第一节 DB结构

DB在boltdb是一个结构体，里面封装了很多属性，部分属性添加了中文注释，其他部分属性，大家可以直接看英文注释，感觉英文表述的很通俗易懂。

  2. // The largest step that can be taken when remapping the mmap.
  3. const maxMmapStep = 1 << 30 // 1GB
  5. // The data file format version.
  6. const version = 2
  8. // Represents a marker value to indicate that a file is a Bolt DB.
  9. const magic uint32 = 0xED0CDAED
  11. // IgnoreNoSync specifies whether the NoSync field of a DB is ignored when
  12. // syncing changes to a file.  This is required as some operating systems,
  13. // such as OpenBSD, do not have a unified buffer cache (UBC) and writes
  14. // must be synchronized using the msync(2) syscall.
  15. const IgnoreNoSync = runtime.GOOS == "openbsd"
  17. // Default values if not set in a DB instance.
  18. const (
  19.     DefaultMaxBatchSize  int =s 1000
  20.     DefaultMaxBatchDelay     = 10 * time.Millisecond
  21.     // 16k
  22.     DefaultAllocSize = 16 * 1024 * 1024
  23. )
  25. // default page size for db is set to the OS page size.
  26. var defaultPageSize = os.Getpagesize()
  28. // DB represents a collection of buckets persisted to a file on disk.
  29. // All data access is performed through transactions which can be obtained through the DB.
  30. // All the functions on DB will return a ErrDatabaseNotOpen if accessed before Open() is called.
  31. type DB struct {
  32.     // When enabled, the database will perform a Check() after every commit.
  33.     // A panic is issued if the database is in an inconsistent state. This
  34.     // flag has a large performance impact so it should only be used for
  35.     // debugging purposes.
  36.     StrictMode bool
  38.     // Setting the NoSync flag will cause the database to skip fsync()
  39.     // calls after each commit. This can be useful when bulk loading data
  40.     // into a database and you can restart the bulk load in the event of
  41.     // a system failure or database corruption. Do not set this flag for
  42.     // normal use.
  43.     //
  44.     // If the package global IgnoreNoSync constant is true, this value is
  45.     // ignored.  See the comment on that constant for more details.
  46.     //
  47.     // THIS IS UNSAFE. PLEASE USE WITH CAUTION.
  48.     NoSync bool
  50.     // When true, skips the truncate call when growing the database.
  51.     // Setting this to true is only safe on non-ext3/ext4 systems.
  52.     // Skipping truncation avoids preallocation of hard drive space and
  53.     // bypasses a truncate() and fsync() syscall on remapping.
  54.     //
  55.     // https://github.com/boltdb/bolt/issues/284
  56.     NoGrowSync bool
  58.     // If you want to read the entire database fast, you can set MmapFlag to
  59.     // syscall.MAP_POPULATE on Linux 2.6.23+ for sequential read-ahead.
  60.     MmapFlags int
  62.     // MaxBatchSize is the maximum size of a batch. Default value is
  63.     // copied from DefaultMaxBatchSize in Open.
  64.     //
  65.     // If <=0, disables batching.
  66.     //
  67.     // Do not change concurrently with calls to Batch.
  68.     MaxBatchSize int
  70.     // MaxBatchDelay is the maximum delay before a batch starts.
  71.     // Default value is copied from DefaultMaxBatchDelay in Open.
  72.     //
  73.     // If <=0, effectively disables batching.
  74.     //
  75.     // Do not change concurrently with calls to Batch.
  76.     MaxBatchDelay time.Duration
  78.     // AllocSize is the amount of space allocated when the database
  79.     // needs to create new pages. This is done to amortize the cost
  80.     // of truncate() and fsync() when growing the data file.
  81.     AllocSize int
  83.     path     string
  84.     file     *os.File  // 真实存储数据的磁盘文件
  85.     lockfile *os.File // windows only
  86.     dataref  []byte   // mmap'ed readonly, write throws SEGV
  87.     // 通过mmap映射进来的地址
  88.     data   *[maxMapSize]byte
  89.     datasz int
  90.     filesz int // current on disk file size
  91.     //  元数据
  92.     meta0 *meta
  93.     meta1 *meta
  95.     pageSize int
  96.     opened   bool
  97.     rwtx     *Tx  // 写事务锁
  98.     txs      []*Tx  // 读事务数组
  99.     freelist *freelist // 空闲列表
  100.     stats    Stats
  102.     pagePool sync.Pool
  104.     batchMu sync.Mutex
  105.     batch   *batch
  107.     rwlock   sync.Mutex   // Allows only one writer at a time.
  108.     metalock sync.Mutex   // Protects meta page access.
  109.     mmaplock sync.RWMutex // Protects mmap access during remapping.
  110.     statlock sync.RWMutex // Protects stats access.
  112.     ops struct {
  113.         writeAt func(b []byte, off int64) (n int, err error)
  114.     }
  116.     // Read only mode.
  117.     // When true, Update() and Begin(true) return ErrDatabaseReadOnly immediately.
  118.     readOnly bool
  119. }