http://www.dbaleading.com/Article.asp?id=40
http://www.ardentperf.com/2007/09/12/gc-buffer-busy-waits-in-rac-finding-hot-blocks/
使用ash
1、从dba_hist_snapshot中查得相关时间窗口
select min(begin_interval_time) min, max(end_interval_time) max from dba_hist_snapshot where snap_id between 12831 and 12838; MIN MAX ------------------------------ ------------------------------ 12-SEP-07 09.00.17.451 AM 12-SEP-07 05.00.03.683 PM 这是我们要使用的时间窗口：今天 9am to 5pm .
2、查看相关的最多的等待类
select wait_class_id, wait_class, count(*) cnt from dba_hist_active_sess_history where snap_id between 12831 and 12838 group by wait_class_id, wait_class order by 3; WAIT_CLASS_ID WAIT_CLASS CNT ------------- ------------------------------ ---------- 3290255840 Configuration 169 2000153315 Network 934 4108307767 System I/O 7199 3386400367 Commit 7809 4217450380 Application 12248 3875070507 Concurrency 14754 1893977003 Other 35499 97762 3871361733 Cluster 104810 1740759767 User I/O 121999 我们看到在 ASH中有相当多的Cluster等待事件。
3、下一步，查看相关的具体等待事件
select event_id, event, count(*) cnt from dba_hist_active_sess_history where snap_id between 12831 and 12838 and wait_class_id=3871361733 group by event_id, event order by 3; EVENT_ID EVENT CNT ---------- ---------------------------------------- ---------- 3905407295 gc current request 4 3785617759 gc current block congested 10 2705335821 gc cr block congested 15 512320954 gc cr request 16 3794703642 gc cr grant congested 17 3897775868 gc current multi block request 17 1742950045 gc current retry 18 1445598276 gc cr disk read 148 1457266432 gc current split 229 2685450749 gc current grant 2-way 290 957917679 gc current block lost 579 737661873 gc cr block 2-way 699 2277737081 gc current grant busy 991 3570184881 gc current block 3-way 1190 3151901526 gc cr block lost 1951 111015833 gc current block 2-way 2078 3046984244 gc cr block 3-way 2107 661121159 gc cr multi block request 4092 3201690383 gc cr grant 2-way 4129 1520064534 gc cr block busy 4576 2701629120 gc current block busy 14379 1478861578 gc buffer busy 67275 我们看到gc buffer busy是最多的.. 我们看到从今天早9到下午5点，在ash采样期间共有67,275个sessions在等待此事件。让我们看下，在被采样的时候，这些session 在执行什么sql.
4、我们看下这些sql的sql_id
select sql_id, count(*) cnt from dba_hist_active_sess_history where snap_id between 12831 and 12838 and event_id in (2701629120, 1478861578) group by sql_id having count(*)>1000 order by 2; SQL_ID CNT ------------- ---------- 6kk6ydpp3u8xw 1011 2hvs3mpab5j0w 1022 292jxfuggtsqh 1168 3mcxaqffnzgfw 1226 a36pf34c87x7s 1328 4vs8wgvpfm87w 1390 22ggtj4z9ak3a 1574 gsqhbt5a6d4uv 1744 cyt90uk11a22c 2240 39dtqqpr7ygcw 4251 8v3b2m405atgy 42292
5、查看相关sql的内容，确定是哪些表引起的。
select sql_text from dba_hist_sqltext where sql_id='8v3b2m405atgy'; SQL_TEXT --------------------------------------------------------------------------- insert into bigtable(id, version, client, cl_business_id, cl_order_id, desc
select count(distinct(current_obj#)) from dba_hist_active_sess_history where snap_id between 12831 and 12838 and event_id=1478861578 and sql_id='8v3b2m405atgy'; COUNT(DISTINCT(CURRENT_OBJ#)) ----------------------------- 14 select current_obj#, count(*) cnt from dba_hist_active_sess_history where snap_id between 12831 and 12838 and event_id=1478861578 and sql_id='8v3b2m405atgy' group by current_obj# order by 2; CURRENT_OBJ# CNT ------------ ---------- 3122841 1 3122868 3 3173166 4 3324924 5 3325122 8 3064307 8 -1 10 3064369 331 0 511 3122795 617 3064433 880 3208619 3913 3208620 5411 3208618 22215 select object_id, owner, object_name, subobject_name, object_type from dba_objects where object_id in (3208618, 3208619, 3208620); OBJECT_ID OWNER OBJECT_NAME SUBOBJECT_NAME OBJECT_TYPE ---------- ---------- ------------------------------ ------------------------------ ------------------- 3208618 JSCHDER BIGTABLE_LOG P_2007_09 TABLE PARTITION 3208619 JSCHDER BIGTABL_LG_X_ID P_2007_09 INDEX PARTITION 3208620 JSCHDER BIGTABL_LG_X_CHANGE_DATE P_2007_09 INDEX PARTITION
6、进一步，我们确认是哪些块引起的，并确定是不是表头块。
select current_file#, current_block#, count(*) cnt from dba_hist_active_sess_history where snap_id between 12831 and 12838 and event_id=1478861578 and sql_id='8v3b2m405atgy' and current_obj# in (3208618, 3208619, 3208620) group by current_file#, current_block# having count(*)>50 order by 3; CURRENT_FILE# CURRENT_BLOCK# CNT ------------- -------------- ---------- 1330 238073 51 1542 22645 55 1487 237914 56 1330 238724 61 1330 244129 76 1487 233206 120 select segment_name, header_file, header_block from dba_segments where owner='JHEIDER' and partition_name='P_2007_09' and segment_name in ('PLACEMENTS_LOG','PLCMNTS_LG_X_ID', 'PLCMNTS_LG_X_CHANGE_DATE'); SEGMENT_NAME HEADER_FILE HEADER_BLOCK ------------------------------ ----------- ------------ BIGTABL_LG_X_CHANGE_DATE 1207 204809 BIGTABL_LG_X_ID 1207 196617 BIGTABLE_LOG 1209 16393
7、同时，也可用如下的脚本来测试：
col object format a60 col i format 99 select * from ( select o.owner||'.'||o.object_name||decode(o.subobject_name,NULL,'','.')|| o.subobject_name||' ['||o.object_type||']' object, instance_number i, stat from ( select obj#||'.'||dataobj# obj#, instance_number, sum( GC_BUFFER_BUSY_DELTA ) stat from dba_hist_seg_stat where (snap_id between 12831 and 12838) and (instance_number between 1 and 6) group by rollup(obj#||'.'||dataobj#, instance_number) having obj#||'.'||dataobj# is not null ) s, dba_hist_seg_stat_obj o where o.dataobj#||'.'||o.obj#=s.obj# order by max(stat) over (partition by s.obj#) desc, o.owner||o.object_name||o.subobject_name, nvl(instance_number,0) ) where rownum<=40;
具体的相关解释，，这里不做具体，自己figure out.