db_files 不要設定的太大,否則影響記憶體的使用
db_files引數限制了資料庫資料檔案總的個數,datafiles數目達到db_files指定後資料庫不能新增新的資料檔案,如果需要修改要重新重啟資料庫
所以這個引數都會有一定的預留,但是如果預先設定太大的話會影響oracle記憶體的使用
下面採取極端的 200 vs 20000 來做一個簡單的比較
首先我們來看其對SGA的影響
db_files=200
SQL> show sga
Total System Global Area 555559728 bytes
Fixed Size 731952 bytes
Variable Size 385875968 bytes
Database Buffers 167772160 bytes
Redo Buffers 1179648 bytes
SQL> create table b as select * from v$sgastat;
Table created.
SQL> show parameter db_files
NAME TYPE VALUE
———————————— ———– ————————————————————
db_files integer 200
db_files=20000
Total System Global Area 1444753456 bytes
Fixed Size 733232 bytes
Variable Size 1275068416 bytes
Database Buffers 167772160 bytes
Redo Buffers 1179648 bytes
Database mounted.
Database opened.
SQL> create table e as select * from v$sgastat;
Table created.
SQL> show parameter db_files
NAME TYPE VALUE
———————————— ———– ————————————————————
db_files integer 20000
可以看出shared pool部分20000的db_files比200的db_files多了848M,這個多出來的值不是固定的,和其他引數還有關係,如果在正式環境上會高出更多
我遇到過db_files引數設定為10000比設定為1000要使用2G空間的情況
上面我對v$sgastat做了兩次snapshot,現在我們可以看看是哪個component佔用了這些空間
SQL> select b.pool,b.name,b.bytes before,e.bytes after,e.bytes-b.bytes delta from b,e
2 where b.pool=e.pool and b.name=e.name and b.bytes!=e.bytes order by delta desc;
POOL NAME BEFORE AFTER DELTA
———– ————————– ———- ———- ———-
shared pool free memory 300344592 948654528 648309936 – 600M
shared pool Checkpoint queue 2053120 204805120 202752000 – 200M
shared pool FileOpenBlock 7517528 16440360 8922832
shared pool enqueue 1833832 4568752 2734920
shared pool enqueue resources 662048 3236424 2574376
shared pool KGK heap 6904 640600 633696
shared pool fixed allocation callback 560 640 80
shared pool trigger inform 344 216 -128
shared pool sim memory hea 1274744 1274040 -704
shared pool KQR M PO 118832 11288 -107544
shared pool KQR L PO 414720 235544 -179176
shared pool KGLS heap 898344 390168 -508176
shared pool miscellaneous 30526568 29338032 -1188536
shared pool library cache 4324488 2925224 -1399264
shared pool sql area 5029320 1234712 -3794608
15 rows selected.
下面來看看對PGA的影響,PGA中有一部分記憶體空間是用來存放opened file descriptors,db_files引數設定越高,這部分預留空間越大
db_files=200
SQL> select min(value) from v$sesstat s,v$statname n
2 where s.statistic# = n.statistic# and
3 n.name = ‘session pga memory’
4 /
MIN(VALUE)
———-
118760
db_files=20000
SQL> select min(value) from v$sesstat s,v$statname n
2 where s.statistic# = n.statistic# and
3 n.name = ‘session pga memory’
4 /
MIN(VALUE)
———-
1866088
如果按照1000個process計算的話,PGA的差距大約為1.6G
SQL> select 1000*(1866088-118760)/power(1024,3) from dual;
1000*(1866088-118760)/POWER(1024,3)
———————————–
1.62732601
所以db_files不要預留太大,否則會大大影響到記憶體空間的使用