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Oracle Study之--Oracle等待事件（7）
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Free buffer waits
当一个会话将数据块从磁盘读到内存中时，它需要到内存中找到空闲的内存空间来存放这些数据块，当内存中没有空闲的空间时，就会产生这个等待；除此之外，还有一种情况就是会话在做一致性读时，需要构造数据块在某个时刻的前映像（p_w_picpath），此时需要申请内存来存放这些新构造的数据块，如果内存中无法找到这样的内存块，也会发生这个等待事件。
当数据库中出现比较严重的free buffer waits等待事件时，可能的原因是：
（1）data buffer 太小，导致空闲空间不够
（2）内存中的脏数据太多，DBWR无法及时将这些脏数据写到磁盘中以释放空间
这个等待事件包含2个参数：
File#： 需要读取的数据块所在的数据文件的文件号。
Block#： 需要读取的数据块块号。
案例分析：
11:14:33 SYS@ prod>show parameter cache NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ db_16k_cache_size big integer 24M db_cache_advice string ON db_cache_size big integer 16M db_keep_cache_size big integer 0 db_recycle_cache_size big integer 12M
11:21:17 SYS@ prod>conn scott/tiger Connected. 11:23:16 SCOTT@ prod>begin 11:25:04 2 for i in 1..100000 loop 11:25:04 3 insert into t1 values (i); 11:25:04 4 end loop; 11:25:04 5 end; 11:25:04 6 / PL/SQL procedure successfully completed. 11:23:29 SYS@ prod>conn tom/tom Connected. 11:23:38 TOM@ prod>create table t1 as select * from scott.t1; Table created. Elapsed: 00:00:02.19 11:23:52 TOM@ prod>begin 11:24:59 2 for i in 1..100000 loop 11:24:59 3 insert into t1 values (i); 11:24:59 4 end loop; 11:24:59 5 end; 11:24:59 6 / PL/SQL procedure successfully completed. 11:25:12 SYS@ prod>select event,TOTAL_WAITS,AVERAGE_WAIT,EVENT_ID from v$system_event 2* where event like '%buffer%' EVENT TOTAL_WAITS AVERAGE_WAIT EVENT_ID ---------------------------------------------------------------- ----------- ------------ ---------- latch: cache buffers chains 3 .2 2779959231 free buffer waits 14 .86 2701153470 buffer busy waits 2 .23 2161531084 log buffer space 7 40.42 3357856061 latch: cache buffers lru chain 17 .32 3401628503 buffer deadlock 11 .03 218992928 6 rows selected.
Latch free
在10g之前的版本里，latch free 等待事件代表了所有的latch等待，在10g以后，一些常用的latch事件已经被独立了出来：
11:25:2name1 SYS@ prod>select name from v$event_name where name like 'latch%' order by 1; NAME ---------------------------------------------------------------- latch activity latch free latch: Change Notification Hash table latch latch: In memory undo latch latch: MQL Tracking Latch latch: PX hash array latch latch: Undo Hint Latch latch: WCR: processes HT latch: WCR: sync latch: cache buffer handles latch: cache buffers chains latch: cache buffers lru chain latch: call allocation latch: change notification client cache latch latch: checkpoint queue latch latch: enqueue hash chains latch: gc element NAME ---------------------------------------------------------------- latch: gcs resource hash latch: ges resource hash list latch: lob segment dispenser latch latch: lob segment hash table latch latch: lob segment query latch latch: messages latch: object queue header operation latch: parallel query alloc buffer latch: redo allocation latch: redo copy latch: redo writing latch: row cache objects latch: session allocation latch: shared pool latch: undo global data latch: virtual circuit queues 33 rows selected. 11:39:21 SYS@ prod>select EVENT#,EVENT_ID,NAME,PARAMETER1,PARAMETER2,PARAMETER3 from v$event_name 2* where name like '%latch free%' EVENT# EVENT_ID NAME PARAMETER1 PARAMETER2 PARAMETER3 ---------- ---------- ------------------------------ ---------- -------------------- ------------------------------ 402 3474287957 latch free address number tries 409 2530878290 wait list latch free address number tries 11:32:33 SYS@ prod>desc v$latchname Name Null? Type ----------------------------------------------------------------- -------- -------------------------------------------- LATCH# NUMBER NAME VARCHAR2(64) HASH NUMBER 所以latch free 等待事件在10g以后的版本中并不常见，而是以具体的Latch 等待事件出现。这个等待事件有三个参数：Address: 会话等待的latch 地址。Number： latch号，通过这个号，可以从v$latchname 视图中找到这个latch 的相关的信息，Tries: 会话尝试获取Latch 的次数。 11:34:25 SYS@ prod>select * from v$latchname 11:34:36 2 where name like '%buffer%'; LATCH# NAME HASH ---------- ---------------------------------------------------------------- ---------- 33 SGA IO buffer pool latch 2719726273 63 IPC stats buffer allocation latch 1449990452 106 KJC global post event buffer 3098969798 145 cache buffers lru chain 3559635447 146 buffer pool 510014793 150 cache buffers chains 3563305585 151 cache buffer handles 892398878 196 media recovery process out of buffers 2731251867 197 mapped buffers lru chain 93631960 208 lock DBA buffer during media recovery 3620457631 350 virtual circuit buffers 1577520421 378 parallel query alloc buffer 291345605 416 p_w_picpath handles of buffered messages latch 3223585260 476 buffer pin latch 3925519355 14 rows selected.
