Oracle Study之--Oracle等待事件（7）
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 12M11:21:17 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.
