《Troubleshooting SQL Server》读书笔记-性能故障诊断方法论

第一章 A Performance Troubleshooting Methodology。

     确定具体的方法论用于SQL Server故障诊断是件很难的事情。因为根据问题和环境的不同，解决方法也不一样。一个准确的故障诊断，不仅是收集各种信息，而且要明白它们的含义。千万不要只见树木不见森林。

作者通过一个简单的诊断示例，演示基本的方法和借助的工具：

1. 首先查看DMV sys.dm_os_wait_stats来确定操作系统层面的主要的资源等待。

2. 假设发现主要等待类型为PAGEIOLATCH_SH。PAGEIOLATCH_SH发生的原因是磁盘子系统的速度不能满足DB对页的请求速度。

     看起来是IO瓶颈。但是我们要考虑周全，也有可能是其它的问题导致的过度IO。而这是表现不是根本原因。

3.  然后通过sys.dm_io_virtual_file_stats得到各个数据库和数据文件的IO量和IO延时。同时查看性能计数器Physical Disk\Avg.  Disk Reads/sec和Physical Disk\Avg.  Disk Writes/sec。

     这样就能把问题定位到具体数据库。

4.  针对上面确定的数据库，通过sys.dm_exec_query_stats查看是哪些查询造成了大量的IO.找出这些语句调整或者采取其它的调优方式。

5.  同时通过观察性能计数器Page Life Expectancy，Free List Stalls/sec和high  Lazy Writes/sec,如果PLE值很低，而后两个计数器常不为0，则可认为是内存瓶颈。

在这个故障诊断的案例中有许多要关注的点，一不小心就会被带坑里去，采用了错误的调优方式。比如，从一始我们就认定是IO瓶颈，于是提高IO子系统的速度。虽然短时会改善性能，但是过段时间这个问题又会出现。

所以千万不要只见树木不见森林。

 

等待统计(Wait Statistics: the Basis for Troubleshooting):

     在故障诊断的第一个检查的项一般会是等待统计。SQL Server有一个伪操作系统SQLOS，管理着内部的各种操作和资源。而SQLOS中的操作和资源的各种等待，可以通过sys.dm_os_wait_stats查询得到。

     因为很多时候对问题现状的描述不是很清楚，比如“数据库好慢”，所以我们搞明白SQL Server在待什么和为什么等待。

     下面的查询已经过滤掉非问题型等待：

SELECT TOP  10  
        wait_type , 
        max_wait_time_ms wait_time_ms , 
        signal_wait_time_ms , 
        wait_time_ms - signal_wait_time_ms AS resource_wait_time_ms , 
        100.0 * wait_time_ms / SUM (wait_time_ms ) OVER  ( ) 
                                    AS percent_total_waits , 
        100.0 * signal_wait_time_ms / SUM (signal_wait_time_ms) OVER  ( ) 
                                    AS percent_total_signal_waits , 
        100.0 * ( wait_time_ms - signal_wait_time_ms ) 
         / SUM (wait_time_ms ) OVER  ( ) AS percent_total_resource_waits  
FROM    sys .dm_os_wait_stats 
WHERE    wait_time_ms > 0  -- remove zero wait_time  
         AND  wait_type NOT  IN -- filter out additional irrelevant waits  
( 'SLEEP_TASK' , 'BROKER_TASK_STOP', 'BROKER_TO_FLUSH' , 
  'SQLTRACE_BUFFER_FLUSH','CLR_AUTO_EVENT', 'CLR_MANUAL_EVENT',  
  'LAZYWRITER_SLEEP', 'SLEEP_SYSTEMTASK', 'SLEEP_BPOOL_FLUSH', 
  'BROKER_EVENTHANDLER', 'XE_DISPATCHER_WAIT' , 'FT_IFTSHC_MUTEX' , 
  'CHECKPOINT_QUEUE', 'FT_IFTS_SCHEDULER_IDLE_WAIT' ,  
  'BROKER_TRANSMITTER' , 'FT_IFTSHC_MUTEX' , 'KSOURCE_WAKEUP', 
  'LAZYWRITER_SLEEP', 'LOGMGR_QUEUE', 'ONDEMAND_TASK_QUEUE', 
  'REQUEST_FOR_DEADLOCK_SEARCH' , 'XE_TIMER_EVENT', 'BAD_PAGE_PROCESS', 
  'DBMIRROR_EVENTS_QUEUE', 'BROKER_RECEIVE_WAITFOR' , 
  'PREEMPTIVE_OS_GETPROCADDRESS', 'PREEMPTIVE_OS_AUTHENTICATIONOPS', 
  'WAITFOR', 'DISPATCHER_QUEUE_SEMAPHORE', 'XE_DISPATCHER_JOIN' , 
  'RESOURCE_QUEUE' ) 
ORDER  BY wait_time_ms DESC

一些常见的等待类型有：

  CXPACKET:并行执行等待

  SOS_SCHEDULER_YIELD：每个worker在占用scheduler一会儿然后退让(Yield)，让其它的worker使用scheduler。此等待通常说明有CPU压力。

  THREADPOOL:每个task需要绑定到一个worker执行。此等待说明worker用尽，有CPU压力，或者阻塞导致大量的task长时间占用woker。

  LCK_*：锁等待。

  PAGEIOLATCH_*,  IO_COMPLETION,  WRITELOG：IO瓶颈的表现。

  PAGELATCH_*：页争用等待。此等待最出名代表就是tempdb的系统页争用问题。

  LATCH_*:是一种轻量短暂同步对象，用于保护访问的内部对象。根据等待类型不同，代表着不同的问题。sys.dm_os_latch_stats

  ASYNC_NETWORK_IO：通常是客户端处理数据性能不够快。

根据不同等待类型确定，来确定我们下一步诊断方向。如果PAGEIOLATCH_*等待过高，接下来就会检查虚拟文件状态，如果LCK_*等待过高，就会检查数据库中的阻塞状况，而不会浪费时间去检查IO配置。

在解决完一个问题后需要清空等待信息，然后等过段时间DMV中的数据积累到一定程度，再来确定问题是否还存在。如下语句清空：

DBCC SQLPERF ('sys.dm_os_wait_stats' , clear )

虚拟文件统计(Virtual File Statistics):

     只看等待统计，很多问题都像是IO瓶颈。所以我们要检查虚拟文件统计。

SELECT  DB_NAME(vfs .database_id) AS database_name  , 
        vfs.database_id  , 
        vfs.FILE_ID , 
        io_stall_read_ms / NULLIF(num_of_reads , 0) AS avg_read_latency , 
        io_stall_write_ms / NULLIF(num_of_writes, 0) 
                                               AS avg_write_latency , 
        io_stall  / NULLIF(num_of_reads + num_of_writes, 0) 
                                               AS avg_total_latency , 
        num_of_bytes_read / NULLIF(num_of_reads , 0) 
                                               AS avg_bytes_per_read  , 
        num_of_bytes_written  / NULLIF(num_of_writes, 0) 
                                               AS avg_bytes_per_write , 
        vfs.io_stall , 
        vfs.num_of_reads , 
        vfs.num_of_bytes_read , 
        vfs.io_stall_read_ms  , 
        vfs.num_of_writes , 
        vfs.num_of_bytes_written , 
        vfs.io_stall_write_ms , 
        size_on_disk_bytes  / 1024 / 1024. AS size_on_disk_mbytes , 
        physical_name 
FROM    sys .dm_io_virtual_file_stats (NULL, NULL)  AS vfs 
         JOIN sys .master_files  AS mf ON vfs.database_id  = mf .database_id 
                                        AND  vfs.FILE_ID = mf .FILE_ID 
ORDER  BY avg_total_latency DESC

发现有高IO等待意味着有IO瓶颈，但是其根本原因可能是磁盘了系统有瓶颈，也可能是索引过多、索引缺失、内存瓶颈和低效的查询等。还要借助其它方式进一步确定根本原因。千万不要只见树木不见森林。

 

性能计数器(Performance Counters)

   在诊断初期，会先收集针对SQL Server的性能计数器，随着深入再收集Windows的计数器。这些数据有助于明白当前的性能状况和界定问题的边界。

推荐的分析工具PAL。可以使用PerfMon收集，也可以通过 sys.dm_os_performance_counters查看。

使用DMV时，要注意计数器数据是增量的，所以要收集两个时间点上的数据副本进行对比。

DECLARE @CounterPrefix NVARCHAR (30) 
SET  @CounterPrefix = CASE WHEN @@SERVICENAME = 'MSSQLSERVER' 
                             THEN 'SQLServer:'  
                          ELSE 'MSSQL$'  + @@SERVICENAME + ':'  
                     END  ; 
-- Capture the first counter set  
SELECT  CAST(1 AS INT ) AS collection_instance , 
        [OBJECT_NAME]  , 
        counter_name , 
        instance_name  , 
        cntr_value , 
        cntr_type , 
         CURRENT_TIMESTAMP  AS collection_time 
INTO    #perf_counters_init 
FROM    sys .dm_os_performance_counters 
WHERE    ( OBJECT_NAME = @CounterPrefix + 'Access Methods' 
           AND  counter_name = 'Full Scans/sec' 
         ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'Access Methods' 
             AND  counter_name = 'Index Searches/sec'  
           ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'Buffer Manager' 
             AND  counter_name = 'Lazy Writes/sec'  
           ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'Buffer Manager' 
             AND  counter_name = 'Page life expectancy'  
           ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'General Statistics'  
             AND  counter_name = 'Processes Blocked' 
           ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'General Statistics'  
             AND  counter_name = 'User Connections' 
           ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'Locks' 
             AND  counter_name = 'Lock Waits/sec' 
           ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'Locks' 
             AND  counter_name = 'Lock Wait Time (ms)' 
           ) 

         OR ( OBJECT_NAME = @CounterPrefix + 'SQL Statistics' 
             AND  counter_name = 'SQL Re-Compilations/sec' 
           ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'Memory Manager' 
             AND  counter_name = 'Memory Grants Pending' 
           ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'SQL Statistics' 
             AND  counter_name = 'Batch Requests/sec'  
           ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'SQL Statistics' 
             AND  counter_name = 'SQL Compilations/sec'  
           ) 
 
-- Wait on Second between data collection  
WAITFOR DELAY  '00:00:01'  
 
-- Capture the second counter set 
SELECT  CAST(2 AS INT ) AS collection_instance , 
         OBJECT_NAME , 
        counter_name , 
        instance_name  , 
        cntr_value , 
        cntr_type , 
         CURRENT_TIMESTAMP  AS collection_time 
INTO    #perf_counters_second 
FROM    sys .dm_os_performance_counters 
WHERE    ( OBJECT_NAME = @CounterPrefix + 'Access Methods' 
           AND  counter_name = 'Full Scans/sec' 
         ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'Access Methods' 
             AND  counter_name = 'Index Searches/sec'  
           ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'Buffer Manager' 
             AND  counter_name = 'Lazy Writes/sec'  
           ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'Buffer Manager' 
             AND  counter_name = 'Page life expectancy'  
           ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'General Statistics'  
             AND  counter_name = 'Processes Blocked' 
           ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'General Statistics'  
             AND  counter_name = 'User Connections' 
           ) 

         OR ( OBJECT_NAME = @CounterPrefix + 'Locks' 
             AND  counter_name = 'Lock Waits/sec' 
           ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'Locks' 
             AND  counter_name = 'Lock Wait Time (ms)' 
           ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'SQL Statistics' 
             AND  counter_name = 'SQL Re-Compilations/sec' 
           ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'Memory Manager' 
             AND  counter_name = 'Memory Grants Pending' 
           ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'SQL Statistics' 
             AND  counter_name = 'Batch Requests/sec'  
           ) 
         OR ( OBJECT_NAME = @CounterPrefix + 'SQL Statistics' 
             AND  counter_name = 'SQL Compilations/sec'  
           ) 
 
-- Calculate the cumulative counter values 
SELECT  i .OBJECT_NAME , 
        i.counter_name , 
        i.instance_name , 
         CASE WHEN i.cntr_type  = 272696576  
                THEN s.cntr_value - i.cntr_value  
             WHEN i.cntr_type  = 65792 THEN s.cntr_value  
         END  AS cntr_value 
FROM    #perf_counters_init  AS i 
         JOIN #perf_counters_second AS s 
              ON i.collection_instance  + 1  = s.collection_instance 
                  AND  i.OBJECT_NAME = s.OBJECT_NAME 
                  AND  i.counter_name = s.counter_name  
                  AND  i.instance_name = s.instance_name 
ORDER  BY OBJECT_NAME 
 
-- Cleanup tables  
DROP TABLE  #perf_counters_init  
DROP TABLE  #perf_counters_second 

收集的计数器包括：

•     SQLServer:Access Methods\Full Scans/sec
•     SQLServer:Access Methods\Index Searches/sec
•     SQLServer:Buffer Manager\Lazy Writes/sec
•     SQLServer:Buffer Manager\Page life expectancy
•     SQLServer:Buffer Manager\Free list stalls/sec
•     SQLServer:General Statistics\Processes Blocked
•     SQLServer:General Statistics\User Connections
•     SQLServer:Locks\Lock Waits/sec
•     SQLServer:Locks\Lock Wait Time (ms)
•     SQLServer:Memory Manager\Memory Grants Pending
•     SQLServer:SQL Statistics\Batch Requests/sec
•     SQLServer:SQL Statistics\SQL Compilations/sec
•     SQLServer:SQL Statistics\SQL Re-Compilations/sec

Index Searches比Full Scans高个800~1000比较理想。对于大于4Gb RAM的机器，Page life expectancy=(RAM/4)*300.

 

计划高速缓存的使用(Plan Cache Usage)

   在SQL Server2005&2008中可以使用sys.dm_exec_query_stats查询得到各种存于计划高速缓存中的执行信息。结合其它的DMV&DMF可以得到各种TOP(N)信息。如：top 10 Physical Reads

SELECT TOP  10  
        execution_count  , 
        statement_start_offset  AS stmt_start_offset , 
         sql_handle  , 
        plan_handle  , 
        total_logical_reads / execution_count  AS avg_logical_reads , 
        total_logical_writes  / execution_count  AS avg_logical_writes  , 
        total_physical_reads  / execution_count  AS avg_physical_reads  , 
        t.text 
FROM    sys .dm_exec_query_stats AS s 
         CROSS  APPLY  sys .dm_exec_sql_text(s.sql_handle ) AS t 
ORDER  BY avg_physical_reads  DESC

总结

   性能诊断最忌片面，千万不要只见树林不见森林。这不仅是一门手艺，更是一门艺术。