InnoDB支援行鎖，有時升級為表鎖。

MyISAM只支援表鎖。

表鎖：開小小，加鎖快，不會出現死鎖；鎖粒度大，鎖衝突機率高，併發度低。

行鎖：開銷大，加鎖慢，會出現死鎖，鎖粒度小，鎖衝突機率低，併發高。

1.1.   InnoDB鎖型別

主要分為：讀鎖（共享鎖），寫鎖（排他鎖），意向鎖，和MDL鎖。

1.1.1.   讀鎖

讀鎖，S鎖，一個事物在讀取一個資料行時，其他事務也可以讀，但不能對該資料行增刪改的操作。兩種select方式的應用。

l   自動提交模式下的select查詢語句，不需加任何鎖返回結果，是一致性非鎖定讀。

l   透過select....lock in share mode在被讀取的行記錄或行記錄的範圍上加一個讀鎖，讓其他事務可讀不可申請加寫鎖。

1.1.2.   寫鎖

寫鎖簡稱X鎖，一個事務獲取一行的寫鎖，其他事務就不能獲取該行其它鎖，優先順序最高。

select for update，會對讀取的行記錄上加一個寫鎖，其他任何事務就不能加任何鎖。

1.1.3.   MDL鎖

mysql5.5引入meta data lock，簡稱MDL鎖，用於保護表中後設資料的資訊。即一個事務查詢表將自動給表加MDL鎖，其他事務不能做任何DDL操作。

1.1.4.   意向鎖

InnoDB引擎中，意向鎖是表級鎖，作用和MDL類似，防止事務進行過程中，執行DDL語句的操作而導致資料不一致。有兩種意向鎖型別：

l   意向共享鎖（IS）：資料行加共享鎖前必須先取得該表的IS鎖。

l   意向排他鎖（IX）：資料行加排他鎖前必須先取得該表的IX鎖。

1.2.   InnoDB行鎖種類

InnoDB預設事務隔離級別為RR，且引數innodb_locks_unsafe_for_binlog=0的模式下，行鎖有三種。

l   單個行記錄的鎖（record lock）,主鍵和唯一索引都是。

l   間隙鎖（GAP lock）

l   記錄鎖和間隙鎖的組合叫next-key lock。普通索引預設。

1.2.1.   單個行記錄的鎖

InnoDB上的行鎖就是加在索引上。有索引，更新只鎖指定行，無索引，更新鎖所有行。

1.2.2.   間隙鎖（GAP lock）

RR隔離級別，為了避免幻讀，引入Gap lock，只鎖定行記錄資料的範圍，不包含記錄本身，即不允許在此範圍內插入任何資料。

RC隔離級別允許出現幻讀現象。

1.2.3.   Next-Key Locks

Next-key lock是記錄鎖（Record Lock）與間隔鎖（Gap Lock）的組合，當InnoDB掃描索引記錄時，會先對選中的索引記錄加上記錄鎖（Record lock）,再對索引記錄兩邊的間隙加上間隙鎖（Gap lock）。

1.3.   鎖等待和死鎖

鎖等待，是一個事務產生鎖，其他事務等待上個事務釋放它的鎖。鎖等待超時閾值innodb_lok_wait_timeout控制，單位秒。

死鎖，多個事務爭奪資源相互等待的現象，即鎖資源請求產生了迴路，就是死迴圈。

避免死鎖的方法：

l   如不同的程式會併發存取多個表，或涉及多汗記錄，儘量約定以相同的順序訪問表。

l   業務中儘量採用小事務，避免大事務，及時提交或回滾。

l   在同一個事務中，儘可能做到一次鎖定所需的所有資源。

l   對容易產生死鎖的業務，可以嘗試使用升級鎖粒度，透過表鎖定減少鎖產生的機率。

 

透過show engine innodb sttus檢視死鎖展示資訊 。

[(none)]>show engine innodb status;

 

| Type   | Name | Status

 

| InnoDB |      |

=====================================

2018-11-07 22:49:40 0x7f1320202700 INNODB MONITOR OUTPUT

=====================================

Per second averages calculated from the last 11 seconds

-----------------

BACKGROUND THREAD

-----------------

srv_master_thread loops: 98 srv_active, 0 srv_shutdown, 49465 srv_idle

srv_master_thread log flush and writes: 49563

----------

SEMAPHORES

----------

OS WAIT ARRAY INFO: reservation count 20

OS WAIT ARRAY INFO: signal count 20

RW-shared spins 0, rounds 34, OS waits 16

RW-excl spins 0, rounds 200, OS waits 2

RW-sx spins 0, rounds 0, OS waits 0

Spin rounds per wait: 34.00 RW-shared, 200.00 RW-excl, 0.00 RW-sx

------------

TRANSACTIONS

------------

Trx id counter 65440

Purge done for trx's n:o < 65438 undo n:o < 0 state: running but idle

History list length 12

LIST OF TRANSACTIONS FOR EACH SESSION:

---TRANSACTION 421197684710112, not started

0 lock struct(s), heap size 1136, 0 row lock(s)

---TRANSACTION 421197684709200, not started

0 lock struct(s), heap size 1136, 0 row lock(s)

--------

FILE I/O

--------

I/O thread 0 state: waiting for completed aio requests (insert buffer thread)

I/O thread 1 state: waiting for completed aio requests (log thread)

I/O thread 2 state: waiting for completed aio requests (read thread)

I/O thread 3 state: waiting for completed aio requests (read thread)

I/O thread 4 state: waiting for completed aio requests (read thread)

I/O thread 5 state: waiting for completed aio requests (read thread)

I/O thread 6 state: waiting for completed aio requests (read thread)

I/O thread 7 state: waiting for completed aio requests (read thread)

I/O thread 8 state: waiting for completed aio requests (read thread)

I/O thread 9 state: waiting for completed aio requests (read thread)

I/O thread 10 state: waiting for completed aio requests (write thread)

I/O thread 11 state: waiting for completed aio requests (write thread)

I/O thread 12 state: waiting for completed aio requests (write thread)

I/O thread 13 state: waiting for completed aio requests (write thread)

I/O thread 14 state: waiting for completed aio requests (write thread)

I/O thread 15 state: waiting for completed aio requests (write thread)

I/O thread 16 state: waiting for completed aio requests (write thread)

I/O thread 17 state: waiting for completed aio requests (write thread)

Pending normal aio reads: [0, 0, 0, 0, 0, 0, 0, 0] , aio writes: [0, 0, 0, 0, 0, 0, 0, 0] ,

 ibuf aio reads:, log i/o's:, sync i/o's:

Pending flushes (fsync) log: 0; buffer pool: 0

271 OS file reads, 61118 OS file writes, 60451 OS fsyncs

0.00 reads/s, 0 avg bytes/read, 0.00 writes/s, 0.00 fsyncs/s

-------------------------------------

INSERT BUFFER AND ADAPTIVE HASH INDEX

-------------------------------------

Ibuf: size 1, free list len 0, seg size 2, 0 merges

merged operations:

 insert 0, delete mark 0, delete 0

discarded operations:

 insert 0, delete mark 0, delete 0

Hash table size 553193, node heap has 0 buffer(s)

Hash table size 553193, node heap has 0 buffer(s)

Hash table size 553193, node heap has 0 buffer(s)

Hash table size 553193, node heap has 0 buffer(s)

Hash table size 553193, node heap has 0 buffer(s)

Hash table size 553193, node heap has 1 buffer(s)

Hash table size 553193, node heap has 0 buffer(s)

Hash table size 553193, node heap has 0 buffer(s)

0.00 hash searches/s, 0.00 non-hash searches/s

---

LOG

---

Log sequence number 20357616

Log flushed up to   20357616

Pages flushed up to 20357616

Last checkpoint at  20357607

0 pending log flushes, 0 pending chkp writes

60048 log i/o's done, 0.00 log i/o's/second

----------------------

BUFFER POOL AND MEMORY

----------------------

Total large memory allocated 2198863872

Dictionary memory allocated 156387

Buffer pool size   131056

Free buffers       130465

Database pages     590

Old database pages 0

Modified db pages  0

Pending reads      0

Pending writes: LRU 0, flush list 0, single page 0

Pages made young 0, not young 0

0.00 youngs/s, 0.00 non-youngs/s

Pages read 238, created 352, written 805

0.00 reads/s, 0.00 creates/s, 0.00 writes/s

No buffer pool page gets since the last printout

Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s

LRU len: 590, unzip_LRU len: 0

I/O sum[0]:cur[0], unzip sum[0]:cur[0]

----------------------

INDIVIDUAL BUFFER POOL INFO

----------------------

---BUFFER POOL 0

Buffer pool size   16382

Free buffers       16279

Database pages     102

Old database pages 0

Modified db pages  0

Pending reads      0

Pending writes: LRU 0, flush list 0, single page 0

Pages made young 0, not young 0

0.00 youngs/s, 0.00 non-youngs/s

Pages read 38, created 64, written 95

0.00 reads/s, 0.00 creates/s, 0.00 writes/s

No buffer pool page gets since the last printout

Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s

LRU len: 102, unzip_LRU len: 0

I/O sum[0]:cur[0], unzip sum[0]:cur[0]

---BUFFER POOL 1

Buffer pool size   16382

Free buffers       16312

Database pages     70

Old database pages 0

Modified db pages  0

Pending reads      0

Pending writes: LRU 0, flush list 0, single page 0

Pages made young 0, not young 0

0.00 youngs/s, 0.00 non-youngs/s

Pages read 6, created 64, written 64

0.00 reads/s, 0.00 creates/s, 0.00 writes/s

No buffer pool page gets since the last printout

Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s

LRU len: 70, unzip_LRU len: 0

I/O sum[0]:cur[0], unzip sum[0]:cur[0]

---BUFFER POOL 2

Buffer pool size   16382

Free buffers       16319

Database pages     63

Old database pages 0

Modified db pages  0

Pending reads      0

Pending writes: LRU 0, flush list 0, single page 0

Pages made young 0, not young 0

0.00 youngs/s, 0.00 non-youngs/s

Pages read 8, created 55, written 56

0.00 reads/s, 0.00 creates/s, 0.00 writes/s

No buffer pool page gets since the last printout

Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s

LRU len: 63, unzip_LRU len: 0

I/O sum[0]:cur[0], unzip sum[0]:cur[0]

---BUFFER POOL 3

Buffer pool size   16382

Free buffers       16303

Database pages     79

Old database pages 0

Modified db pages  0

Pending reads      0

Pending writes: LRU 0, flush list 0, single page 0

Pages made young 0, not young 0

0.00 youngs/s, 0.00 non-youngs/s

Pages read 73, created 6, written 59

0.00 reads/s, 0.00 creates/s, 0.00 writes/s

No buffer pool page gets since the last printout

Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s

LRU len: 79, unzip_LRU len: 0

I/O sum[0]:cur[0], unzip sum[0]:cur[0]

---BUFFER POOL 4

Buffer pool size   16382

Free buffers       16265

Database pages     117

Old database pages 0

Modified db pages  0

Pending reads      0

Pending writes: LRU 0, flush list 0, single page 0

Pages made young 0, not young 0

0.00 youngs/s, 0.00 non-youngs/s

Pages read 76, created 41, written 120

0.00 reads/s, 0.00 creates/s, 0.00 writes/s

No buffer pool page gets since the last printout

Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s

LRU len: 117, unzip_LRU len: 0

I/O sum[0]:cur[0], unzip sum[0]:cur[0]

---BUFFER POOL 5

Buffer pool size   16382

Free buffers       16307

Database pages     75

Old database pages 0

Modified db pages  0

Pending reads      0

Pending writes: LRU 0, flush list 0, single page 0

Pages made young 0, not young 0

0.00 youngs/s, 0.00 non-youngs/s

Pages read 11, created 64, written 91

0.00 reads/s, 0.00 creates/s, 0.00 writes/s

No buffer pool page gets since the last printout

Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s

LRU len: 75, unzip_LRU len: 0

I/O sum[0]:cur[0], unzip sum[0]:cur[0]

---BUFFER POOL 6

Buffer pool size   16382

Free buffers       16363

Database pages     19

Old database pages 0

Modified db pages  0

Pending reads      0

Pending writes: LRU 0, flush list 0, single page 0

Pages made young 0, not young 0

0.00 youngs/s, 0.00 non-youngs/s

Pages read 13, created 6, written 12

0.00 reads/s, 0.00 creates/s, 0.00 writes/s

No buffer pool page gets since the last printout

Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s

LRU len: 19, unzip_LRU len: 0

I/O sum[0]:cur[0], unzip sum[0]:cur[0]

---BUFFER POOL 7

Buffer pool size   16382

Free buffers       16317

Database pages     65

Old database pages 0

Modified db pages  0

Pending reads      0

Pending writes: LRU 0, flush list 0, single page 0

Pages made young 0, not young 0

0.00 youngs/s, 0.00 non-youngs/s

Pages read 13, created 52, written 308

0.00 reads/s, 0.00 creates/s, 0.00 writes/s

No buffer pool page gets since the last printout

Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s

LRU len: 65, unzip_LRU len: 0

I/O sum[0]:cur[0], unzip sum[0]:cur[0]

--------------

ROW OPERATIONS

--------------

0 queries inside InnoDB, 0 queries in queue

0 read views open inside InnoDB

Process ID=21556, Main thread ID=139720374998784, state: sleeping

Number of rows inserted 60824, updated 0, deleted 0, read 121836

0.00 inserts/s, 0.00 updates/s, 0.00 deletes/s, 0.00 reads/s

----------------------------

END OF INNODB MONITOR OUTPUT

============================

 

1.4.   鎖問題的監控

透過show full processlist和show engine Innodb status來判斷事務中鎖問題情況，另外還有三張表可查：

information_schema.INNODB_TRX

information_schema.INNODB_LOCKS

information_schema.INNODB_LOCK_WAITS

 

innodb_trx表部分欄位

trx_id唯一的事務id號；

trx_state:事務狀態；

trx_wait_started:事務開始等待時間。

trx_mysql_thread_id:執行緒ID，與show full processlist相互對應。

trx_query:事務執行的SQL;

trx_operation_state:事務執行的狀態。

 

[(none)]>show full processlist;

+----+------+-----------+------+---------+------+----------+-----------------------+

| Id | User | Host      | db   | Command | Time | State    | Info                  |

+----+------+-----------+------+---------+------+----------+-----------------------+

| 57 | root | localhost | NULL | Query   |    0 | starting | show full processlist |

+----+------+-----------+------+---------+------+----------+-----------------------+