db file sequential read等待事件有三个参数，属于User I/O类的等待：
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SQL> select name,parameter1,parameter2,parameter3,wait_class from v$event_name where name = 'db file sequential read';
NAME PARAMETER1 PARAMETER2 PARAMETER3 WAIT_CLASS
------------------------------ ---------- ---------- ---------- ----------
db file sequential read file# block# blocks User I/O
file#：要读取的数据块锁在数据文件的文件号。
block#：要读取的起始数据块号。
blocks：要读取的数据块数目（这里应该等于1）。
当进程需要访问一个在SGA中不存在的block时，进程会等待Oracle将此block从disk读取到SGA中。在这个过程中发生的等待事件即db file sequential read，过程如图：
db file sequential read等待事件是发生的常见原因：
1.索引的访问及回表
2.访问undo segment或者回滚操作
3.直接以ROWID方式访问表中的数据
在此，我们来分别对3种情况进行验证。
首先，创建测试表及索引：
SQL> create table scott.tb_test(id int,age int,name varchar2(20));
Table created.
SQL> create index scott.idx_id on scott.tb_test(id);
Index created.
插入测试数据：
SQL> begin
2 for i in 1..1000 loop
3 insert into scott.tb_test values(i,i+1,'test');
4 end loop;
5 commit;
6 end;
7 /
PL/SQL procedure successfully completed.
在另外的session中对当前session进行10046追踪：
SQL> oradebug setospid 3836
Oracle pid: 22, Unix process pid: 3836, image: oracle@zhuga (TNS V1-V3)
SQL> oradebug event 10046 trace name context forever,level 12
Statement processed.
SQL> oradebug tracefile_name
/opt/app/oracle/diag/rdbms/bddev2/BDDEV2/trace/BDDEV2_ora_3836.trc
持续观察追踪文件的输出
1.索引的访问
查看当前session的累积等待信息：
SQL> SELECT INST_ID
2 ,SID
3 ,EVENT
4 ,TOTAL_WAITS WAITS_CNT
5 ,TIME_WAITED
6 ,AVERAGE_WAIT/100 "AVERAGE_WAIT(S)"
7 ,MAX_WAIT/100 "MAX_WAIT(S)"
8 FROM GV$SESSION_EVENT
9 WHERE SID=&INPUT_SID
10 ORDER BY TOTAL_WAITS DESC;
Enter value for input_sid: 812
old 9: WHERE SID=&INPUT_SID
new 9: WHERE SID=812
INST_ID SID EVENT WAITS_CNT TIME_WAITED AVERAGE_WAIT(S) MAX_WAIT(S)
---------- ---------- ------------------------------ ---------- ----------- --------------- -----------
1 812
db file sequential read 207
15 .0007 .01
1 812 SQL*Net message to client 29 0 0 0
1 812 SQL*Net message from client 28 215952 77.1256 680.52
1 812 Disk file operations I/O 6 0 .0001 0
1 812 log file sync 5 0 .0005 0
1 812 events in waitclass Other 3 5 .0156 .02
1 812 db file scattered read 1 0 0 0
当前session中执行如下语句：
SQL> alter system flush buffer_cache;
System altered.
SQL> select /*+ index(tb_test,idx_id)*/ age from scott.tb_test where id = 500;
追踪文件内容：
PARSING IN CURSOR #140128573892064 len=72 dep=0 uid=0 oct=3 lid=0 tim=1541577174155088 hv=968314915 ad='55a3880b0' sqlid='d53f2unwvfn13'
select /*+ index(tb_test,idx_id)*/ age from scott.tb_test where id = 500
END OF STMT
PARSE #140128573892064:c=94,e=93,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=1923716061,tim=1541577174155086
EXEC #140128573892064:c=42,e=42,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=1923716061,tim=1541577174155379
WAIT #140128573892064: nam='SQL*Net message to client' ela= 2 driver id=1650815232 #bytes=1 p3=0 obj#=91021 tim=1541577174155417
WAIT #140128573892064: nam='db file sequential read' ela= 11 file#=5 block#=147 blocks=1 obj#=91022 tim=1541577174155531
WAIT #140128573892064: nam='db file sequential read' ela= 11 file#=5 block#=150 blocks=1 obj#=91022 tim=1541577174155602
WAIT #140128573892064: nam='db file sequential read' ela= 11 file#=5 block#=134 blocks=1 obj#=91021 tim=1541577174155659
FETCH #140128573892064:c=227,e=239,p=3,cr=3,cu=0,mis=0,r=1,dep=0,og=1,plh=1923716061,tim=1541577174155687
WAIT #140128573892064: nam='SQL*Net message from client' ela= 154 driver id=1650815232 #bytes=1 p3=0 obj#=91021 tim=1541577174155878
FETCH #140128573892064:c=101,e=100,p=0,cr=1,cu=0,mis=0,r=0,dep=0,og=1,plh=1923716061,tim=1541577174156014
STAT #140128573892064 id=1 cnt=1 pid=0 pos=1 obj=91021 op='TABLE ACCESS BY INDEX ROWID TB_TEST (cr=4 pr=3 pw=0 time=217 us cost=1 size=26 card=1)'
STAT #140128573892064 id=2 cnt=1 pid=1 pos=1 obj=91022 op='INDEX RANGE SCAN IDX_ID (cr=3 pr=2 pw=0 time=256 us cost=1 size=0 card=1)'
WAIT #140128573892064: nam='SQL*Net message to client' ela= 1 driver id=1650815232 #bytes=1 p3=0 obj#=91021 tim=1541577174156117
SQL> select owner,object_name,object_type from dba_objects where object_id = 91022;
OWNER OBJECT_NAME OBJECT_TYPE
-------------------- -------------------- -------------------
SCOTT IDX_ID INDEX
或者
SQL> select owner,segment_name,segment_type from dba_extents where file_id = 5 and 134 between block_id and block_id+blocks-1;
OWNER SEGMENT_NAME SEGMENT_TYPE
-------------------- -------------- ------------------
SCOTT TB_TEST TABLE
可看到发生了
三次db file sequential read等待，其中两次是访问索引(一次访问索引头块，另一次是访问真正数据块)，一次是回表
此时查看当前session的累积等待信息：
INST_ID SID EVENT WAITS_CNT TIME_WAITED AVERAGE_WAIT(S) MAX_WAIT(S)
---------- ---------- -------------------------------- ---------- ----------- --------------- -----------
1 812
db file sequential read 210
15 .0007 .01
1 812 SQL*Net message to client 32 0 0 0
1 812 SQL*Net message from client 31 233915 75.4564 680.52
1 812 Disk file operations I/O 6 0 .0001 0
1 812 log file sync 5 0 .0005 0
1 812 events in waitclass Other 4 6 .0152 .02
1 812 db file scattered read 1 0 0 0
可看到信息是一致的。
2.访问undo或回滚
在另外的session中修改scott.tb_test数据，不提交：
SQL> update scott.tb_test set age = 100 where id = 500;
1 row updated.
在当前session执行：
SQL> alter system flush buffer_cache;
System altered.
SQL> select age from scott.tb_test where id = 500;
AGE
----------
501
PARSING IN CURSOR #140128575016040 len=44 dep=0 uid=0 oct=3 lid=0 tim=1541577933156059 hv=2526155070 ad='55a80a9a0' sqlid='0x9dgqqb9449y'
select age from scott.tb_test where id = 500
END OF STMT
PARSE #140128575016040:c=0,e=125,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=1923716061,tim=1541577933156058
EXEC #140128575016040:c=0,e=49,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=1923716061,tim=1541577933156190
WAIT #140128575016040: nam='SQL*Net message to client' ela= 3 driver id=1650815232 #bytes=1 p3=0 obj#=91021 tim=1541577933156224
WAIT #140128575016040: nam='db file sequential read' ela= 18 file#=5 block#=147 blocks=1 obj#=91022 tim=1541577933156326
WAIT #140128575016040: nam='db file sequential read' ela= 12 file#=5 block#=150 blocks=1 obj#=91022 tim=1541577933156396
WAIT #140128575016040: nam='db file sequential read' ela= 11 file#=5 block#=134 blocks=1 obj#=91021 tim=1541577933156456
WAIT #140128575016040: nam='db file sequential read' ela= 12 file#=3 block#=176 blocks=1 obj#=0 tim=1541577933156512
WAIT #140128575016040: nam='db file sequential read' ela= 12 file#=3 block#=32616 blocks=1 obj#=0 tim=1541577933156594
FETCH #140128575016040:c=140,e=410,p=5,cr=5,cu=0,mis=0,r=1,dep=0,og=1,plh=1923716061,tim=1541577933156658
WAIT #140128575016040: nam='SQL*Net message from client' ela= 164 driver id=1650815232 #bytes=1 p3=0 obj#=0 tim=1541577933156863
FETCH #140128575016040:c=88,e=88,p=0,cr=1,cu=0,mis=0,r=0,dep=0,og=1,plh=1923716061,tim=1541577933156988
STAT #140128575016040 id=1 cnt=1 pid=0 pos=1 obj=91021 op='TABLE ACCESS BY INDEX ROWID TB_TEST (cr=6 pr=5 pw=0 time=409 us cost=1 size=26 card=1)'
STAT #140128575016040 id=2 cnt=1 pid=1 pos=1 obj=91022 op='INDEX RANGE SCAN IDX_ID (cr=3 pr=2 pw=0 time=262 us cost=1 size=0 card=1)'
WAIT #140128575016040: nam='SQL*Net message to client' ela= 1 driver id=1650815232 #bytes=1 p3=0 obj#=0 tim=1541577933157090
可看到，除了
两次索引访问和一次回表
之外，还有
两次db file sequential read等待是发生在file#=3,block#为176和32616上
，其中176是对象的头块，查看此对象(dba_extents或者DBA_UNDO_EXTENTS)：
SQL> select segment_name,segment_type from dba_extents where file_id = 3 and 176 between block_id and block_id + blocks - 1
2 union all
3 select segment_name,segment_type from dba_extents where file_id = 3 and 32616 between block_id and block_id + blocks - 1
4 ;
SEGMENT_NAME SEGMENT_TYPE
--------------------------------------------------------------------------------- ------------------
_SYSSMU4_1254879796$ TYPE2 UNDO
_SYSSMU4_1254879796$ TYPE2 UNDO
说明对undo段的访问可引起db file sequential read等待。
3.通过ROWID访问表数据其实与索引回表是一样的，因此不再单独验证。
注意：
1.对db file sequential read等待来说，绝大多数系统无法避免。db file sequential read等待比较多也不算是坏事，关键要看是否合理
2.平均等待时间不应该超过7ms, 否则证明存储存在争用。
3.优化方式：加大SGA, 降低总体物理读，提高存储性能，选择合理的索引，对频繁访问的表做cache等
查看各session的db file sequential read等待情况：
select a.sid,
a.event,
a.total_waits,
a.time_waited,
a.time_waited / c.sum_time_waited * 100 pct_wait_time,
round((sysdate - b.logon_time) * 24) hours_connected
from v$session_event a, v$session b,
(select sid, sum(time_waited) sum_time_waited
from v$session_event
where wait_class <> 'Idle'
having sum(time_waited) > 0 group by sid) c
where a.sid = b.sid
and a.sid = c.sid
and a.time_waited > 0
and a.event = 'db file sequential read'
order by hours_connected desc, pct_wait_time;
查看db file sequential read等待的对象：
select b.session_id,
nvl(substr(a.object_name,1,30),
'P1='||b.p1||' P2='||b.p2||' P3='||b.p3) object_name,
a.subobject_name,
a.object_type
from dba_objects a, v$active_session_history b, x$bh c
where c.obj = a.object_id(+)
and b.p1 = c.file#(+)
and b.p2 = c.dbablk(+)
and b.event = 'db file sequential read'
union
select b.session_id,
nvl(substr(a.object_name,1,30),
'P1='||b.p1||' P2='||b.p2||' P3='||b.p3) object_name,
a.subobject_name,
a.object_type
from dba_objects a, v$active_session_history b, x$bh c
where c.obj = a.data_object_id(+)
and b.p1 = c.file#(+)
and b.p2 = c.dbablk(+)
and b.event = 'db file sequential read'
order by 1;
查看数据文件上单块读的平均等待时间：
select a.file#,
b.file_name,
a.singleblkrds, --总等待次数
a.singleblkrdtim, --总等待时间(厘秒)
a.singleblkrdtim/a.singleblkrds average_wait
from v$filestat a, dba_data_files b
where a.file# = b.file_id
and a.singleblkrds > 0
order by average_wait;
某个session自登录以来所有等待事件的情况：
SELECT INST_ID,
SID,
EVENT,
TOTAL_WAITS WAITS_CNT,
TIME_WAITED,
AVERAGE_WAIT/100 "AVERAGE_WAIT(S)",
MAX_WAIT/100 "MAX_WAIT(S)"
FROM GV$SESSION_EVENT
WHERE SID=&INPUT_SID
ORDER BY TOTAL_WAITS DESC;
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