第七节 db.allocate()和db.grow()分析

  1. // allocate returns a contiguous block of memory starting at a given page.
  2. func (db *DB) allocate(count int) (*page, error) {
  3.     // Allocate a temporary buffer for the page.
  4.     var buf []byte
  5.     if count == 1 {
  6.         buf = db.pagePool.Get().([]byte)
  7.     } else {
  8.         buf = make([]byte, count*db.pageSize)
  9.     }
  10.     // 转成*page
  11.     p := (*page)(unsafe.Pointer(&buf[0]))
  12.     p.overflow = uint32(count - 1)
  14.     // Use pages from the freelist if they are available.
  15.     // 先从空闲列表中找
  16.     if p.id = db.freelist.allocate(count); p.id != 0 {
  17.         return p, nil
  18.     }
  20.     // 找不到的话，就按照事务的pgid来分配
  21.     // 表示需要从文件内部扩大
  23.     // Resize mmap() if we're at the end.
  24.     p.id = db.rwtx.meta.pgid
  25.     // 因此需要判断是否目前所有的页数已经大于了mmap映射出来的空间
  26.     // 这儿计算的页面总数是从当前的id后还要计算count+1个
  27.     var minsz = int((p.id+pgid(count))+1) * db.pageSize
  28.     if minsz >= db.datasz {
  29.         if err := db.mmap(minsz); err != nil {
  30.             return nil, fmt.Errorf("mmap allocate error: %s", err)
  31.         }
  32.     }
  34.     // Move the page id high water mark.
  35.     // 如果不是从freelist中找到的空间的话，更新meta的id，也就意味着是从文件中新扩展的页
  36.     db.rwtx.meta.pgid += pgid(count)
  38.     return p, nil
  39. }
  41. // grow grows the size of the database to the given sz.
  42. func (db *DB) grow(sz int) error {
  43.     // Ignore if the new size is less than available file size.
  44.     if sz <= db.filesz {
  45.         return nil
  46.     }
  48.     // 满足这个条件sz>filesz
  50.     // If the data is smaller than the alloc size then only allocate what's needed.
  51.     // Once it goes over the allocation size then allocate in chunks.
  52.     if db.datasz < db.AllocSize {
  53.         sz = db.datasz
  54.     } else {
  55.         sz += db.AllocSize
  56.     }
  58.     // Truncate and fsync to ensure file size metadata is flushed.
  59.     // https://github.com/boltdb/bolt/issues/284
  60.     if !db.NoGrowSync && !db.readOnly {
  61.         if runtime.GOOS != "windows" {
  62.             if err := db.file.Truncate(int64(sz)); err != nil {
  63.                 return fmt.Errorf("file resize error: %s", err)
  64.             }
  65.         }
  66.         if err := db.file.Sync(); err != nil {
  67.             return fmt.Errorf("file sync error: %s", err)
  68.         }
  69.     }
  71.     db.filesz = sz
  72.     return nil
  73. }