如何設(shè)定ASHbuffer大小

Ash Buffer是SGA里的一塊循環(huán)使用的內(nèi)存區(qū)域，用于存放MMON捕捉到的active session信息，MMNL每隔60min將Ash buffer的內(nèi)容經(jīng)過(guò)過(guò)

Ash Buffer是SGA里的一塊循環(huán)使用的內(nèi)存區(qū)域，用于存放MMON捕捉到的active session信息，MMNL每隔60min將Ash buffer的內(nèi)容經(jīng)過(guò)過(guò)濾后dump到AWR，對(duì)應(yīng)的視圖是DBA_HIST_ACTIVE_SESS_HISTORY，以騰出Ash buffer空間用于之后的采集。Ash Buffer的大小在instance啟動(dòng)時(shí)就已經(jīng)決定了，并且在instance運(yùn)行期間不能被動(dòng)態(tài)調(diào)整，那么Ash Buffer的大小是由哪些因素決定的？
關(guān)于ASH buffer的大小MOS 243132.1上有如下描述
Size of ASH Circular Buffer = Max [Min [ #CPUs * 2 MB, 5% of Shared Pool Size, 30MB ], 1MB ]

###看下實(shí)際環(huán)境里和ash buffer有關(guān)的參數(shù)
SQL> show parameter cpu_count

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
cpu_count integer 16

SQL> show parameter shared_pool_size
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
shared_pool_size big integer 1312M

---計(jì)算一下ash buffer應(yīng)該為多少大小
Max [Min [ 16 * 2 MB, 5%*1312M, 30MB ], 1MB ]=30MB

---實(shí)際Ash buffer大小可以從v$sgastat、v$ash_info獲得，均為32MB，和公式提供的稍有差別，選擇32MB作為Ash Buffer
select * from v$sgastat where;
POOL NAME BYTES
------------ -------------------------- ----------
shared pool ASH buffers 33554432

select total_size,fixed_size,sampling_interval from v$ash_info;
TOTAL_SIZE FIXED_SIZE SAMPLING_INTERVAL
---------- ---------- -----------------
33554432 33554432 1000

###更改一下cpu_count
alter system set cpu_count=8 scope=spfile;

startup force

show parameter cpu_count
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
cpu_count integer 8

---按照公式Size of ASH Circular Buffer = Max [Min [ #CPUs * 2 MB, 5% of Shared Pool Size, 30MB ], 1MB ]計(jì)算
Ash buffer=Max [Min [ 8 * 2 MB, 5%*1312M, 30MB ], 1MB ]=16MB

---下面的查詢(xún)結(jié)果應(yīng)驗(yàn)了上面的計(jì)算
select total_size,fixed_size,sampling_interval from v$ash_info;
TOTAL_SIZE FIXED_SIZE SAMPLING_INTERVAL
---------- ---------- -----------------
16777216 16777216 1000

select * from v$sgastat where;
POOL NAME BYTES
------------ -------------------------- ----------
shared pool ASH buffers 16777216

###"_ash_size"隱含參數(shù)控制ash buffer的大小
公式Size of ASH Circular Buffer = Max [Min [ #CPUs * 2 MB, 5% of Shared Pool Size, 30MB ], 1MB ]里的1MB是_ash_size的默認(rèn)值
---_ash_size的默認(rèn)值為1MB
select ksppinm,ksppstvl from x$ksppi x, x$ksppcv y where (x.indx = y.indx) and ksppinm like '_ash_size%'

KSPPINM KSPPSTVL
------------------------------ ------------------------------
_ash_size 1048618

---我們把_ash_size改大，改成64MB
alter system set "_ash_size"=67108864 scope=spfile;

startup force

---計(jì)算一下此時(shí)的Ash buffer值
ASH Circular Buffer = Max [Min [ 8 * 2 MB, 5%*1315M, 30MB ], 64MB ]=64MB

---下面的查詢(xún)結(jié)果應(yīng)驗(yàn)了上面的計(jì)算
SQL> select total_size,fixed_size,sampling_interval from v$ash_info;

TOTAL_SIZE FIXED_SIZE SAMPLING_INTERVAL
---------- ---------- -----------------
67108864 67108864 1000

SQL> select * from v$sgastat where;

POOL NAME BYTES
------------ -------------------------- ----------
shared pool ASH buffers 67108864

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