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direct path read 测试

原创 Linux操作系统 作者:wei-xh 时间:2011-03-09 14:09:32 0 删除 编辑

CRMG的数据库等待事件TOP 5里,总会出现一个10G以前几乎不可能在TOP里的等待,direct path read

10G以前这个等待跟并行查询有关。

在执行parallel query时,slave session所执行的direct path I/O引发的。如果这个等待比较严重,需要提高parallel query本身性能。

让我不解的是,我们crmg上没有并行查询。

11G莫非发生了什么变化?

答案即将揭晓:

 

 

先看下下面一段话,摘抄地址:http://oraclue.com/2009/07/17/direct-path-reads-and-serial-table-scans-in-11g/

There have been changes in 11g in the heuristics to choose between direct path reads or reads through buffer cache for serial table scans.

In 10g, serial table scans for "large" tables used to go through cache (by default) which is not the case anymore. 

In 11g, this decision to read via direct path or through cache is based on the size of the table, buffer cache size and various other stats.

Direct path reads are faster than scattered reads and have less impact on other processes because they avoid latches.

11G以后,ORACLE会根据表大小来智能的选择全表扫描是通过从buffer cache读还是以direct path read方式读。direct path read这种方式需要更少的latch

 

我们来测试一把:

1) 构建一个大表,足够大于_small_table_threshold的值。

SQL> set linesize 120

SQL> col name for a30

SQL> col value for a20

SQL> col describ for a60

SQL> SELECT x.ksppinm NAME, y.ksppstvl VALUE, x.ksppdesc describ

  2    FROM SYS.x$ksppi x, SYS.x$ksppcv y

  3   WHERE x.inst_id = USERENV ('Instance')

  4     AND y.inst_id = USERENV ('Instance')

  5     AND x.indx = y.indx

  6     AND x.ksppinm LIKE '%&par%'

  7  /

输入 par 的值:  small_table

原值    6:    AND x.ksppinm LIKE '%&par%'

新值    6:    AND x.ksppinm LIKE '%small_table%'

 

NAME                           VALUE                DESCRIB

------------------------------ -------------------- ------------------------------------------------------------

_small_table_threshold         1300                 threshold level of table size for direct reads

这里我选择了ord_order_item_detail,是crmg上一个非常大的表。

然后开两个session ,分别用10046跟一下:

 

在开始跟之前需要介绍一个事件,10949事件,这个事件可以取消direct path read

 

SESSION 1, direct path read:

alter system flush buffer_cache;

alter session set events '10046 trace name context forever, level 8';

select /*+ full(a) */ count(*) from  ord_order_item_detail a;

alter session set events '10046 trace name context off';

 

SESSION 2,取消direct path read,采取散列读。

alter system flush buffer_cache;

alter session set events '10046 trace name context forever, level 8';

alter session set events '10949 trace name context forever, level 1';

select /*+ full(a) */ count(*) from  ord_order_item_detail a;

alter session set events '10046 trace name context off';

 

 

我们看一下两个session 的跟踪文件:

SESSION 1的跟踪文件:

call     count       cpu    elapsed       disk      query    current        rows

------- ------  -------- ---------- ---------- ---------- ----------  ----------

Parse        1      0.00       0.00          0          0          0           0

Execute      1      0.00       0.00          0          0          0           0

Fetch        2      3.00      53.37      97515      97639          0           1

------- ------  -------- ---------- ---------- ---------- ----------  ----------

total        4      3.00      53.37      97515      97639          0           1

Elapsed times include waiting on following events:

  Event waited on                             Times   Max. Wait  Total Waited

  ----------------------------------------   Waited  ----------  ------------

  SQL*Net message to client                       3        0.00          0.00

  SQL*Net message from client                     3        0.00          0.00

  db file sequential read                        13        0.01          0.07

 direct path read                            12250        1.39         49.74

********************************************************************************

 

SESSION 2的跟踪文件:

call     count       cpu    elapsed       disk      query    current        rows

------- ------  -------- ---------- ---------- ---------- ----------  ----------

Parse        1      0.00       0.00          0          0          0           0

Execute      1      0.00       0.00          0          0          0           0

Fetch        2      2.97       3.42      97515      98129          0           1

------- ------  -------- ---------- ---------- ---------- ----------  ----------

total        4      2.97       3.42      97515      98129          0           1

Elapsed times include waiting on following events:

  Event waited on                             Times   Max. Wait  Total Waited

  ----------------------------------------   Waited  ----------  ------------

  SQL*Net message to client                       7        0.00          0.00

  SQL*Net message from client                     7        0.00          0.00

  db file sequential read                        13        0.00          0.00

  db file scattered read                       6125        0.00          0.55

  latch free                                      3        0.00          0.01

 latch: cache buffers chains                     1        0.00          0.00

 

不过不幸的是,单纯从我的示例中来看,我们发现direct path readdb file scattered read方式,性能差好多

不过latch的争用,direct path read方式没有产生latch争用,db file scattered read方式存在latch争用。

 

_serial_direct_read,这个参数,文档中记载,如果这个参数为true的话,将开启direct path read。但是我测试的结果是,不管这个参数的值为何,

只要表大小达到一个值,都会用direct path read方式来进行表扫描。

还有一点需要说明的是,direct path read方式,会触发对这个表的检查点动作,即把这个表的相关脏数据写回disk.

 

可能我的库有问题。Direct read的读取时间差异非常大,而scater 读的时间非常平。

 

WAIT #2: nam='db file scattered read' ela= 94 file#=53 block#=272 blocks=16 obj#=12153 tim=1299644760722303

WAIT #2: nam='db file scattered read' ela= 87 file#=54 block#=273 blocks=15 obj#=12153 tim=1299644760725285

WAIT #2: nam='db file scattered read' ela= 92 file#=55 block#=272 blocks=16 obj#=12153 tim=1299644760728224

WAIT #2: nam='db file scattered read' ela= 90 file#=56 block#=272 blocks=16 obj#=12153 tim=1299644760731193

 

 

WAIT #2: nam='direct path read' ela= 7997 file number=11 first dba=26186 block cnt=15 obj#=12153 tim=1299644699977054

WAIT #2: nam='direct path read' ela= 4 file number=10 first dba=26474 block cnt=15 obj#=12153 tim=1299644699977115

WAIT #2: nam='direct path read' ela= 8752 file number=12 first dba=26058 block cnt=15 obj#=12153 tim=1299644699988878

WAIT #2: nam='direct path read' ela= 11 file number=11 first dba=26186 block cnt=15 obj#=12153 tim=1299644699988963

WAIT #2: nam='direct path read' ela= 13831 file number=29 first dba=124378 block cnt=15 obj#=12153 tim=1299644700005480

在青岛测试库测试了一下,direct path read扫描时间远低于db file scattered read

 

22G的表

direct path read扫描需要                        Elapsed: 00:03:20.44

db file scattered read扫描需要  Elapsed: 00:05:20.44

刚好差了两分钟。

 

 

对比两种方式发生的IO次数,比较接近:

oracle11@qdstandby2:/opt/oracle/admin/diag/rdbms/crmgsb/crmg/trace>grep 'direct path read' crmg_ora_455120.trc|  wc -l

   17479

oracle11@qdstandby2:/opt/oracle/admin/diag/rdbms/crmgsb/crmg/trace> grep 'db file scattered read'  crmg_ora_455120.trc|  wc -l                   <

   18068

 

不过IO响应时间的大范围抖动在direct path read里依然存在:

WAIT #2: nam='direct path read' ela= 21 file number=16 first dba=214529 block cnt=127 obj#=23236 tim=26576037710961

WAIT #2: nam='direct path read' ela= 30413 file number=17 first dba=214400 block cnt=128 obj#=23236 tim=26576037744576

WAIT #2: nam='direct path read' ela= 16 file number=18 first dba=214401 block cnt=127 obj#=23236 tim=26576037747520

WAIT #2: nam='direct path read' ela= 13121 file number=16 first dba=214656 block cnt=128 obj#=23236 tim=26576037763559

WAIT #2: nam='direct path read' ela= 15 file number=17 first dba=214530 block cnt=126 obj#=23236 tim=26576037766438

WAIT #2: nam='direct path read' ela= 29255 file number=18 first dba=214528 block cnt=128 obj#=23236 tim=26576037798557

WAIT #2: nam='direct path read' ela= 15 file number=16 first dba=214785 block cnt=127 obj#=23236 tim=26576037801488

WAIT #2: nam='direct path read' ela= 17960 file number=17 first dba=214656 block cnt=128 obj#=23236 tim=26576037822200

WAIT #2: nam='direct path read' ela= 1 file number=18 first dba=214657 block cnt=127 obj#=23236 tim=26576037825246

WAIT #2: nam='direct path read' ela= 22402 file number=16 first dba=214912 block cnt=128 obj#=23236 tim=26576037850470

WAIT #2: nam='direct path read' ela= 15 file number=17 first dba=214785 block cnt=127 obj#=23236 tim=26576037853535

WAIT #2: nam='direct path read' ela= 31519 file number=18 first dba=214784 block cnt=128 obj#=23236 tim=26576037892702

WAIT #2: nam='direct path read' ela= 16 file number=16 first dba=215041 block cnt=127 obj#=23236 tim=26576037895685

WAIT #2: nam='direct path read' ela= 24008 file number=17 first dba=214912 block cnt=128 obj#=23236 tim=26576037922490

WAIT #2: nam='direct path read' ela= 16 file number=18 first dba=214913 block cnt=127 obj#=23236 tim=26576037925382

 

 

db file scattered readIO响应时间比较平稳

WAIT #1: nam='db file scattered read' ela= 31296 file#=16 block#=202242 blocks=126 obj#=23236 tim=26576310523644

WAIT #1: nam='db file scattered read' ela= 25830 file#=17 block#=202114 blocks=126 obj#=23236 tim=26576310554176

WAIT #1: nam='db file scattered read' ela= 30326 file#=18 block#=202114 blocks=126 obj#=23236 tim=26576310588374

WAIT #1: nam='db file scattered read' ela= 19134 file#=16 block#=202370 blocks=126 obj#=23236 tim=26576310611310

WAIT #1: nam='db file scattered read' ela= 20454 file#=17 block#=202242 blocks=126 obj#=23236 tim=26576310635544

WAIT #1: nam='db file scattered read' ela= 22665 file#=18 block#=202242 blocks=126 obj#=23236 tim=26576310661976

WAIT #1: nam='db file scattered read' ela= 20304 file#=16 block#=202498 blocks=126 obj#=23236 tim=26576310687267

WAIT #1: nam='db file scattered read' ela= 7812 file#=17 block#=202370 blocks=126 obj#=23236 tim=26576310698848

WAIT #1: nam='db file scattered read' ela= 11577 file#=18 block#=202370 blocks=126 obj#=23236 tim=26576310715361

WAIT #1: nam='db file scattered read' ela= 10604 file#=16 block#=202626 blocks=126 obj#=23236 tim=26576310730434

WAIT #1: nam='db file scattered read' ela= 14844 file#=17 block#=202498 blocks=126 obj#=23236 tim=26576310749031

WAIT #1: nam='db file scattered read' ela= 14715 file#=18 block#=202498 blocks=126 obj#=23236 tim=26576310767578

WAIT #1: nam='db file scattered read' ela= 20183 file#=16 block#=202754 blocks=126 obj#=23236 tim=26576310792739

WAIT #1: nam='db file scattered read' ela= 14721 file#=17 block#=202626 blocks=126 obj#=23236 tim=26576310811619

WAIT #1: nam='db file scattered read' ela= 29155 file#=18 block#=202626 blocks=126 obj#=23236 tim=26576310845827

WAIT #1: nam='db file scattered read' ela= 22199 file#=16 block#=202882 blocks=126 obj#=23236 tim=26576310880130

WAIT #1: nam='db file scattered read' ela= 16471 file#=17 block#=202754 blocks=126 obj#=23236 tim=26576310901276

WAIT #1: nam='db file scattered read' ela= 22103 file#=18 block#=202754 blocks=126 obj#=23236 tim=26576310927107

WAIT #1: nam='db file scattered read' ela= 26420 file#=16 block#=203010 blocks=126 obj#=23236 tim=26576310958408

 

还测试了一下,表大小达到多大的时候会出现direct path read

结论是至少大于_small_table_threshold*5,也就是_small_table_threshold的五倍。

 

当是1----4倍的时候,SESSION的等待事件里都是:

EVENT                          TOTAL_WAITS TIME_WAITED

------------------------------ ----------- -----------

SQL*Net message from client             29        9752

db file scattered read            504       15

events in waitclass Other                2          11

db file sequential read                 23           3

SQL*Net message to client               30           0

 

一达到五倍,就会出现direct path read

SQL*Net message from client             37       24145

events in waitclass Other                3          20

direct path read                 448        13

db file sequential read                 33           3

SQL*Net message to client               38           0

 

 

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