sql server 会记录IO等待时间超过15 seconds的请求，这时application会有 time out 现象，dba需要判断是workload,concurrecy 所致还是sql server配置没有最优导致？二者之间的比例各占多少？
1：判断sql server的配置是否最优化，相对容易，可以有以下项目检查：
a: 通过datafile/logfile放置在不同的drive上，可以分离random io 和 sequence io 操作，可以极大的缓解读写IO，对于写IO，只需立即做logfile 的 sequence io即可，至于何时把data page 真正写入到datafile,
sql server自己把握。根据我自己的观察，分离datafile/logfile，一个月有个2到3次的 IO stall, 如果都在同一个磁盘，有个8到9次 IO stall 也很常见。
b: 尽量合理的设置[max server memory (MB)], 一般为总内存的7/8即可。
c: tempdb最好在单独的磁盘上，如果不行的话，也要做datafile/logfile分离。
2：如果出现了问题，那如何判断是workload导致的呢？第一要做好IO activity的统计工作，第二要通知开发，是否是业务的变化导致IO增多，困难是这个IO stall 到底涉及到哪些表？如何得到？
a: 通过以下的sp, 利用sys.dm_io_virtual_file_stats DMV 记录每一分钟sql server IO读写操作
USE [master] GO SET ANSI_NULLS ON GO SET QUOTED_IDENTIFIER ON GO create proc [dbo].[usp_monitor_virtual_file_latency] as begin if OBJECT_ID('master.dbo.tbl_virtual_file_latency','U') is null begin create table master.dbo.tbl_virtual_file_latency ( id int identity(1,1) primary key, collect_dt datetime not null, [database_name] [nvarchar](128) NULL, [file_id] [smallint] NOT NULL, [type_desc] [nvarchar](60) NULL, [sample_ms] [int] NOT NULL, [num_of_reads] [bigint] NOT NULL, [num_of_bytes_read] [bigint] NOT NULL, [io_stall_read_ms] [bigint] NOT NULL, [num_of_writes] [bigint] NOT NULL, [num_of_bytes_written] [bigint] NOT NULL, [io_stall_write_ms] [bigint] NOT NULL, [io_stall] [bigint] NOT NULL, [size_on_disk_bytes] [bigint] NOT NULL, [file_handle] [varbinary](8) NOT NULL, [physical_name] [nvarchar](260) NOT NULL, [state_desc] [nvarchar](60) NULL ) create index idx_collect_dt on tbl_virtual_file_latency(collect_dt) with(online=on) end insert into master.dbo.tbl_virtual_file_latency SELECT --virtual file latency GETDATE(), db_name(vfs.[database_id]) as database_name ,vfs.[file_id] ,mf.[type_desc] ,vfs.[sample_ms] ,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] ,vfs.[io_stall] ,vfs.[size_on_disk_bytes] ,vfs.[file_handle] ,mf.[physical_name] ,mf.[state_desc] FROM sys.dm_io_virtual_file_stats (NULL,NULL) AS [vfs] JOIN sys.master_files(nolock) AS [mf] ON [vfs].[database_id] = [mf].[database_id] AND [vfs].[file_id] = [mf].[file_id] WHERE DB_NAME(vfs.database_id) not in('master','model','msdb') --delete old data delete from master.dbo.tbl_virtual_file_latency where collect_dt <DATEADD(MONTH,-2,getdate()) end
b: 通过以下的sp,求每分钟内每个数据文件的IO的size和读写情况
USE [master] GO SET ANSI_NULLS ON GO SET QUOTED_IDENTIFIER ON GO create proc [dbo].[usp_calculate_virtual_file_stats_PerMinutes] @physical_name nvarchar(1000), @collect_starttime datetime, @collect_endtime datetime as with snap as ( select row_number() over (order by id asc) as row_num, id ,[collect_dt] ,[database_name] ,[file_id] ,[type_desc] ,[sample_ms] ,[num_of_reads] ,[num_of_bytes_read] ,[io_stall_read_ms] ,[num_of_writes] ,[num_of_bytes_written] ,[io_stall_write_ms] ,[io_stall] from master.dbo.tbl_virtual_file_latency(nolock) AS [vfs1] WHERE vfs1.physical_name=@physical_name and vfs1.collect_dt>=@collect_starttime and vfs1.collect_dt<=@collect_endtime ) select cur.collect_dt, read_io_num= case when (cur.num_of_reads-pre.num_of_reads)=0 then 0 else cur.num_of_reads-pre.num_of_reads end , write_io_num= case when (cur.num_of_writes-pre.num_of_writes)=0 then 0 else cur.num_of_writes-pre.num_of_writes end , read_latency_MS =case when (cur.num_of_reads-pre.num_of_reads)=0 then 0 else cast((cur.io_stall_read_ms-pre.io_stall_read_ms)*1./(cur.num_of_reads-pre.num_of_reads) as decimal(10,1)) end , write_latency_MS= case when (cur.num_of_writes-pre.num_of_writes)=0 then 0 else cast((cur.io_stall_write_ms-pre.io_stall_write_ms)*1./(cur.num_of_writes-pre.num_of_writes) as decimal(10,1)) end , average_latency_MS=case when (cur.num_of_reads-pre.num_of_reads+cur.num_of_writes-pre.num_of_writes)=0 then 0 else cast((cur.io_stall-pre.io_stall)*1./(cur.num_of_reads-pre.num_of_reads+cur.num_of_writes-pre.num_of_writes) as decimal(10,1)) end, averageKB_read=case when (cur.num_of_reads-pre.num_of_reads)=0 then 0 else cast((cur.num_of_bytes_read-pre.num_of_bytes_read)*1./(cur.num_of_reads-pre.num_of_reads)/1024 as decimal(10,1))end , averageKB_write=case when (cur.num_of_writes-pre.num_of_writes)=0 then 0 else cast((cur.num_of_bytes_written-pre.num_of_bytes_written)*1./(cur.num_of_writes-pre.num_of_writes)/1024 as decimal(10,1)) end, averageKB_Transfer=case when (cur.num_of_reads-pre.num_of_reads+cur.num_of_writes-pre.num_of_writes)=0 then 0 else cast((cur.num_of_bytes_read-pre.num_of_bytes_read+cur.num_of_bytes_written-pre.num_of_bytes_written)*1./(cur.num_of_reads-pre.num_of_reads+cur.num_of_writes-pre.num_of_writes)/1024 as decimal(10,1)) end from snap as pre join snap as cur on pre.row_num=cur.row_num-1
c: 最后根据历史数据来看IO读写的趋势
exec usp_calculate_virtual_file_stats_PerMinutes 'E:TestDB est.mdf', '2014-12-14 13:49:01.000', '2014-12-14 15:49:01.000'
3：根据IO stall的信息找出sql server 读取哪个表导致了IO Stall.
sql server log text:
SQL Server has encountered 727 occurrence(s) of I/O requests taking longer than 15 seconds to complete on file [F:MSSQLTest_1.ndf:MSSQL_DBCC7] in database [Test] (7). The OS file handle is 0x0000000000001A40. The offset of the latest long I/O is: 0x000023101f0000
a: 把偏移量0x000023101f0000转化为十进制为：150594322432
b: 把该值除以8192即为page number: 150594322432/8192=18383096
c:在通过page number求的table name, 再与开发讨论IO的问题.
select object_name(object_id) as tblname,* from sys.partitions (nolock)as sp where sp.partition_id in ( select au.container_id from sys.dm_os_buffer_descriptors(nolock) as obd join sys.allocation_units(nolock) as au on obd.allocation_unit_id=au.allocation_unit_id where obd.page_id=18383096)
d: 如果该page number不再内存中的化，该SQL 是没有结果的。可以使用dbcc page(dbid,fileid,pageid,3)来得到该表的object_id.
结论：IO stall 只可能缓解，是没法避免的，业务量导致的IO永远在某一点上是超过存储的容量的，dba能通过各种事前事后方法到找到root cause, 据此提出解决的方法。希望以上的方法能对大家有所启发。