ASM由于其高度的封装性，使得我们很难知道窥探其内部的原理。可以通过一下视图和数据字典来来查看ASM 的信息。

一. 相关视图和数据字典

This list is obtained querying v$fixed_view_definitionwhere view_name like '%ASM%' which exposes all the v$ and gv$ views with theirdefinition. Fixed tables are exposed by querying v$fixed_table where name like'x$kf%' (ASM fixed tables use the 'X$KF' prefix).

SQL>select * fromv$fixed_view_definition whereview_name like '%ASM%';

SQL>select * from sys.v$fixed_tablewhere name like 'X$KF%' ;

Noteon 11g there are additional V$views: , and X$tables: *

New in 11g:

二. Striping and Mirroring with ASM, extentsand allocation units

Abasic example, using ASM and normal redundancy: the available storage, say 64HDs over FC SAN, are used to create the main DB diskgroup: DATADG. DATADG islogically divided into 2 evenly sized groups of disks: 32 disks in failgroupN.1 and 32 in failgroup N.2. Oracle datafiles created in DATADG are 'striped'into smaller pieces, extents of 1MB in size. Extents are allocated to thestorage in 2 (mirrored) allocation units (AU): one AU in failgroup N.1 theother in failgroup N.2.

Allocation Units

EveryASM disk is divided into allocation units (AU). An AU is the fundamental unitof allocation within a disk group. A file extent consists of one or more AU. AnASM file consists of one or more file extents.

When you create a disk group, youcan set the ASM AU size to be between 1 MB and 64 MB in powers of two, such as,1, 2, 4, 8, 16, 32, or 64. Larger AU sizes typically provide performanceadvantages for data warehouse applications that use large sequential reads.

默认的AU 大小是1M。

三. X$KFFXP

ThisX$ table contains the mapping between files, extents and allocation units. Itallows to track the position of all the extents of a given file striped andmirrored across storage.

Note:RDBMS read operations access only the primary extent of a mirrored couple(unless there is an IO error) . Write operations instead write all mirroredextents to disk.

X$KFFXP Column Name

Description

Example1 - reading ASM files with direct OS access

（1）Find the 2 mirrored extents of an ASM file (thespfile in this example)

sys@+ASM1>selectGROUP_KFFXP,DISK_KFFXP,AU_KFFXP from x$kffxp where number_kffxp=(selectfile_number from v$asm_alias where name='spfiletest1.ora');

GROUP_KFFXP DISK_KFFXP AU_KFFXP

1 20 379

1 3 101

（2）find the diskname

sys@+ASM1> select disk_number,path fromv$asm_disk where GROUP_NUMBER=1 anddisk_number in (3,20);

DISK_NUMBER PATH

3 /dev/mpath/itstor417_2p1

20 /dev/mpath/itstor419_2p1

（3）access the data directly from disk with dd

ddif=/dev/mpath/itstor417_2p1 bs=1024k count=1 skip=101|strings|more

四. X$KFDAT

This X$ table contains details of all allocation units (free and used).

X$KFDAT Column Name

Description

Example2 - listallocation units of a given file from x$kfdat

similarly to example 1 above, another wayto retrieve ASM file allocation maps:

sys@+ASM1> selectGROUP_KFDAT,NUMBER_KFDAT,AUNUM_KFDAT from x$kfdat where fnum_kfdat=(selectfile_number from v$asm_alias where name='spfiletest1.ora');

GROUP_KFDAT NUMBER_KFDAT AUNUM_KFDAT-

1 3 101

1 20 379

Example3 - from strace data of an oracle userprocess

（1）from the strace file of a user(shadow) process identify IO operations:

ex: strace-p 30094 2>&1|grep -v time

read64(15,"#\242\0\0\33\0@\2\343\332\177\303s\5\1\4\211\330\0\0\0"..., 8192,473128960) = 8192

itis a read operation of 8KB (oracle block) at the offset 473128960 (=451 MB +27*8KB) from file descriptor FD=15

（2）using /proc/30094/fd -> findFD=15 is /dev/mpath/itstor420_1p1

（3）I find the group and disk number ofthe file:

sys@+ASM1> selectGROUP_NUMBER,DISK_NUMBER from v$asm_disk

where path='/dev/mpath/itstor420_1p1';

GROUP_NUMBER DISK_NUMBER

1 30

（4）using the disk number, group numberand offset (from strace above) I find the file number and extent number:

sys@+ASM1> select number_kffxp,XNUM_KFFXP from x$kffxp where group_kffxp=1 and disk_kffxp=20 and au_kffxp=451;

NUMBER_KFFXP XNUM_KFFXP

268 17

（5）from v$asm_file fnum=268 is file ofthe users' tablesspace:

sys@+ASM1> select name from v$asm_aliaswhere FILE_NUMBER=268

NAME

USERS.268.612033477

sys@DB> select file#,name fromv$datafile where upper(name) like '%USERS.268.612033477';

FILE# NAME

9 +TEST1_DATADG1/test1/datafile/users.268.612033477

（6）from dba extents finally find theowner and segment name relative to the original IO operation:

sys@TEST1> selectowner,segment_name,segment_type from dba_extents

where FILE_ID=9 and 27+17*1024*1024 betweenblock_id and block_id+blocks;

OWNER SEGMENT_NAME SEGMENT_TYPE

SCOTT EMP TABLE

五. X$KFDPARTNER

ThisX$ table contains the disk-to-partner (1-N) relationship. Two disks of a givenASM diskgroup are partners if they each contain a mirror copy of the sameextent. Therefore partners must belong to different failgroups of the samediskgroup. From a few liveexamples I can see that typically disks have 10 partners each atdiskgroup creation and fluctuate around 10 partners following ASM operations.This mechanism is in place to reduce the chance of losing both sides of themirror in case of double disk failure.

X$KFDPARTNER Column Name

Description

六. X$KFFIL and metadata files

Three types of metadata:

（1）diskgroup metadata: files with NUMBER_KFFIL <256 ASM metadata andASMlog files. These files have high redundancy (3 copies) and block size =4KB.

1）ASM log files are used for ASMinstance and crash recovery when a crash happens with metadata operations (seebelow COD and ACD)

2）at diskgroup creation 6 files withmetadata are visible from x$kffil

（2）disk metadata: disk headers (typically the first 2 AU of each disk)are not listed in x$kffil (they appear as file number 0 in x$kfdat). Containdisk membership information. This part of the disk has to be 'zeroed out'before the disk can be added to ASM diskgroup as a new disk.

（3）file metadata: 3 mirrored extents with file metadata, visible fromx$kffxp and x$kfdat

Example: list all files,system and users' with their sizes:

SYS@+ASM2(rac2)> select group_kffil group#,number_kffil file#, filsiz_kffil filesize_after_mirr, filspc_kffilraw_file_size from x$kffil;

GROUP# FILE# FILESIZE_AFTER_MIRRRAW_FILE_SIZE

1 1 2097152 2097152

1 2 1048576 1048576

1 3 88080384 89128960

1 4 1392640 2097152

1 5 1048576 1048576

1 6 1048576 1048576

1 256 545267712 547356672

1 257 629153792 631242752

1 258 351281152 353370112

1 259 7872512 8388608

1 260 15319040 16777216

Example: List all filesincluding metadata allocated in the ASM diskgroups

SYS@+ASM2(rac2)> select group_kfdat group#,FNUM_KFDAT file#, sum(1) AU_used from x$kfdat where v_kfdat='V' group by group_kfdat,FNUM_KFDAT,v_kfdat;

GROUP# FILE# AU_USED

1 0 2

1 1 2

1 2 1

1 3 85

1 4 2

1 5 1

1 6 1

1 256 522

1 257 602

1 258 337

1 259 8

Descriptionof metadata files

This paragraph is from: Oracle AutomaticStorage Management, Oracle Press Nov 2007, N. Vengurlekar, M. Vallath, R.Long

（1）. File#0, AU=0: disk header (disk name, etc), Allocation Table (AT)and Free Space Table (FST)

（2）. File#0, AU=1: Partner Status Table (PST)

（3）. File#1: File Directory (files and their extent pointers)

（4）. File#2: Disk Directory

（5）. File#3: Active Change Directory (ACD) The ACD is analogous to a redolog, where changes to the metadata are logged. Size=42MB * number of instances

（6）. File#4: Continuing Operation Directory (COD). The COD is analogousto an undo tablespace. It maintains the state of active ASM operations such asdisk or datafile drop/add. The COD log record is either committed or rolledback based on the success of the operation.

（7）. File#5: Template directory

（8）. File#6: Alias directory

（9）. 11g, File#9: Attribute Directory

（10）. 11g, File#12: Stalenessregistry, created when needed to track offline disks

七. DBMS_DISKGROUP, an internal ASM package

dbms_diskgroupis an Oracle 'internal package' (it doesn't show up as an object being that ASMhas no dictionary) called dbms_diskgroup. It is used to access the ASM withfilesystem-like calls. 11g asmcmd uses this package to implement the cpcommand. A list of procedures:

dbms_diskgroup.open(:fileName, :openMode, :fileType, :blkSz, :hdl,:plkSz, :fileSz)

dbms_diskgroup.createfile(:fileName, :fileType, :blkSz, :fileSz, :hdl, :plkSz, :fileGenName)

dbms_diskgroup.close(:hdl)

dbms_diskgroup.read(:hdl, :offset, :blkSz, :data_buf)

dbms_diskgroup.commitfile(:handle)

dbms_diskgroup.resizefile(:handle,:fsz)

dbms_diskgroup.remap(:gnum, :fnum, :virt_extent_num)

dbms_diskgroup.getfileattr(:fileName, :fileType, :fileSz, :blkSz)

dbms_diskgroup.checkfile(?)

dbms_diskgroup.patchfile(?)

八. ASM parameters and underscore parameters

可以使用SQL 查看ASM 参数：

/* Formatted on 2011/8/30 16:28:54(QP5 v5.163.1008.3004) */

SELECT a.ksppinm"Parameter", c.ksppstvl"Instance Value"

FROMx$ksppi a,x$ksppcv b,x$ksppsv c

WHERE a.indx = b.indx AND a.indx = c.indx ANDksppinm LIKE '%asm%'

ORDER BY a.ksppinm;

或者：

select * fromall_parameters where name like '%asm%';

Oracle all_parameters 视图

http://blog.csdn.net/tianlesoftware/article/details/6641281

注意这里的_asm_ausize =1M

Oracle 11g 里新增加的参数：

九. ASM-related acronyms 相关名词解释

（1）. PST - Partner Status Table. Maintains info on disk-to-diskgroupmembership.

（2）. COD - Continuing Operation Directory. The COD structuremaintains the state of active ASM operations or changes, such as disk ordatafile drop/add. The COD log record is either committed or rolled back basedon the success of the operation. (source Oracle whitepaper)

（3）. ACD - Active Change Directory. The ACD is analogous to a redolog, where changes to the metadata are logged. The ACD log record is used todetermine point of recovery in the case of ASM operation failures or instancefailures. (source Oracle whitepaper)

（4）. OSM Oracle Storage Manager, legacy name, synonymous of ASM

（5）. CSS Cluster Synchronization Services. Part of Oracleclusterware, mandatory with ASM even in single instance. CSS is used toheartbeat the health of the ASM instances.

（6）. RBAL - Oracle backgroud process. In an ASM instance coordinatedrebalancing operations. In a DB instance, opens and mount diskgroups from thelocal ASM instance.

（7）. ARBx - Oracle backgroud processes. In an ASM instance, a slavefor rebalancing operations

（8）. PSPx - Oracle backgroud processes. In an ASM instance, ProcessSpawners

（9）. GMON - Oracle backgroud processes. In an ASM instance,diskgroup monitor.

（10）. ASMB - Oracle backgroudprocess. In an DB instance, keeps a (bequeath) persistent DB connection to thelocal ASM instance. Provides hearthbeat and ASM statistics. During a diskgrouprebalancing operation ASM communicates to the DB AU changes via this connection.

（11）. O00x - Oracle backgroudprocesses. Slaves used to connected from the DB to the ASM instance for 'shortoperations'.