0, 0, uintptr(sz))
	if addr == 0 {
		return os.NewSyscallError("MapViewOfFile", errno)
	}

	// Close mapping handle.
	if err := syscall.CloseHandle(syscall.Handle(h)); err != nil {
		return os.NewSyscallError("CloseHandle", err)
	}

	// Convert to a byte array.
	db.data = ((*[maxMapSize]byte)(unsafe.Pointer(addr)))
	db.datasz = sz

	return nil
}

// unix
// mmap memory maps a DB's data file.
func mmap(db *DB, sz int) error {
	// Map the data file to memory.
	b, err := unix.Mmap(int(db.file.Fd()), 0, sz, syscall.PROT_READ, syscall.MAP_SHARED|db.MmapFlags)
	if err != nil {
		return err
	}

	// Advise the kernel that the mmap is accessed randomly.
	err = unix.Madvise(b, syscall.MADV_RANDOM)
	if err != nil && err != syscall.ENOSYS {
		// Ignore not implemented error in kernel because it still works.
		return fmt.Errorf("madvise: %s", err)
	}

	// Save the original byte slice and convert to a byte array pointer.
	db.dataref = b
	db.data = (*[maxMapSize]byte)(unsafe.Pointer(&b[0]))
	db.datasz = sz
	return nil
}