ORACLE ASM中的几个概念
一.ASM磁盘
下面是对ASM磁盘的英文解释：
Oracle ASM disks are the storage devices that are provisioned to Oracle ASM disk groups. Examples of Oracle ASM disks include:
A disk or partition from a storage array
An entire disk or the partitions of a disk
Logical volumes
Network-attached files (NFS)
When you add a disk to a disk group, you can assign an Oracle ASM disk name or Oracle ASM assigns the Oracle ASM disk name automatically. This name is different from the path
name ud by the operating system. In a cluster, a disk may be assigned different operating system device names on different nodes, but the disk has the same Oracle ASM disk name on all of the nodes. In a cluster, an Oracle ASM disk must be accessible from all of the instances that share the disk group.
Oracle ASM spreads the files proportionally across all of the disks in the disk group. This allocation pattern maintains every disk at the same capacity level and ensures that all of the disks in a disk group have the same I/O load. Becau Oracle ASM load balances among all of the disks in a disk group, different Oracle ASM disks should not share the same physical drive.
二.ASM磁盘组
A disk group consists of multiple disks and is the fundamental object that Oracle ASM manages. Each disk group contains the metadata that is required for the management of space in the disk group. Disk group components include disks, files, and allocation units.
三.ASM故障组
它是用来存储数据的镜像副本的，它是磁盘组的一个子集，通过显式的创建语句（如下所示）更便于理解故障组的位置。
create diskgroup dgA normal redundancy
failgroup fg1 disk 'ORCL:VOL01','ORCL:VOL02'
failgroup fg2 disk 'ORCL:VOL03','ORCL:VOL04'
;
下面是故障组的英文解释：
Failure groups are ud to store mirror copies of data. When Oracle ASM allocates an extent for a normal redundancy file, Oracle ASM allocates a primary copy and a condary copy. Oracle ASM choos the disk on which to store the condary copy so that it is in a different failure group than the primary copy. Each copy is on a disk in a different failure group so that the simultaneous failure of all disks in a failure group does not result in data loss.
A failure group is a subt of the disks in a disk group, which could fail at the same time becau they share hardware. The failure of common hardware must be tolerated. Four drives that are in a single removable tray of a large JBOD (Just a Bunch of Disks) array should be in the same failure group becau the tray could be removed making all four drives fail at the same time. Drives in the same cabinet could be in multiple failure groups if the cabinet has redundant power and cooling so that it is not necessary to protect against failure of the entire cabinet. However, Oracle ASM mirroring is not intended to protect against a fire in the computer room that destroys the entire cabinet.
There are always failure groups even if they are not explicitly created. If you do not specify a failure group for a disk, then Oracle automatically creates a new failure group containing just that disk, except for disk groups containing disks on Oracle Exadata cells.
A normal redundancy disk group must contain at least two failure groups. A high redundancy disk group must contain at least three failure groups. However, Oracle recommends using veral failure groups. A small number of failure groups, or failure gro
ups of uneven capacity, can create allocation problems that prevent full u of all of the available storage.
四.AU(Allocation Units分配单元)
它是ASM磁盘组的最基本的逻辑管理单元，在创建ASM磁盘组的时候指定，创建之后不可更改；可变的是extents，按照规律extent能够实现可变大小，详见后面对extent的说明。
下面是AU的英文解释：
Every Oracle ASM disk is divided into allocation units (AU). An allocation unit is the fundamental unit of allocation within a disk group. A file extent consists of one or more allocation units. An Oracle ASM file consists of one or more file extents.
When you create a disk group, you can t the Oracle ASM allocation unit size with the AU_SIZE disk group attribute. The values can be 1, 2, 4, 8, 16, 32, or 64 MB, depending on the specific disk group compatibility level. Larger AU sizes typically provide performance advantages for data warehou applications that u large quential reads.
Oracle recommends that the allocation unit (AU) size for a disk group be t to 4 megabytes (MB). In addition to this AU size recommendation, the operating system (OS) I/O size should be t to the largest possible size.
Some benefits of a 4 MB allocation unit are:
1.Incread I/O through the I/O subsystem if the I/O size is incread to the AU size.
2.Reduced SGA size to manage the extent maps in the databa instance.
3.Faster datafile initialization if the I/O size is incread to the AU size.
4.Incread file size limits.
5.Reduced databa open time.
The allocation unit size is specified with the disk group attribute AU_SIZE. The AU size cannot be altered after a disk group is created. Example 4-1 shows how the AU_SIZE is specified with the CREATE DISKGROUP SQL statement.
五.Extents
下面是对extents的英文解释：
