MySQL：理解MDLLock

本文基于源碼版本5.7.14
水平有限，有誤請諒解
筆者已經(jīng)將加好MDL 獲取過程和釋放過程的版本放到了github如下：
https://github.com/gaopengcarl/percona-server-locks-detail-5.7.22
供參考

作者深入理解主從原理專欄

公司主營業(yè)務(wù)：成都網(wǎng)站建設(shè)、網(wǎng)站制作、移動網(wǎng)站開發(fā)等業(yè)務(wù)。幫助企業(yè)客戶真正實現(xiàn)互聯(lián)網(wǎng)宣傳，提高企業(yè)的競爭能力。創(chuàng)新互聯(lián)建站是一支青春激揚、勤奮敬業(yè)、活力青春激揚、勤奮敬業(yè)、活力澎湃、和諧高效的團隊。公司秉承以“開放、自由、嚴謹、自律”為核心的企業(yè)文化，感謝他們對我們的高要求，感謝他們從不同領(lǐng)域給我們帶來的挑戰(zhàn)，讓我們激情的團隊有機會用頭腦與智慧不斷的給客戶帶來驚喜。創(chuàng)新互聯(lián)建站推出崇信免費做網(wǎng)站回饋大家。

https://www.jianshu.com/nb/43148932

一、MDL Lock綜述

MySQL中MDL鎖一直是一個比較讓人比較頭疼的問題，我們談起堵塞一般更加傾向于InnoDB層的row lock（gap lock/next key lock/key lock），因為它很好理解也很好觀察。而對于MDL Lock考慮就少一些，因為它實在不好觀察，只有出現(xiàn)問題查看show processlist的時候，可以看到簡單的所謂的‘Waiting for table metadata lock’之類的狀態(tài)，其實MDL Lock是MySQL上層一個非常復雜的子系統(tǒng)，有自己的死鎖檢測機制。
大家一般說是不是鎖表了很大一部分就和MDL Lock有關(guān)，可見的它的關(guān)鍵性和嚴重性，筆者也是根據(jù)自己的需求學習了一些，且沒有能力閱讀全部的代碼，但是筆者通過增加一個TICKET的打印函數(shù)讓語句的MDL Lock加鎖流程全部打印出來方便學習，下面從一些基礎(chǔ)概念說起然后告訴大家筆者是如何做的打印功能，最后對每種MDL TYPE可能出現(xiàn)的語句進行測試和分析。如果大家對基本概念和增加打印函數(shù)不感興趣可直接參考第五部分語句加MDL Lock測試和分析，希望這些測試能夠幫助到大家診斷問題。

剛好最近筆者遇到一次MDL Lock出現(xiàn)死鎖的情況會在下篇文章中給出案例，本文只看理論。

• 處于層次：MySQL層，實際上早在open_table函數(shù)中MDL LOCK就開始獲取了。
• 最早獲取階段: THD::enter_stage: ‘Opening tables’
• 調(diào)用棧幀

  #0 open_table_get_mdl_lock(thd=0x7fffd0000df0,ot_ctx=0x7fffec06fb00,
  table_list=0x7fffd00067d8,flags=0,mdl_ticket=0x7fffec06f950)
  at/root/MySQL5.7.14/percona-server-5.7.14-7/sql/sql_base.cc:2789
  #1 0x0000000001516e17inopen_table(thd=0x7fffd0000df0,
  table_list=0x7fffd00067d8,ot_ctx=0x7fffec06fb00)
  at/root/MySQL5.7.14/percona-server-5.7.14-7/sql/sql_base.cc:3237
  

• 死鎖檢測出錯碼

  { "ER_LOCK_DEADLOCK", 1213, "Deadlock found when trying to get lock; try restarting transaction" },
  ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction
  

  可以發(fā)現(xiàn)MDL Lock的死鎖拋錯和Innodb死鎖一模一樣，不同的只是‘show engine innodb status’沒有死鎖信息。

二、重要數(shù)據(jù)結(jié)構(gòu)和概念

1、MDL Lock類型

我們主要研究的類型如下：

MDL_INTENTION_EXCLUSIVE(IX)
MDL_SHARED(S)
MDL_SHARED_HIGH_PRIO(SH)
MDL_SHARED_READ(SR)
MDL_SHARED_WRITE(SW)
MDL_SHARED_WRITE_LOW_PRIO(SWL)
MDL_SHARED_UPGRADABLE(SU)
MDL_SHARED_READ_ONLY(SRO)
MDL_SHARED_NO_WRITE(SNW)
MDL_SHARED_NO_READ_WRITE(SNRW)
MDL_EXCLUSIVE(X)

第五部分會對每種類型進行詳細的測試和解釋。

2、MDL Lock namespace

在MDL中MDL_KEY按照namespace+DB+OBJECT_NAME的方式進行表示，所謂的namespace也比較重要下面是namespace的分類：

• GLOBAL is used for the global read lock.
  • TABLESPACE is for tablespaces.
  • SCHEMA is for schemas (aka databases).
  • TABLE is for tables and views.
  • FUNCTION is for stored functions.
  • PROCEDURE is for stored procedures.
  • TRIGGER is for triggers.
  • EVENT is for event scheduler events.
  • COMMIT is for enabling the global read lock to block commits.
  • USER_LEVEL_LOCK is for user-level locks.
  • LOCKING_SERVICE is for the name plugin RW-lock service

本文我們主要對GLOBAL/SCHEMA/TABLE namespace進行描述，而對于COMMIT namespace是提交的時候會用到的如果遇到等待，狀態(tài)為‘Waiting for commit lock’，一般為FTWRL堵塞COMMIT?？蓞⒖嘉业摹渡钊肜斫釳ySQL主從原理》15節(jié)。其他namespace不做描述。

3、MDL Lock實現(xiàn)分類

• scope lock:一般對應(yīng)全局MDL Lock，如flush table with read lock 會獲取namespace space：GLOBAL type：S和namespace space：COMMIT type：S的MDL Lock。它包含GLOBAL, COMMIT, TABLESPACE和SCHEMA
• object lock:如其名字所示，對象級別的MDL Lock，比如TABLE級別的MDL Lock，這也是本文的討論核心。 它包含其他的namespace。

下面是源碼注釋：

  /**
    Helper struct which defines how different types of locks are handled
    for a specific MDL_lock. In practice we use only two strategies: "scoped"
    lock strategy for locks in GLOBAL, COMMIT, TABLESPACE and SCHEMA namespaces
    and "object" lock strategy for all other namespaces.
  */

4、MDL Lock兼容矩陣

這里兼容矩陣是學習鎖堵塞的重點，類型很多比Innodb row lock類型要多很多，不用記住，只需要遇到能知道。

5、MDL Lock duration（MDL Lock持續(xù)周期）

這個對應(yīng)源碼的enum_mdl_duration，通常我們需要關(guān)注MDL Lock是事務(wù)提交后釋放還是語句結(jié)束后釋放，實際上就是這個，這對MDL lock堵塞的范圍很重要。我直接復制源碼的解釋。

• MDL_STATEMENT：Locks with statement duration are automatically released at the end of statement or transaction.
• MDL_TRANSACTION：Locks with transaction duration are automatically released at the end of transaction.
• MDL_EXPLICIT:Locks with explicit duration survive the end of statement and transaction.They have to be released explicitly by calling MDL_context::release_lock().

6、MDL Lock的FAST PATH（unobtrusive）和SLOW PATH（obtrusive）

使用兩種不同的方式目的在于優(yōu)化MDL Lock的實現(xiàn)，下面是源碼的注釋，可做適當了解：

• A) “unobtrusive” lock types
  1) Each type from this set should be compatible with all other types from the set (including itself).
  2) These types should be common for DML operations Our goal is to optimize acquisition and release of locks of this type by avoiding complex checks and manipulations on m_waiting/m_granted bitmaps/lists. We replace them with a check of and increment/decrement of integer counters.We call the latter type of acquisition/release “fast path”.Use of “fast path” reduces the size of critical section associated with MDL_lock::m_rwlock lock in the common case and thus increases scalability.The amount by which acquisition/release of specific type “unobtrusive” lock increases/decreases packed counter in MDL_lock::m_fast_path_state is returned by this function.
• B) “obtrusive” lock types
  1) Granted or pending lock of those type is incompatible withsome other types of locks or with itself.
  2) Not common for DML operations These locks have to be always acquired involving manipulations on m_waiting/m_granted bitmaps/lists, i.e. we have to use “slow path” for them. Moreover in the presence of active/pending locks from “obtrusive” set we have to acquire using “slow path” even locks of”unobtrusive” type.

7、MDL_request結(jié)構(gòu)部分屬性

也就是通過語句解析后需要獲得的MDL Lock的需求，然后通過這個類對象在MDL子系統(tǒng)中進行MDL Lock申請，大概包含如下一些屬性：

/** Type of metadata lock. */
  enum enum_mdl_type type; //需求的類型
  /** Duration for requested lock. */
  enum enum_mdl_duration duration; //持續(xù)周期
  /**
    Pointers for participating in the list of lock requests for this context.
  */
  MDL_request *next_in_list; //雙向鏈表實現(xiàn)
  MDL_request **prev_in_list;
  /**
    Pointer to the lock ticket object for this lock request.
    Valid only if this lock request is satisfied.
  */
  MDL_ticket *ticket; //注意這里如果申請成功(沒有等待)，會指向一個實際的TICKET，否則為NULL
  /** A lock is requested based on a fully qualified name and type. */

8、MDL_key結(jié)構(gòu)部分屬性

就是實際的namespace+DB+OBJECT_NAME，整個放到一個char數(shù)組里面，他會在MDL_LOCK和MDL_REQUEST中出現(xiàn)。

private:
uint16m_length;
uint16m_db_name_length;
charm_ptr[MAX_MDLKEY_LENGTH];//放到了這里

9、MDL_ticket結(jié)構(gòu)部分屬性

如同門票一樣，如果獲取了MDL Lock必然給MDL_request返回一張門票，如果等待則不會分配。源碼MDL_context::acquire_lock可以觀察到。部分屬性如下：

/**
    Pointers for participating in the list of lock requests for this context.
    Context private.正如解釋這里是context中鏈表鏈表的形成，是線程私有的
  */
  MDL_ticket *next_in_context;
  MDL_ticket **prev_in_context;
  /**
    Pointers for participating in the list of satisfied/pending requests
    for the lock. Externally accessible.正如解釋這里是MDL_LOCK中鏈表鏈表的形成，是全局的
  */
  MDL_ticket *next_in_lock;
  MDL_ticket **prev_in_lock;
/**
    Context of the owner of the metadata lock ticket. Externally accessible.
    很明顯這里指向了這個ticket的擁有者也就是MDL_context，它是線程的屬性
  */
  MDL_context *m_ctx; 
  /**
    Pointer to the lock object for this lock ticket. Externally accessible.
    很明顯這里是一個指向MDL_LOCK的一個指針
  */
  MDL_lock *m_lock;
  /**
    Indicates that ticket corresponds to lock acquired using "fast path"
    algorithm. Particularly this means that it was not included into
    MDL_lock::m_granted bitmap/list and instead is accounted for by
    MDL_lock::m_fast_path_locks_granted_counter
    這里就代表了是否是FAST PATH從注釋來看fast path方式不會在MDL LOCK中
    占用granted位圖和鏈表取而代之代之的是一個統(tǒng)計器m_fast_path_locks_granted_counter
    這樣一來開銷肯定更小
  */
  bool m_is_fast_path;
  /**
    Indicates that ticket corresponds to lock request which required
    storage engine notification during its acquisition and requires
    storage engine notification after its release.
  */

10、MDL_lock結(jié)構(gòu)部分屬性

每一個MDL_key都會對應(yīng)一個MDL_lock，其中包含了所謂的GRANTED鏈表和WAIT鏈表，考慮它的復雜性，可以直接參考源碼注釋也非常詳細，這里給出我所描述的幾個屬性如下：

/** The key of the object (data) being protected. */
  MDL_key key;
/** List of granted tickets for this lock. */
  Ticket_list m_granted;
/** Tickets for contexts waiting to acquire a lock. */
  Ticket_list m_waiting;

11、MDL_context結(jié)構(gòu)部分屬性

這是整個MySQL線程和MDL Lock子系統(tǒng)進行交互的一個所謂的上下文結(jié)構(gòu),其中包含了很多方法和屬性，我比較關(guān)注的屬性如下：

/**
    If our request for a lock is scheduled, or aborted by the deadlock
    detector, the result is recorded in this class.
  */
  MDL_wait m_wait;
/**
    Lists of all MDL tickets acquired by this connection.
    這是一個不同MDL lock持續(xù)時間的一個鏈表數(shù)組。實際就是
    MDL_STATEMENT一個鏈表
    MDL_TRANSACTION一個鏈表
    MDL_EXPLICIT一個鏈表
  */
Ticket_list m_tickets[MDL_DURATION_END];
//這是一個父類指針指向子類對象，虛函數(shù)重寫的典型,實際他就指向了一個線程
/*
class THD :public MDL_context_owner,
           public Query_arena,
           public Open_tables_state
*/
MDL_context_owner *m_owner;

12、所有等待狀態(tài)

源碼給出了所有的等待標記如下：

PSI_stage_info MDL_key::m_namespace_to_wait_state_name[NAMESPACE_END]=
{
  {0, "Waiting for global read lock", 0},
  {0, "Waiting for tablespace metadata lock", 0},
  {0, "Waiting for schema metadata lock", 0},
  {0, "Waiting for table metadata lock", 0},
  {0, "Waiting for stored function metadata lock", 0},
  {0, "Waiting for stored procedure metadata lock", 0},
  {0, "Waiting for trigger metadata lock", 0},
  {0, "Waiting for event metadata lock", 0},
  {0, "Waiting for commit lock", 0},
  {0, "User lock", 0}, /* Be compatible with old status. */
  {0, "Waiting for locking service lock", 0},
  {0, "Waiting for backup lock", 0},
  {0, "Waiting for binlog lock", 0}
};

我們常見的是：

• “Waiting for table metadata lock”：通常就是namespace TABLE級別的MDL Lock，具體根據(jù)兼容矩陣判斷參考第五節(jié)。
• “Waiting for global read lock”：通常就是namespace GLOBAL級別的MDL Lock，通常和flush table with read lock有關(guān)，參考第五節(jié)。
• “Waiting for commit lock”：通常就是namespace COMMIT級別的MDL Lock，通常和flush table with read lock有關(guān)，參考第五節(jié)。

三、增加打印函數(shù)my_print_ticket

學習MDL Lock最好的方式當然是獲取一條語句鎖加的所有MDL Lock，包含加鎖、升級、降級和釋放的流程。雖然5.7加入診斷MDL Lock的方法：

UPDATE performance_schema.setup_consumers SET ENABLED = 'YES' WHERE NAME ='global_instrumentation';
UPDATE performance_schema.setup_instruments SET ENABLED = 'YES' WHERE NAME ='wait/lock/metadata/sql/mdl';
select * from performance_schema.metadata_locks

但是對于每個語句獲取的所有MDL Lock的流程仍然不好觀察，因此我加入了打印函數(shù)：

/*p_ticket in parameter*/
int my_print_ticket(const MDL_ticket* p_ticket)

并且在mdl_ticket類中增加了這個函數(shù)原型為友元函數(shù)：

friend int my_print_ticket(const MDL_ticket* p_ticket);

主要捕獲MDL Lock的加鎖信息打印到err日志中，包含的信息如下：

• 線程id：通過p_ticket->m_ctx->get_thd(); 獲取
• mdl lock database name：通過p_ticket->m_lock->key.db_name()獲取
• mdl lock object name：通過p_ticket->m_lock->key.name()獲取
• mdl lock namespace：通過p_ticket->m_lock->key.mdl_namespace()獲取
• mdl lock fast path：通過p_ticket->m_is_fast_path獲取判斷是則輸出否則不輸出
• mdl lock type：通過p_ticket->m_type獲取
• mdl lock duration：通過p_ticket->m_duration獲取

上面這些信息都在前面進行過描述了。具體的輸出信息如下：

2017-08-03T07:34:21.720583Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T07:34:21.720601Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T07:34:21.720619Z 3 [Note] (-->MDL PRINT) OBJ_name is:test 
2017-08-03T07:34:21.720637Z 3 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T07:34:21.720655Z 3 [Note] (---->MDL PRINT) Fast path is:(Y)
2017-08-03T07:34:21.720673Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED_WRITE(SW) 
2017-08-03T07:34:21.720692Z 3 [Note] (------>MDL PRINT) Mdl  duration is:MDL_TRANSACTION

這實際上和metadata_locks中的信息差不多，如下：

MySQL>   select * from performance_schema.metadata_locks\G
*************************** 1. row ***************************
          OBJECT_TYPE: TABLE
        OBJECT_SCHEMA: test
          OBJECT_NAME: test
OBJECT_INSTANCE_BEGIN: 140734412907760
            LOCK_TYPE: SHARED_WRITE
        LOCK_DURATION: TRANSACTION
          LOCK_STATUS: GRANTED
               SOURCE: sql_parse.cc:6314
      OWNER_THREAD_ID: 39
       OWNER_EVENT_ID: 241

一旦有了這個函數(shù)我們只需要在加鎖、升級、降級和釋放的位置進行適當添加就可以了。

四、在合適的位置增加my_print_ticket打印函數(shù)

既然我們要研究MDL Lock的加鎖?升級?降級，那么我們就必要找到他們的函數(shù)入口，然后在合適的位置增加打印函數(shù)my_print_ticket進行觀察，下面標示出打印位置。

1、加鎖：MDL_context::acquire_lock

bool
MDL_context::acquire_lock(MDL_request *mdl_request, ulong lock_wait_timeout)
{
  if (mdl_request->ticket) //獲取成功獲得ticket
  {
    /*
      We have managed to acquire lock without waiting.
      MDL_lock, MDL_context and MDL_request were updated
      accordingly, so we can simply return success.
    */
    //REQUESET獲取TICKET成功 此處打印
    return FALSE;
  }
  /*
    Our attempt to acquire lock without waiting has failed.
    As a result of this attempt we got MDL_ticket with m_lock
    member pointing to the corresponding MDL_lock object which
    has MDL_lock::m_rwlock write-locked.
  */
  //獲取不成功加入MDL_lock 等待隊列
  lock= ticket->m_lock;
  lock->m_waiting.add_ticket(ticket);
  will_wait_for(ticket); //死鎖檢測
  /* There is a shared or exclusive lock on the object. */
  DEBUG_SYNC(get_thd(), "mdl_acquire_lock_wait");
  find_deadlock();
  //此處打印TICKET進入了等待流程
  if (lock->needs_notification(ticket) || lock->needs_connection_check())
  {
   }
  done_waiting_for();//等待完成對死鎖檢測等待圖進行調(diào)整去掉本等待邊edge(無向圖)
  //當然到這里也是通過等待后獲得成功了狀態(tài)為GRANTED
  DBUG_ASSERT(wait_status == MDL_wait::GRANTED);
  m_tickets[mdl_request->duration].push_front(ticket);
  mdl_request->ticket= ticket;
  MySQL_mdl_set_status(ticket->m_psi, MDL_ticket::GRANTED);
  //此處打印通過等待REQUEST獲得了TICKET
  return FALSE;
}

2、降級：void MDL_ticket::downgrade_lock(enum_mdl_type new_type)

void MDL_ticket::downgrade_lock(enum_mdl_type new_type)
{
  /* Only allow downgrade from EXCLUSIVE and SHARED_NO_WRITE. */
  DBUG_ASSERT(m_type == MDL_EXCLUSIVE ||
              m_type == MDL_SHARED_NO_WRITE);
//此處打印出降級前的TICKET
  if (m_hton_notified)
  {
    MySQL_mdl_set_status(m_psi, MDL_ticket::POST_RELEASE_NOTIFY);
    m_ctx->get_owner()->notify_hton_post_release_exclusive(&m_lock->key);
    m_hton_notified= false;
    MySQL_mdl_set_status(m_psi, MDL_ticket::GRANTED);
  }
//函數(shù)結(jié)尾答應(yīng)出降級后的TICKET
}

3、升級：MDL_context::upgrade_shared_lock(MDL_ticket *mdl_ticket,enum_mdl_type new_type, ulong lock_wait_timeout)

bool
MDL_context::upgrade_shared_lock(MDL_ticket *mdl_ticket,
                                 enum_mdl_type new_type,
                                 ulong lock_wait_timeout)
{
  MDL_REQUEST_INIT_BY_KEY(&mdl_new_lock_request,
                          &mdl_ticket->m_lock->key, new_type,
                          MDL_TRANSACTION);//構(gòu)造一個request
 //此處打印出來的TICKET類型 
  if (acquire_lock(&mdl_new_lock_request, lock_wait_timeout)) //嘗試使用新的LOCK_TYPE進行加鎖
    DBUG_RETURN(TRUE);
  is_new_ticket= ! has_lock(mdl_svp, mdl_new_lock_request.ticket);
  lock= mdl_ticket->m_lock;
  //下面進行一系列對MDL_LOCK的維護并且對所謂的合并操作
  /* Code below assumes that we were upgrading to "obtrusive" type of lock. */
  DBUG_ASSERT(lock->is_obtrusive_lock(new_type));
  /* Merge the acquired and the original lock. @todo: move to a method. */
  MySQL_prlock_wrlock(&lock->m_rwlock);
  if (is_new_ticket)
  {
    m_tickets[MDL_TRANSACTION].remove(mdl_new_lock_request.ticket);
    MDL_ticket::destroy(mdl_new_lock_request.ticket);
  }
 //此處打印出來的升級后TICKET類型 
  DBUG_RETURN(FALSE);
}

4、釋放：略

五、常見MDL Lock類型加鎖測試

1、MDL_INTENTION_EXCLUSIVE(IX)

這個鎖會在很多操作的時候都會出現(xiàn)，比如做任何一個DML/DDL操作都會觸發(fā)，實際上DELTE/UPDATE/INSERT/FOR UPDATE等DML操作會在GLOBAL 上加IX鎖，然后才會在本對象上加鎖。而DDL 語句至少會在GLOBAL 上加IX鎖，對象所屬 SCHEMA上加IX鎖，本對象加鎖。

下面是 DELETE 觸發(fā)的 GLOABL IX MDL LOCK：

2017-08-03T18:22:38.092205Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T18:22:38.092242Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T18:22:38.092276Z 3 [Note] (--->MDL PRINT) Namespace is:GLOBAL 
2017-08-03T18:22:38.092310Z 3 [Note] (---->MDL PRINT) Fast path is:(Y)
2017-08-03T18:22:38.092344Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_INTENTION_EXCLUSIVE(IX) 
2017-08-03T18:22:38.092380Z 3 [Note] (------>MDL PRINT) Mdl  duration is:MDL_STATEMENT 
2017-08-03T18:22:38.092551Z 3 [Note] (------->MDL PRINT) Mdl  status is:EMPTY

我們注意一樣它的持續(xù)周期為語句級別。

下面是 ALETER 語句觸發(fā)的GLOABL IX MDL Lock：

2017-08-03T18:46:05.894871Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T18:46:05.894915Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T18:46:05.894948Z 3 [Note] (--->MDL PRINT) Namespace is:GLOBAL 
2017-08-03T18:46:05.894980Z 3 [Note] (---->MDL PRINT) Fast path is:(Y)
2017-08-03T18:46:05.895012Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_INTENTION_EXCLUSIVE(IX) 
2017-08-03T18:46:05.895044Z 3 [Note] (------>MDL PRINT) Mdl  duration is:MDL_STATEMENT 
2017-08-03T18:46:05.895076Z 3 [Note] (------->MDL PRINT) Mdl  status is:EMPTY

所以這個MDL Lock無所不在，而只有是否兼容問題，如果不兼容則堵塞。scope lock的IX類型一般都是兼容的除非遇到S類型，下面討論。

這把鎖一般用在flush tables with read lock中，如下：

MySQL> flush tables with read lock;
Query OK, 0 rows affected (0.01 sec)
2017-08-03T18:19:11.603911Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T18:19:11.603947Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T18:19:11.603971Z 3 [Note] (--->MDL PRINT) Namespace is:GLOBAL 
2017-08-03T18:19:11.603994Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED(S) 
2017-08-03T18:19:11.604045Z 3 [Note] (------>MDL PRINT) Mdl  duration is:MDL_EXPLICIT 
2017-08-03T18:19:11.604073Z 3 [Note] (------->MDL PRINT) Mdl  status is:EMPTY 
2017-08-03T18:19:11.604133Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T18:19:11.604156Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T18:19:11.604194Z 3 [Note] (--->MDL PRINT) Namespace is:COMMIT 
2017-08-03T18:19:11.604217Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED(S) 
2017-08-03T18:19:11.604240Z 3 [Note] (------>MDL PRINT) Mdl  duration is:MDL_EXPLICIT 
2017-08-03T18:19:11.604310Z 3 [Note] (------->MDL PRINT) Mdl  status is:EMPTY

我們注意到其namspace為GLOBAL和COMMIT顯然他們是scope lock ，他們的TYPE為S，那么很顯然根據(jù)兼容性原則scope lock的MDL IX和MDL S 不兼容， flush tables with read lock 就會堵塞所有DELTE/UPDATE/INSERT/FOR UPDATE等DML和DDL操作，并且也會堵塞commit操作。

3、MDL_SHARED_HIGH_PRIO(SH)

這個鎖基本上大家也是經(jīng)常用到只是沒感覺到而已，比如我們一般desc操作，兼容矩陣如下：

操作記錄如下：

MySQL> desc test.testsort10;
2017-08-03T19:06:05.843277Z 4 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T19:06:05.843324Z 4 [Note] (>MDL PRINT) Thread id is 4: 
2017-08-03T19:06:05.843359Z 4 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T19:06:05.843392Z 4 [Note] (-->MDL PRINT) OBJ_name is:testsort10 
2017-08-03T19:06:05.843425Z 4 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T19:06:05.843456Z 4 [Note] (---->MDL PRINT) Fast path is:(Y)
2017-08-03T19:06:05.843506Z 4 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED_HIGH_PRIO(SH) 
2017-08-03T19:06:05.843538Z 4 [Note] (------>MDL PRINT) Mdl  duration is:MDL_TRANSACTION 
2017-08-03T19:06:05.843570Z 4 [Note] (------->MDL PRINT) Mdl  status is:EMPTY

這中類型的優(yōu)先級比較高，但是其和X不兼容。注意持續(xù)時間為MDL_TRANSACTION 。

4、MDL_SHARED_READ(SR)

這把鎖一般用在非當前讀取的select中，兼容性如下：

操作記錄如下：

MySQL> select * from test.testsort10 limit 1;
2017-08-03T19:13:52.338764Z 4 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T19:13:52.338813Z 4 [Note] (>MDL PRINT) Thread id is 4: 
2017-08-03T19:13:52.338847Z 4 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T19:13:52.338883Z 4 [Note] (-->MDL PRINT) OBJ_name is:testsort10 
2017-08-03T19:13:52.338917Z 4 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T19:13:52.338950Z 4 [Note] (---->MDL PRINT) Fast path is:(Y)
2017-08-03T19:13:52.339025Z 4 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED_READ(SR) 
2017-08-03T19:13:52.339062Z 4 [Note] (------>MDL PRINT) Mdl  duration is:MDL_TRANSACTION 
2017-08-03T19:13:52.339097Z 4 [Note] (------->MDL PRINT) Mdl  status is:EMPTY

這里還是要提及一下平時我們偶爾會出現(xiàn)select也堵住的情況(比如DDL的某個階段需要對象MDL X鎖)。我們不得不抱怨MySQL居然會堵塞select其實這里也就是object mdl lock X 和SR 不兼容的問題(參考前面的兼容矩陣)。注意持續(xù)時間為MDL_TRANSACTION 。

5、MDL_SHARED_WRITE(SW)

這把鎖一般用于DELTE/UPDATE/INSERT/FOR UPDATE等操作對table的加鎖(當前讀)，不包含DDL操作，但是要注意DML操作實際上還會有一個GLOBAL的IX的鎖，前面已經(jīng)提及過了，這把鎖只是對象上的，兼容性如下：

操作記錄如下：

MySQL> select * from test.testsort10 limit 1 for update;
2017-08-03T19:25:41.218428Z 4 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T19:25:41.218461Z 4 [Note] (>MDL PRINT) Thread id is 4: 
2017-08-03T19:25:41.218493Z 4 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T19:25:41.218525Z 4 [Note] (-->MDL PRINT) OBJ_name is:testsort10 
2017-08-03T19:25:41.218557Z 4 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T19:25:41.218588Z 4 [Note] (---->MDL PRINT) Fast path is:(Y)
2017-08-03T19:25:41.218620Z 4 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED_WRITE(SW) 
2017-08-03T19:25:41.218677Z 4 [Note] (------>MDL PRINT) Mdl  duration is:MDL_TRANSACTION 
2017-08-03T19:25:41.218874Z 4 [Note] (------->MDL PRINT) Mdl  status is:EMPTY

注意持續(xù)時間為MDL_TRANSACTION 。

6、MDL_SHARED_WRITE_LOW_PRIO(SWL)

這把鎖很少用到源碼注釋只有如下：

Used by DML statements modifying tables and using the LOW_PRIORITY clause

不做解釋了。

7、MDL_SHARED_UPGRADABLE(SU)

這把鎖一般在ALTER TABLE語句中會用到，他可以升級為SNW,、SNRW、X，同時至少X鎖也可以降級為SU實際上在Innodb ONLINE DDL中非常依賴它，由于它的存在那么DML(SW)和SELECT(SR)都不會堵塞，兼容性如下：

我們有必要研究一下他的兼容性，可以看到 OBJECT LOCK中(SELECT)SR和(DML)SW都是允許的，而在SCOPED LOCK中雖然DML DDL都會在GLOBAL 上鎖，但是其類型都是IX。所以這個SU鎖不堵塞DML/SELECT 讀寫操作進入Innodb引擎層，它是ONLINE DDL的基礎(chǔ)。如果不兼容你都進入不了Innodb引擎層，更談不上什么ONLINE DDL，注意我這里說的ALGORITHM=INPLACE的ONLINE DDL。

操作日志記錄：

MySQL>  alter table testsort12 add column it int not null;
Query OK, 0 rows affected (6.27 sec)
Records: 0  Duplicates: 0  Warnings: 0
2017-08-03T19:46:54.781453Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T19:46:54.781487Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T19:46:54.781948Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T19:46:54.781990Z 3 [Note] (-->MDL PRINT) OBJ_name is:testsort12 
2017-08-03T19:46:54.782026Z 3 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T19:46:54.782060Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED_UPGRADABLE(SU) 
2017-08-03T19:46:54.782096Z 3 [Note] (------>MDL PRINT) Mdl  duration is:MDL_TRANSACTION 
2017-08-03T19:46:54.782175Z 3 [Note] (------->MDL PRINT) Mdl  status is:EMPTY 
2017-08-03T19:46:54.803898Z 3 [Note] (upgrade_shared_lock)THIS MDL LOCK will upgrade
2017-08-03T19:46:54.804201Z 3 [Note] (upgrade_shared_lock)THIS MDL LOCK  upgrade TO
2017-08-03T19:46:54.804240Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T19:46:54.804254Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T19:46:54.804267Z 3 [Note] (-->MDL PRINT) OBJ_name is:testsort12 
2017-08-03T19:46:54.804280Z 3 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T19:46:54.804293Z 3 [Note] (----->MDL PRINT) Mdl type :MDL_EXCLUSIVE(X) 
2017-08-03T19:46:54.804306Z 3 [Note] (------>MDL PRINT) Mdl  duration is:MDL_TRANSACTION 
2017-08-03T19:46:54.804319Z 3 [Note] (------->MDL PRINT) Mdl  status is:EMPTY 
2017-08-03T19:46:54.855563Z 3 [Note] (downgrade_lock)THIS MDL LOCK will downgrade
2017-08-03T19:46:54.855693Z 3 [Note] (downgrade_lock) to this MDL lock
2017-08-03T19:46:54.855706Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T19:46:54.855717Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T19:46:54.856053Z 3 [Note] (-->MDL PRINT) OBJ_name is:testsort12 
2017-08-03T19:46:54.856069Z 3 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T19:46:54.856082Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED_UPGRADABLE(SU) 
2017-08-03T19:46:54.856094Z 3 [Note] (------>MDL PRINT) Mdl  duration is:MDL_TRANSACTION 
2017-08-03T19:46:54.856214Z 3 [Note] (------->MDL PRINT) Mdl  status is:EMPTY 
2017-08-03T19:47:00.260166Z 3 [Note] (upgrade_shared_lock)THIS MDL LOCK will upgrade
2017-08-03T19:47:00.304057Z 3 [Note] (upgrade_shared_lock)THIS MDL LOCK  upgrade TO
2017-08-03T19:47:00.304090Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T19:47:00.304105Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T19:47:00.304119Z 3 [Note] (-->MDL PRINT) OBJ_name is:testsort12 
2017-08-03T19:47:00.304132Z 3 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T19:47:00.304181Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_EXCLUSIVE(X) 
2017-08-03T19:47:00.304196Z 3 [Note] (------>MDL PRINT) Mdl  duration is:MDL_TRANSACTION 
2017-08-03T19:47:00.304211Z 3 [Note] (------->MDL PRINT) Mdl  status is:EMPTY 
2017-08-03T19:47:01.032329Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!

我們需要簡單分析一下，獲得testsort12表上的MDL Lock大概流程如下：

2017-08-03T19:46:54.781487 獲得 MDL_SHARED_UPGRADABLE(SU)
2017-08-03T19:46:54.804293 升級 MDL_EXCLUSIVE(X)  準備階段
2017-08-03T19:46:54.855563 降級 MDL_SHARED_UPGRADABLE(SU) 執(zhí)行階段
2017-08-03T19:47:00.304057 升級 MDL_EXCLUSIVE(X)  提交階段

不管如何這個ALTER操作還是比較費時的，從時間我們看到2017-08-03T19:46:54降級完成(SU)到2017-08-03T19:47:00這段時間，實際上是最耗時的實際上這里就是實際的Inplace重建，但是這個過程實際在MDL SU模式下所以不會堵塞DML/SELECT操作。這里再給大家提個醒，所謂的ONLINE DDL只是在Inplace重建階段不堵塞DML/SELECT操作，還是盡量在數(shù)據(jù)庫壓力小的時候操作，如果有DML沒有提交或者SELECT沒有做完這個時候SW或者SR必然堵塞X，而X為高優(yōu)先級能夠堵塞所有操作。這樣導致的現(xiàn)象就是由于DML未提交會堵塞DDL操作，而DDL操作會堵塞所有操作，基本對于這個TABLE的表全部操作堵塞（SW堵塞X，X堵塞所有操作）。
而對于ALGORITHM=COPY 在COPY階段用的是SNW鎖，接下來我就先來看看SNW鎖。

8、MDL_SHARED_NO_WRITE(SNW)

SU可以升級為SNW而SNW可以升級為X，如前面所提及的用于ALGORITHM=COPY 中，保護數(shù)據(jù)的一致性。先看看它的兼容性如下：

從兼容矩陣可以看到，本鎖不會堵塞SR，但是堵塞SW，當然也就堵塞了DML(SW)而SELECT(SR)不會堵塞，下面是部分操作記錄日志：

MySQL>  alter table testsort12 add column ik int not null, ALGORITHM=COPY  ;
2017-08-03T20:07:58.413215Z 3 [Note] (upgrade_shared_lock)THIS MDL LOCK  upgrade TO
2017-08-03T20:07:58.413241Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T20:07:58.413257Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T20:07:58.413273Z 3 [Note] (-->MDL PRINT) OBJ_name is:testsort12 
2017-08-03T20:07:58.413292Z 3 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T20:07:58.413308Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED_NO_WRITE(SNW) 
2017-08-03T20:07:58.413325Z 3 [Note] (------>MDL PRINT) Mdl  duration is:MDL_TRANSACTION 
2017-08-03T20:07:58.413341Z 3 [Note] (------->MDL PRINT) Mdl  status is:EMPTY 
2017-08-03T20:08:25.392006Z 3 [Note] (upgrade_shared_lock)THIS MDL LOCK  upgrade TO
2017-08-03T20:08:25.392024Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T20:08:25.392086Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T20:08:25.392159Z 3 [Note] (-->MDL PRINT) OBJ_name is:testsort12 
2017-08-03T20:08:25.392199Z 3 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T20:08:25.392214Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_EXCLUSIVE(X) 
2017-08-03T20:08:25.392228Z 3 [Note] (------>MDL PRINT) Mdl  duration is:MDL_TRANSACTION 
2017-08-03T20:08:25.392242Z 3 [Note] (------->MDL PRINT) Mdl  status is:EMPTY

我們可以發(fā)現(xiàn)如下：

2017-08-03T20:07:58.413308 獲得了MDL_SHARED_NO_WRITE(SNW) 
2017-08-03T20:08:25.392006 升級為MDL_EXCLUSIVE(X)

2017-08-03T20:07:58.413308到2017-08-03T20:08:25.392006就是實際COPY的時間，可見整個COPY期間只能SELECT，而不能DML。也是ALGORITHM=COPY和ALGORITHM=INPLACE的一個關(guān)鍵區(qū)別。

9、MDL_SHARED_READ_ONLY(SRO)

用于LOCK TABLES READ 語句，兼容性如下：

根據(jù)兼容性可以發(fā)現(xiàn)，堵塞DML(SW)但是SELECT(SR)還是可以的。下面是操作日志：

MySQL> lock table testsort12 read;
Query OK, 0 rows affected (0.01 sec)
2017-08-03T21:08:27.267947Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T21:08:27.267979Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T21:08:27.268009Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T21:08:27.268040Z 3 [Note] (-->MDL PRINT) OBJ_name is:testsort12 
2017-08-03T21:08:27.268070Z 3 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T21:08:27.268113Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED_READ_ONLY(SRO) 
2017-08-03T21:08:27.268145Z 3 [Note] (------>MDL PRINT) Mdl  duration is:MDL_TRANSACTION 
2017-08-03T21:08:27.268175Z 3 [Note] (------->MDL PRINT) Mdl  status is:EMPTY

10、MDL_SHARED_NO_READ_WRITE(SNRW)

用于LOCK TABLES WRITE語句，兼容性如下：

可以看到DML(SW)和SELECT(SR)都被它堵塞，但是還可以DESC(SH)。
操作日志記錄如下：

MySQL> lock table testsort12 write;
Query OK, 0 rows affected (0.00 sec)
2017-08-03T21:13:07.113347Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T21:13:07.113407Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T21:13:07.113435Z 3 [Note] (--->MDL PRINT) Namespace is:GLOBAL 
2017-08-03T21:13:07.113458Z 3 [Note] (---->MDL PRINT) Fast path is:(Y)
2017-08-03T21:13:07.113482Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_INTENTION_EXCLUSIVE(IX) 
2017-08-03T21:13:07.113505Z 3 [Note] (------>MDL PRINT) Mdl  duration is:MDL_STATEMENT 
2017-08-03T21:13:07.113604Z 3 [Note] (------->MDL PRINT) Mdl  status is:EMPTY 
2017-08-03T21:13:07.113637Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T21:13:07.113660Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T21:13:07.113681Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T21:13:07.113703Z 3 [Note] (-->MDL PRINT) OBJ_name is: 
2017-08-03T21:13:07.113725Z 3 [Note] (--->MDL PRINT) Namespace is:SCHEMA 
2017-08-03T21:13:07.113746Z 3 [Note] (---->MDL PRINT) Fast path is:(Y)
2017-08-03T21:13:07.113768Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_INTENTION_EXCLUSIVE(IX) 
2017-08-03T21:13:07.113791Z 3 [Note] (------>MDL PRINT) Mdl  duration is:MDL_TRANSACTION 
2017-08-03T21:13:07.113813Z 3 [Note] (------->MDL PRINT) Mdl  status is:EMPTY 
2017-08-03T21:13:07.113842Z 3 [Note] (acquire_lock)THIS MDL LOCK acquire ok!
2017-08-03T21:13:07.113865Z 3 [Note] (>MDL PRINT) Thread id is 3: 
2017-08-03T21:13:07.113887Z 3 [Note] (->MDL PRINT) DB_name is:test 
2017-08-03T21:13:07.113922Z 3 [Note] (-->MDL PRINT) OBJ_name is:testsort12 
2017-08-03T21:13:07.113945Z 3 [Note] (--->MDL PRINT) Namespace is:TABLE 
2017-08-03T21:13:07.113975Z 3 [Note] (----->MDL PRINT) Mdl type is:MDL_SHARED_NO_READ_WRITE(SNRW) 
2017-08-03T21:13:07.113998Z 3 [Note] (------>MDL PRINT) Mdl  duration is:MDL_TRANSACTION 
2017-08-03T21:13:07.114021Z 3 [Note] (------->MDL PRINT) Mdl  status is:EMPTY

除此之外可以發(fā)現(xiàn)語句還需要GLOBAL和SCHEMA上的IX鎖，換句話說flush tables with read lock; 會堵塞‘lock table testsort12 write’，但是‘lock table testsort12 read’卻不會堵塞。

11、MDL_EXCLUSIVE(X)

用于各種DDL操作，實際上基本全部的DDL都會涉及到這個鎖，即便是ONLINE DDL也會在準備和提交階段獲取本鎖，因此ONLINE DDL不是完全不堵塞的，只是堵塞時間很短很短，兼容性如下：

我們在驗證SU和SNW MDL Lock類型的時候已經(jīng)看到了操作記錄，不做補充了。

作者微信：gp_22389860