Key lock 的秘密

研究死锁，或者观察sp_lock，有时候最恼人的莫过于你看到下面研究成果的key lock，但是却不知道究竟是哪个page 哪个row被lock住了：

Exec sp_lock:

 
 
就说上面的key (9dd27be994c0) 吧，能不能知道这个key，究竟是对应于那个table，那个data page，甚至哪一行（row）呢？

可以的。且听我慢慢说来。

先说这一行：

52        20        978102525      2          KEY      (9dd27be994c0)                      X          GRANT

其中20就是dbid了，indid 2表示是nonclustered  index，而978102525就是objectid了。这个比较好懂。你可以使用Select object_name(978102525)来得到表的名字：

而Key (9dd27be994c0)其实是键值的哈希（hash）值。Hash出来的值，你无法逆向得到它的键值。但是幸运的是，你可以使用微软的undocumented的宏，%%lockres%%，来对表的所有键进行一次同样的hash运算，然后你就可以从hash的结果集里面找到你想要的键值啦。首先，需要知道index为2的是那个index：

select name,index_id,type_desc from sys.indexes where object_id=object_id('test')

name           
index_id    type_desc

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

cidx1           1           CLUSTERED

idx1            2           NONCLUSTERED

然后就可以使用%%lockres%%了：

SELECT   %%lockres%% 'key', test.*  FROM  test with(index(idx1))

或者：

SELECT   %%lockres%% 'key', test.*  FROM  test with(index=2)

结果如下：

你可以看到，对应sp_lock 的key的hash为9dd27be994c0的行，就是c1=2的那行。简单点你甚至可以使用下面 的语句直接得到某个key hash值的行：

SELECT   %%lockres%% 'key', test.*  FROM  test with(index(idx1)) where %%lockres%% ='(9dd27be994c0)'

再进一步，能否得到该行对应的page和fileid呢？这个就需要使用另一个undocumented的宏．%%physloc%%了：

select %%physloc%%,test.*  from test with(index(idx1))

上面的返回值是一个十六进制的值，表示page，file，和slot id。可以使用sys.fn_physlocformatter函数将它转换成一个可读的(file:page:slot)格式的值：

select %%physloc%%,sys.fn_physlocformatter(%%physloc%%) 'physical location',test.*  from test with(index(idx1))

结果如下：

对应第二行数据的，就在37828这个page了。这个时候，你可以使用DBCC PAGE来打印出它的内容，探究slot为2 的那行：

dbcc traceon(3604)

dbcc page(20,1,37828,1)

结果：

PAGE: (1:37828)

BUFFER:

BUF @0x00000000803BB400

bpage = 0x000000002CAA6000         
bhash = 0x0000000000000000         
bpageno = (1:37828)

bdbid = 20                         
breferences = 0                    
bcputicks = 0

bsampleCount = 0                   
bUse1 = 42097                       bstat = 0x10b

blog = 0x5ab21c9a                  
bnext = 0x0000000000000000

PAGE HEADER:

Page @0x000000002CAA6000

m_pageId = (1:37828)               
m_headerVersion = 1                
m_type = 2

m_typeFlagBits = 0x0                m_level = 0                         m_flagBits = 0x0

m_objId (AllocUnitId.idObj) = 194  
m_indexId (AllocUnitId.idInd) = 256

Metadata: AllocUnitId = 72057594050641920

Metadata: PartitionId = 72057594046251008                                Metadata: IndexId = 2

Metadata: ObjectId = 978102525     
m_prevPage = (0:0)                 
m_nextPage = (0:0)

pminlen = 5                        
m_slotCnt = 5                      
m_freeCnt = 7997

m_freeData = 217                    m_reservedCnt = 0                   m_lsn = (17704:185:18)

m_xactReserved = 0                 
m_xdesId = (0:20808688)            
m_ghostRecCnt = 0

m_tornBits = -1577081955           
DB Frag ID = 1

Allocation Status

GAM (1:2) = ALLOCATED              
SGAM (1:3) = ALLOCATED

PFS (1:32352) = 0x60 MIXED_EXT ALLOCATED   0_PCT_FULL                    DIFF (1:6) = CHANGED

ML (1:7) = NOT MIN_LOGGED

DATA:

Slot 0, Offset 0x60, Length 16, DumpStyle BYTE

Record Type = INDEX_RECORD         
Record Attributes =  NULL_BITMAP
VARIABLE_COLUMNS

Record Size = 16

Memory Dump @0x000000005166A060

0000000000000000:   36010000
00030000 01001000 61626364          
6...........abcd

Slot 1, Offset 0x70, Length 22, DumpStyle BYTE

Record Type = INDEX_RECORD         
Record Attributes =  NULL_BITMAP
VARIABLE_COLUMNS

Record Size = 22

Memory Dump @0x000000005166A070

0000000000000000:   36010000
00030000 02001200 16006162 63640100 
6.............abcd..

0000000000000014:   0000                                         
..

Slot 2, Offset 0xc9, Length 16, DumpStyle BYTE

Record Type = INDEX_RECORD          Record Attributes =  NULL_BITMAP VARIABLE_COLUMNS

Record Size = 16

Memory Dump @0x000000005166A0C9

0000000000000000:   36020000
00030000 01001000 31323334          
6...........1234

Slot 3, Offset 0x96, Length 18, DumpStyle BYTE

Record Type = INDEX_RECORD         
Record Attributes =  NULL_BITMAP
VARIABLE_COLUMNS

Record Size = 18

Memory Dump @0x000000005166A096

0000000000000000:   36030000
00030000 01001200 65656566 6666     
6...........eeefff

Slot 4, Offset 0xa8, Length 17, DumpStyle BYTE

Record Type = INDEX_RECORD         
Record Attributes =  NULL_BITMAP
VARIABLE_COLUMNS

Record Size = 17

Memory Dump @0x000000005166A0A8

0000000000000000:   36040000
00030000 01001100 38397070 70       
6...........89ppp

OFFSET TABLE:

Row - Offset

4 (0x4) - 168 (0xa8)

3 (0x3) - 150 (0x96)

2 (0x2) - 201 (0xc9)

1 (0x1) - 112 (0x70)

0 (0x0) - 96 (0x60)

DBCC execution completed. If DBCC printed error messages, contact your system administrator.

注意，这个行就是index record，和indid=2完全吻合。

参考：http://blogs.msdn.com/b/apgcdsd/archive/2014/08/20/key-lock.aspx