Question

我每天都有一个运行dbcc checkdb语句的作业，而不需要在每个系统数据库上使用其他参数。这项工作在非高峰时段运行，通常需要5秒或更短时间才能运行。

然而，最后一次运行仅用了1秒钟，因为死锁而失败。我有一个警告，可以为我保存一个死锁的xml图表，我将其中包含更多详细信息。

我的主要问题是：为什么会发生这样的僵局并且可以避免？

    <TextData>
      <deadlock-list>
     <deadlock victim="process290fd861088">
      <process-list>
       <process id="process290fd861088" taskpriority="0" logused="0" waitresource="OBJECT: 2:5:0 " ownerId="1250115008" transactionname="CheckDb" lasttranstarted="2017-03-20T01:00:01.427" XDES="0x2b277040bd8" lockMode="S" schedulerid="7" kpid="12760" status="suspended" spid="78" sbid="0" ecid="0" priority="0" trancount="1" lastbatchstarted="2017-03-20T01:00:00.060" lastbatchcompleted="2017-03-20T01:00:00.060" lastattention="1900-01-01T00:00:00.060" clientapp="SQLAgent - TSQL JobStep (Job 0xB425122DD6C28D4BBE42D7F0AF76FC40 : Step 1)" hostname="0000-DB-0000" hostpid="8040" loginname="0000\0000" isolationlevel="read committed (2)" xactid="1250115008" currentdb="2" lockTimeout="4294967295" clientoption1="673185824" clientoption2="128056">
        <executionStack>
         <frame procname="0000_Local.server.CheckSystemDatabases" line="19" stmtstart="740" stmtend="776" sqlhandle="0x030006006a934a11b3ebcd0023a7000001000000000000000000000000000000000000000000000000000000">
    dbcc checkdb(@dbId     </frame>
         <frame procname="adhoc" line="1" stmtend="70" sqlhandle="0x010006006688101b405fcfceb602000000000000000000000000000000000000000000000000000000000000">
    exec [server].[CheckSystemDatabases     </frame>
        </executionStack>
        <inputbuf>
    exec [server].[CheckSystemDatabases];    </inputbuf>
       </process>
       <process id="process2b59a715468" taskpriority="0" logused="952" waitresource="OBJECT: 2:3:0 " ownerId="1250114957" transactionname="droptemp" lasttranstarted="2017-03-20T01:00:01.423" XDES="0x29b8755ce58" lockMode="IX" schedulerid="8" kpid="9440" status="suspended" spid="67" sbid="0" ecid="0" priority="0" trancount="0" lastbatchstarted="2017-03-20T01:00:01.410" lastbatchcompleted="2017-03-20T01:00:01.410" lastattention="1900-01-01T00:00:00.410" clientapp="0000-API-0000" hostname="0000-0000-WEB-0000" hostpid="42180" loginname="0000\0000" isolationlevel="read committed (2)" xactid="0" currentdb="9" lockTimeout="4294967295" clientoption1="673185824" clientoption2="128056">
        <executionStack>
         <frame procname="mssqlsystemresource.sys.sp_executesql" line="1" stmtstart="-1" sqlhandle="0x0400ff7f427f99d9010000000000000000000000000000000000000000000000000000000000000000000000">
    sp_executesql     </frame>
         <frame procname="0000.dbo.SomeProcName" line="93" stmtstart="8320" stmtend="8496" sqlhandle="0x030009002ac137082fa8b20029a7000001000000000000000000000000000000000000000000000000000000">
    exec sp_executesql @selectSql, N'@rowcount int output', @rowcount = @TotalRowCount outpu     </frame>
        </executionStack>
        <inputbuf>
    Proc [Database Id = 9 Object Id = 137871658]    </inputbuf>
       </process>
      </process-list>
      <resource-list>
       <objectlock lockPartition="0" objid="5" subresource="FULL" dbid="2" objectname="tempdb.sys.sysrowsets" id="lock2b4103b8380" mode="IX" associatedObjectId="5">
        <owner-list>
         <owner id="process2b59a715468" mode="IX" />
        </owner-list>
        <waiter-list>
         <waiter id="process290fd861088" mode="S" requestType="wait" />
        </waiter-list>
       </objectlock>
       <objectlock lockPartition="0" objid="3" subresource="FULL" dbid="2" objectname="tempdb.sys.sysrscols" id="lock291f3d8a900" mode="S" associatedObjectId="3">
        <owner-list>
         <owner id="process290fd861088" mode="S" />
        </owner-list>
        <waiter-list>
         <waiter id="process2b59a715468" mode="IX" requestType="wait" />
        </waiter-list>
       </objectlock>
      </resource-list>
     </deadlock>
    </deadlock-list></TextData>

解决方案我已经实现了通过每日checkDB for tempDB来优化用户事务：

set nocount on;  
set deadlock_priority low;  
 declare @dbId int;  
declare loopCheckDB cursor fast_forward   
for select [d].[database_id] from [sys].[databases] as [d] where   [d].[database_id] < 5  
 order by [d].[name]  
 open [loopCheckDB]   
fetch next from [loopCheckDB] into @dbId;   
while @@FETCH_STATUS = 0  
 begin dbcc checkdb(@dbId); 
fetch next from [loopCheckDB] into @dbId; 
end 
close [loopCheckDB];
 deallocate [loopCheckDB];

Answer 1

为什么系统数据库上的dbcc checkdb和用户数据库上的sp_executesql导致死锁？

DBCC checkDB在tempdb.sys.sysrowsets上获取了Intent Exclusive锁，正在等待tempdb.sys.sysrscols上的共享锁..

你的用户proc，也正在访问TEMPDB资源。这个用户proc在tempdb.sys.sysrscols上获得了IX锁，正在等待tempdb.sys.sysrowsets上的共享锁..

因此发生了死锁，这是一个简单的死锁案例

通常，DBCC CheckDB会在分析之前获取数据库的快照，它可以在此快照上运行以避免锁定，阻塞..

在这种情况下，as per this post .. TEMPDB无法实现快照，因此在您的情况下，两个事务都会获得不兼容的锁，这就是您看到死锁的原因。

为什么系统数据库上的dbcc checkdb和用户数据库上的sp_executesql导致死锁？

1 个答案: