db file sequential read等待事件有三个参数，属于User I/O类的等待：
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;
