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Rebalance Disk Group + ASM Disk Discovery

简介: Disk discovery is the mechanism used to find the operating system names for disks Oracle ASM can access.

Disk discovery is the mechanism used to find the operating system names for disks Oracle ASM can access. It is used tofind all the disks that comprise a disk group to be 
mounted, the disks an administrator wants to add to a disk group,
or the disks the administrator might consider adding to a disk group. 

How A Disk is Discovered
When an Oracle ASM instance is initialized, Oracle ASM discovers and examines the contents of all of the disks that are in the paths that you designated with values in the 
ASM_DISKSTRING initialization parameter. Disk discovery also occurs when you:
■ Run the following SQL statements
– Mount a disk group with ALTER DISKGROUP ... MOUNT
– Online a disk with ALTER DISKGROUP ... ONLINE DISK 
– Add a disk to a disk group with CREATE or ALTER DISKGROUP...ADD DISK
– Resize a disk in a disk group with ALTER DISKGROUP...RESIZE DISK 
– Query with SELECT ... FROM V$ASM_DISKGROUP or V$ASM_DISK views
■ Run Oracle Enterprise Manager or Oracle ASM Configuration Assistant (ASMCA) operations that invoke the SQL statements previously listed
■ Run ASMCMD commands that perform the same operations as the SQL statements previously listed

After Oracle ASM successfully discovers a disk, the disk appears in the V$ASM_DISK view. 

Disks that belong to a disk group, that is, disks that have a disk group name in the disk header, show a header status of MEMBER. 

Disks that were discovered, but that have not yet been assigned to a disk group, have a status of either CANDIDATE or PROVISIONED. 

Disks that previously belonged to a disk group and were dropped cleanly from the disk group have a status of FORMER

Disk Discovery Rules
The rules for discovering Oracle ASM disks are as follows:
■ Oracle ASM can discover up to 10,000 disks. That is, if more than 10,000 disks match the ASM_DISKSTRING initialization parameter, then Oracle ASM discovers 
only the first 10,000. (只支持10000个磁盘)

■ Oracle ASM only discovers disk partitions. Oracle ASM does not discover partitions that include the partition table.

■ From the perspective of the installation, candidate disks are those that have the CANDIDATE, PROVISIONED, or FORMER header status. These disks with a CANDIDATE, PROVISIONED, or FORMER status can be added to Oracle ASM disk groups without using the FORCE flag.

■ When adding a disk, the FORCE option must be used if Oracle ASM recognizes that the disk was managed by Oracle. Such a disk appears in the V$ASM_DISK view with 
a status of FOREIGN. In this case, you can only add the disk to a disk group by 
using the FORCE keyword.
■ MEMBER disks can usually be added to a disk group by specifying the FORCE flag, if 
the disks are not part of a currently mounted disk group.