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。

// The largest step that can be taken when remapping the mmap.
const maxMmapStep = 1 << 30 // 1GB

// The data file format version.
const version = 2

// Represents a marker value to indicate that a file is a Bolt DB.
const magic uint32 = 0xED0CDAED

// IgnoreNoSync specifies whether the NoSync field of a DB is ignored when
// syncing changes to a file.  This is required as some operating systems,
// such as OpenBSD, do not have a unified buffer cache (UBC) and writes
// must be synchronized using the msync(2) syscall.
const IgnoreNoSync = runtime.GOOS == "openbsd"

// Default values if not set in a DB instance.
const (
    DefaultMaxBatchSize  int =s 1000
    DefaultMaxBatchDelay     = 10 * time.Millisecond
    // 16k
    DefaultAllocSize = 16 * 1024 * 1024
)

// default page size for db is set to the OS page size.
var defaultPageSize = os.Getpagesize()

// DB represents a collection of buckets persisted to a file on disk.
// All data access is performed through transactions which can be obtained through the DB.
// All the functions on DB will return a ErrDatabaseNotOpen if accessed before Open() is called.
type DB struct {
    // When enabled, the database will perform a Check() after every commit.
    // A panic is issued if the database is in an inconsistent state. This
    // flag has a large performance impact so it should only be used for
    // debugging purposes.
    StrictMode bool

    // Setting the NoSync flag will cause the database to skip fsync()
    // calls after each commit. This can be useful when bulk loading data
    // into a database and you can restart the bulk load in the event of
    // a system failure or database corruption. Do not set this flag for
    // normal use.
    //
    // If the package global IgnoreNoSync constant is true, this value is
    // ignored.  See the comment on that constant for more details.
    //
    // THIS IS UNSAFE. PLEASE USE WITH CAUTION.
    NoSync bool

    // When true, skips the truncate call when growing the database.
    // Setting this to true is only safe on non-ext3/ext4 systems.
    // Skipping truncation avoids preallocation of hard drive space and
    // bypasses a truncate() and fsync() syscall on remapping.
    //
    // https://github.com/boltdb/bolt/issues/284
    NoGrowSync bool

    // If you want to read the entire database fast, you can set MmapFlag to
    // syscall.MAP_POPULATE on Linux 2.6.23+ for sequential read-ahead.
    MmapFlags int

    // MaxBatchSize is the maximum size of a batch. Default value is
    // copied from DefaultMaxBatchSize in Open.
    //
    // If <=0, disables batching.
    //
    // Do not change concurrently with calls to Batch.
    MaxBatchSize int

    // MaxBatchDelay is the maximum delay before a batch starts.
    // Default value is copied from DefaultMaxBatchDelay in Open.
    //
    // If <=0, effectively disables batching.
    //
    // Do not change concurrently with calls to Batch.
    MaxBatchDelay time.Duration

    // AllocSize is the amount of space allocated when the database
    // needs to create new pages. This is done to amortize the cost
    // of truncate() and fsync() when growing the data file.
    AllocSize int

    path     string
    file     *os.File  // 真实存储数据的磁盘文件
    lockfile *os.File // windows only
    dataref  []byte   // mmap'ed readonly, write throws SEGV
    // 通过mmap映射进来的地址
    data   *[maxMapSize]byte
    datasz int
    filesz int // current on disk file size
    //  元数据
    meta0 *meta
    meta1 *meta

    pageSize int
    opened   bool
    rwtx     *Tx  // 写事务锁
    txs      []*Tx  // 读事务数组
    freelist *freelist // 空闲列表
    stats    Stats

    pagePool sync.Pool

    batchMu sync.Mutex
    batch   *batch

    rwlock   sync.Mutex   // Allows only one writer at a time.
    metalock sync.Mutex   // Protects meta page access.
    mmaplock sync.RWMutex // Protects mmap access during remapping.
    statlock sync.RWMutex // Protects stats access.

    ops struct {
        writeAt func(b []byte, off int64) (n int, err error)
    }

    // Read only mode.
    // When true, Update() and Begin(true) return ErrDatabaseReadOnly immediately.
    readOnly bool
}