Question

io_stall_read_ms的值似乎高于查询的实际运行时间。设置：

create database filestats;

use filestats;

create table t1_primary(
 a  int     identity
,b  int
,c  datetime
,d  char(780)
,constraint pk_p_a primary key clustered (a));

set nocount on;
declare @n int = 0;

while @n < 12800
begin
    insert into t1_primary select @n, CURRENT_TIMESTAMP, ABS(CHECKSUM(NEWID()))
select @n += 1
end

set statistics io, time on;

declare @before datetime 

dbcc dropcleanbuffers
checkpoint

if object_id('tempdb..#vfsBefore') is not null
    drop table #vfsBefore

if object_id('tempdb..#vfsAfter') is not null
    drop table #vfsAfter

select * 
into #vfsBefore
from sys.dm_io_virtual_file_stats(db_id('filestats'), null)

select @before = CURRENT_TIMESTAMP

select * from dbo.t1_primary 

select DATEDIFF(ms, @before, CURRENT_TIMESTAMP)

select * 
into #vfsAfter
from sys.dm_io_virtual_file_stats(db_id('filestats'), null)

select 
 db_name(b.database_id)
,mf.name
,a.num_of_reads-b.num_of_reads              as NumReads
,a.num_of_bytes_read-b.num_of_bytes_read    as NumBytesRead
,a.io_stall_read_ms-b.io_stall_read_ms      as IOStallReadMS
,a.num_of_writes-b.num_of_writes            as NumWrites
,a.num_of_bytes_written-b.num_of_bytes_written  as NumBytesWritten
,a.io_stall_write_ms-b.io_stall_write_ms    as IOStallWriteMS
,a.io_stall-b.io_stall                      as IOStallTotal
,a.size_on_disk_bytes-b.size_on_disk_bytes  as SizeOnDiskBytes
,mf.physical_name
from #vfsBefore b
join #vfsAfter a on b.database_id = a.database_id and b.file_id =     a.file_id
join sys.master_files mf on b.database_id = mf.database_id and b.file_id = mf.file_id

运行此结果会产生以下结果：

虚拟文件统计信息： num_of_reads = 26，io_stall_read_ms = 2153

统计IO：表't1_primary'。扫描计数1，逻辑读取2571，物理读取3，预读读取2552

统计时间： CPU时间= 46毫秒，已用时间= 595毫秒。

运行时间，DATEDIFF函数： 593 ms

毫不奇怪，执行聚簇索引扫描，单线程。为什么io_stall_read_ms值高于执行查询的时间。这是怎么回事？

可能是2153（io_stall_ms）/ 26（num_reads）= ~83ms每次读取操作停止吗？

此致

戈德

来自sys.dm_io_virtual_filestats的奇怪IO值

0 个答案: