oracle内存分配机制及查看oracle内存使用率

Oracle内存组件中，有一个叫做SGA的部分，这个部分如果设置了7G，启动之后，立马从内存中分割出7G，哪怕当前Oracle只用了100M。这7G也是不可以被其他任何机制回收和利用的。

而且Oracle是典型的耗内存应用，SGA组件里的BufferCache放的是真实的用户数据。可以避免分散的磁盘操作，而直接在SGA中完成对数据库的修改加工。

SGA是主要占内存的组件。其他PGA等等为用多少分配多少的基本原则。会发生回收机制的。

除此之外，你可以通过show parameter sga查看SGA的具体设置。
select name,total,round(total-free,2) used, round(free,2) free,round((total-free)/total*100,2) pctused from 

(select 'SGA' name,(select sum(value/1024/1024) from v$sga) total,

(select sum(bytes/1024/1024) from v$sgastat where name='free memory')free from dual)

union

select name,total,round(used,2)used,round(total-used,2)free,round(used/total*100,2)pctused from (

select 'PGA' name,(select value/1024/1024 total from v$pgastat where name='aggregate PGA target parameter')total,

(select value/1024/1024 used from v$pgastat where name='total PGA allocated')used from dual)

  union

select name,round(total,2) total,round((total-free),2) used,round(free,2) free,round((total-free)/total*100,2) pctused from (

select 'Shared pool' name,(select sum(bytes/1024/1024) from v$sgastat where pool='shared pool')total,

(select bytes/1024/1024 from v$sgastat where name='free memory' and pool='shared pool') free from dual)

union

select name,round(total,2)total,round(total-free,2) used,round(free,2) free,round((total-free)/total,2) pctused from (

select 'Default pool' name,( select a.cnum_repl*(select value from v$parameter where name='db_block_size')/1024/1024 total from x$kcbwds a, v$buffer_pool p

  where a.set_id=p.LO_SETID and p.name='DEFAULT' and p.block_size=(select value from v$parameter where name='db_block_size')) total,

(select a.anum_repl*(select value from v$parameter where name='db_block_size')/1024/1024 free from x$kcbwds a, v$buffer_pool p 

where a.set_id=p.LO_SETID and p.name='DEFAULT' and p.block_size=(select value from v$parameter where name='db_block_size')) free from dual)

union

  select name,nvl(round(total,2),0)total,nvl(round(total-free,2),0) used,nvl(round(free,2),0) free,nvl(round((total-free)/total,2),0) pctused from (

select 'KEEP pool' name,(select a.cnum_repl*(select value from v$parameter where name='db_block_size')/1024/1024 total from x$kcbwds a, v$buffer_pool p

  where a.set_id=p.LO_SETID and p.name='KEEP' and p.block_size=(select value from v$parameter where name='db_block_size')) total,

(select a.anum_repl*(select value from v$parameter where name='db_block_size')/1024/1024 free from x$kcbwds a, v$buffer_pool p 

where a.set_id=p.LO_SETID and p.name='KEEP' and p.block_size=(select value from v$parameter where name='db_block_size')) free from dual)

union

select name,nvl(round(total,2),0)total,nvl(round(total-free,2),0) used,nvl(round(free,2),0) free,nvl(round((total-free)/total,2),0) pctused from (

select 'RECYCLE pool' name,( select a.cnum_repl*(select value from v$parameter where name='db_block_size')/1024/1024 total from x$kcbwds a, v$buffer_pool p

  where a.set_id=p.LO_SETID and p.name='RECYCLE' and p.block_size=(select value from v$parameter where name='db_block_size')) total,

(select a.anum_repl*(select value from v$parameter where name='db_block_size')/1024/1024 free from x$kcbwds a, v$buffer_pool p 

where a.set_id=p.LO_SETID and p.name='RECYCLE' and p.block_size=(select value from v$parameter where name='db_block_size')) free from dual)

union

select name,nvl(round(total,2),0)total,nvl(round(total-free,2),0) used,nvl(round(free,2),0) free,nvl(round((total-free)/total,2),0) pctused from(

select 'DEFAULT 16K buffer cache' name,(select a.cnum_repl*16/1024 total from x$kcbwds a, v$buffer_pool p

  where a.set_id=p.LO_SETID and p.name='DEFAULT' and p.block_size=16384) total,

  (select a.anum_repl*16/1024 free from x$kcbwds a, v$buffer_pool p 

where a.set_id=p.LO_SETID and p.name='DEFAULT' and p.block_size=16384) free from dual)

union

select name,nvl(round(total,2),0)total,nvl(round(total-free,2),0) used,nvl(round(free,2),0) free,nvl(round((total-free)/total,2),0) pctused from(

select 'DEFAULT 32K buffer cache' name,(select a.cnum_repl*32/1024 total from x$kcbwds a, v$buffer_pool p

  where a.set_id=p.LO_SETID and p.name='DEFAULT' and p.block_size=32768) total,

  (select a.anum_repl*32/1024 free from x$kcbwds a, v$buffer_pool p 

where a.set_id=p.LO_SETID and p.name='DEFAULT' and p.block_size=32768) free from dual)

union

select name,total,total-free used,free, (total-free)/total*100 pctused from (

select 'Java Pool' name,(select sum(bytes/1024/1024) total from v$sgastat where pool='java pool' group by pool)total,

( select bytes/1024/1024 free from v$sgastat where pool='java pool' and name='free memory')free from dual)

union

select name,Round(total,2),round(total-free,2) used,round(free,2) free, round((total-free)/total*100,2) pctused from (

select 'Large Pool' name,(select sum(bytes/1024/1024) total from v$sgastat where pool='large pool' group by pool)total,

( select bytes/1024/1024 free from v$sgastat where pool='large pool' and name='free memory')free from dual)

order by pctused desc;