#### 4.1 空間及磁碟路徑
``` sql
--ASM磁碟組使用情況  used_percent, asm例項無法使用計算
col name format a15
set lines 200
with  temp_asm as (select name,trunc(free_mb/1024,2) free_gb,trunc(total_mb/1024,2) total_gb,trunc(REQUIRED_MIRROR_FREE_MB/1024,2) req_free_gb,trunc(USABLE_FILE_MB/1024,2) uf_gb,type,state from v$asm_diskgroup)
select * from (
select t.*,trunc((total_gb-free_gb)/total_gb*100,2) as used_percent from temp_asm t where type='EXTERN' union all
select t.*,trunc(((total_gb-free_gb)/2)/((total_gb-req_free_gb)/2)*100,2) as used_percent from temp_asm t where type='NORMAL' union all
select t.*,trunc(((total_gb-free_gb)/3)/((total_gb-req_free_gb)/3)*100,2) as used_percent from temp_asm t where type='HIGH')
order by used_percent desc;

--or
select name,free_mb,total_mb,type,state from v$asm_diskgroup;

--檢查磁碟資訊
set pages 999
col path format a20;
col group_name format a15
col name format a20
col FAILGROUP_TYPE format a15
col FAILGROUP format a20
col state for a10
select a.group_number,b.name as group_name,a.FAILGROUP,a.name,a.path,a.state,a.total_mb from v$asm_disk a,v$asm_diskgroup b where a.group_number=b.group_number;
```
#### 4.2 磁碟修復檢查
``` 
#驗證asm磁碟組
kfod asm_diskstring='/dev/mapper/*' disks=all
kfed read /dev/mapper/data1 |grep grpname

#asm 讀取磁碟頭
kfed read /dev/raw/raw2 text=/tmp/asmdisk_raw2.txt  
kfed read /dev/raw/raw2 aun=0 blkn=0
kfed read /dev/raw/raw2  blkn=510 > /tmp/1.txt 
kfed read /dev/raw/raw2 aun=1 blkn=254
#修復磁碟頭
kfed write /dev/raw/raw2 aunum=0 blknum=0 text=/tmp/asmdisk_raw2.txt
#or
dd if=/dev/raw/raw2 of=/tmp/raw2.txt   skip=510 bs=4096 count=1
dd if=/tmp/raw2.txt of=/dev/raw/raw2  
```
#### 4.3 ASM相關檢視

View Name | X$ Table name  |  Description
---|---|---
V$ASM_DISKGROUP | X$KFGRP  |  performs disk discovery and lists diskgroups
V$ASM_DISKGROUP_STAT | X$KFGRP_STAT  |  diskgroup stats without disk discovery
V$ASM_DISK | X$KFDSK, X$KFKID  |  performs disk discovery, lists disks and their usage metrics
V$ASM_DISK_STAT | X$KFDSK_STAT, X$KFKID  |  lists disks and their usage metrics
V$ASM_FILE | X$KFFIL  |  lists ASM files, including metadata/asmdisk files
V$ASM_ALIAS | X$KFALS  |  lists ASM aliases, files and directories
V$ASM_TEMPLATE | X$KFTMTA  |  lists the available templates and their properties
V$ASM_CLIENT | X$KFNCL  |  lists DB instances connected to ASM
V$ASM_OPERATION | X$KFGMG  |  lists rebalancing operations
N.A. | X$KFKLIB  |  available libraries, includes asmlib path
N.A. | X$KFDPARTNER  |  lists disk-to-partner relationships
N.A. | X$KFFXP  |  extent map table for all ASM files
N.A. | X$KFDAT  |  extent list for all ASM disks
N.A. | X$KFBH  |  describes the ASM cache (buffer cache of ASM in blocks of 4K (_asm_blksize)
N.A. | X$KFCCE  |  a linked list of ASM blocks. to be further investigated

#### 4.4 ASM內部檢視X$KFFXP
X$KFFXP是ASM(Automatic Storage Management)自動儲存管理特性的重要內部檢視，該檢視反應了File Extent Map對映關係，ASM會將檔案split成多個多個piece分片，這些分片被稱為Extents。 在Disk上存放這些Extent的位置，就是我們常說的”Allocation Unit”。

KFF意為Kernel File，X$KFFXP即Kernel File Extent Maps， 該內部檢視的一條記錄代表一個Extent

欄位含義如下：
```
GROUP_KFFXP        diskgroup number (1 - 63) ASM disk group number. Join with v$asm_disk and v$asm_diskgroup

NUMBER_KFFXP      file number for the extent ASM file number. Join with v$asm_file and v$asm_alias

COMPOUND_KFFXP    (group_kffxp << 24) + file # File identifier. Join with compound_index in v$asm_file

INCARN_KFFXP      file incarnation number File incarnation id. Join with incarnation in v$asm_file

PXN_KFFXP            physical extent number  Extent number per file

XNUM_KFFXP          extent number bit 31 set if indirect Logical extent
number per file (mirrored extents have the same value)

LXN_KFFXP            logical extent number 0,1 used to identify primary/mirror extent,
2 identifies file header allocation unit (hypothesis) used in the query such that
we go after only the primary extents, not secondary extents

DISK_KFFXP          disk on which AU is located  Disk number where the extent is allocated.
Join with v$asm_disk Relative position of the allocation unit from the beginning of the disk.

AU_KFFXP              AU number on disk of AU allocation unit size (1 MB) in v$asm_diskgroup

#從11g開始加入了CHK_KFFXP SIZE_KFFXP 2個新的欄位

CHK_KFFXP   未知 可能是範圍為[0-256]的某種校驗值

SIZE_KFFXP  size_kffxp is used such that we account for variable sized extents. 
sum(size_kffxp) provides the number of AUs that are on that disk.
```
在例項級別控制ASM Diskgroup AU 和 stripe size的是2個隱藏引數 _asm_ausize 1048576 以及 _asm_stripesize 131072。從11g開始一個Extent可能包含多個AU。

可以透過以下指令碼查詢檔案與Extent等ASM屬性的對映關係:
``` sql
set linesize 140 pagesize 1400
col "FILE NAME" format a40
set head on
select NAME         "FILE NAME",
       NUMBER_KFFXP "FILE NUMBER",
       XNUM_KFFXP   "EXTENT NUMBER",
       DISK_KFFXP   "DISK NUMBER",
       AU_KFFXP     "AU NUMBER",
       SIZE_KFFXP   "NUMBER of AUs"
  from x$kffxp, v$asm_alias
 where GROUP_KFFXP = GROUP_NUMBER
   and NUMBER_KFFXP = FILE_NUMBER
   and system_created = 'Y'
   and lxn_kffxp = 0
 order by name;
```
#### 4.5 條帶化原理和rebalance

##### 條帶化
ASM的條帶化有兩種：coarse和fine-gained 。AU是最小空間分配單元，預設是1M，每個AU預設由8個128K條帶空間組成。
- 在coarse條帶化中，一個磁碟對應一個AU
所以該條帶化適合連續的I/O讀寫，比如全表掃描表
- 在fine-gained條帶化裡，一個AU中的8個128K條帶空間，平均打散在磁碟上
也就是說，4個磁碟，1個AU是平均分佈在4個磁碟，每個磁碟2個128K
適合於對資料讀寫延遲比較敏感的檔案。如redo日誌，控制檔案，spfile等

##### 重平衡
從diskgroup中新增或刪除disk時候，將觸發RBAL程式建立rebalance計劃、計算時間開銷.而後、發訊息給ARBn程式（ASM Reblance）處理該請求
ARBn程式對每個extents進行locked, relocated和unlocked操作，執行過程中可以參考v$asm_operation檢視.
ARBn程式的數量由引數ASM_POWER_LIMIT決定,
COD (Continuting Operation Directory)用於記錄rebalances情況
如果rebalance失敗，在重啟instance時候，將從COD讀取記錄，重新啟動rebalance
注意事項：

- rebalance僅僅在diskgroup發生改變時候才進行的，並不是定時執行
如果rebalance執行過程中，server當機，重啟後會自動進行rebalance

- 每個disk的大小必須是相同，如果存在一個小盤，因為rebalance將對每個盤的分配相同比例的空間，可能造成rebalance時候空間不足
如果執行過程中，空閒空間不足，造成rebalance失敗，將出現ORA-15041錯誤，需要再新增disk
如果需要頻繁新增disk，每一次都可能造成資料的頻繁移動，為提高效率，最好批次新增
如果磁碟大小一樣，仍然沒有進行rebalance，需要檢視asm_power_limit

**參考**
- https://www.askmaclean.com/archives/know-oracle-asm-basic-html.html
- https://blog.csdn.net/iteye_4537/article/details/82130124

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