Oracle ASM的AU（Allocation units）分配

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ASM在分配空间时，以AU为单位进行，AU即Allocation units，是组成ASM disk的基本单元。

在Oracle 10gR2中，ASM AU的缺省单位大小是1M，相应的条带大小是128K。
在ASM中，可以通过调整_asm_ausize隐含参数的大小来进行调整，_asm_stripesize控制相应的条带化参数。

从v$asm_diskgroup视图可以查询得到相关的信息：

SQL> select group_number,name,sector_size,block_size,allocation_unit_size from v$asm_diskgroup;

GROUP_NUMBER NAME   SECTOR_SIZE BLOCK_SIZE ALLOCATION_UNIT_SIZE
------------ ------ ----------- ---------- --------------------
           1 ARCHDG         512       4096              1048576
           2 DATADG         512       4096              1048576

在Oracle Database 11g中，可以通过类似如下语句在创建磁盘组时指定au_size:

SQL> CREATE DISKGROUP DATA DISK '/dev/raw/raw15', '/dev/raw/raw16',
'/dev/raw/raw17' ATTRIBUTE 'au_size' = '16M', 'compatible.asm' = '11.1' 'compatible.rdbms' = '11.1';

ASM允许的相关参数可以从DB的实例中查询获得，以下是一系列和asm相关的初始化参数：

SQL> SELECT x.ksppinm NAME, y.ksppstvl VALUE, x.ksppdesc describ
2    FROM SYS.x$ksppi x, SYS.x$ksppcv y
3   WHERE x.indx = y.indx AND x.ksppinm LIKE '%&par%';
Enter value for par: asm
old   3: WHERE x.indx = y.indx AND x.ksppinm LIKE '%&par%'
new   3: WHERE x.indx = y.indx AND x.ksppinm LIKE '%asm%'

NAME                           VALUE                DESCRIB
------------------------------ -------------------- ------------------------------------------------------------
asm_diskstring                                      disk set locations for discovery
_asm_disk_repair_time          14400                seconds to wait before dropping a failing disk
asm_diskgroups                                      disk groups to mount automatically
asm_power_limit                1                    number of processes for disk rebalancing
_asm_ausize                    1048576              allocation unit size
_asm_blksize                   4096                 metadata block size
_asm_acd_chunks                1                    initial ACD chunks created
_asm_libraries                 ufs                  library search order for discovery
_asm_maxio                     1048576              Maximum size of individual I/O request
_asm_allow_only_raw_disks      TRUE                 Discovery only raw devices
_asmlib_test                   0                    Osmlib test event

NAME                           VALUE                DESCRIB
------------------------------ -------------------- ------------------------------------------------------------
_asm_allow_resilver_corruption FALSE                Enable disk resilvering for external redundancy
_asmsid                        asm                  ASM instance id
_asm_wait_time                 18                   Max/imum time to wait before asmb exits
_asm_stripewidth               8                    ASM file stripe width
_asm_stripesize                131072               ASM file stripe size
_asm_droptimeout               60                   timeout before offlined disks get dropped (in 3s ticks)
_asm_emulmax                   10000                max number of concurrent disks to emulate I/O errors
_asm_emultimeout               0                    timeout before emulation begins (in 3s ticks)
_asm_kfdpevent                 0                    KFDP event

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