what is oracle asm

oracle asm refers to "automatic storage management" and is a volume manager that automatically manages disk groups and provides effective data redundancy functions; it is implemented and deployed as a separate Oracle instance. The advantages of asm: 1. Simple configuration, which can maximize the storage resource utilization of database merger; 2. Support BIGFILE files, etc.

The operating environment of this tutorial: Windows 7 system, Oracle 10g version, Dell G3 computer.

1. What is ASM

ASM (Automatic Storage Management, automatic storage management) is a feature in Oracle10g R2 to simplify the management of Oracle databases. A new feature launched is the volume manager provided by Oracle itself. It is mainly used to replace the LVM provided by the operating system. It not only supports single instances, but also has very good support for RAC.

ASM can automatically manage disk groups and provide effective data redundancy functions. After using ASM (Automatic Storage Management), database administrators no longer need to manage and classify thousands of data files in ORACLE, thus simplifying the DBA's workload and greatly improving work efficiency.

Using ASM, storage can be managed as a small number of storage pools called ASM disk groups. Database-related files are assigned to ASM disk groups, and ASM manages the layout and data organization, ensuring optimal performance and protection against storage hardware failures. Using ASM disk groups makes it easy and convenient to consolidate storage and simplify management tasks that previously required DBAs and system administrators. ASM provides the underlying storage management for ASM Cluster File System (ACFS), which centrally stores data outside of the database.

ASM provides platform-independent file system, logical volume management and software RAID services. ASM can support striping and disk mirroring, allowing you to add or remove disks while the database is loaded and automatically balance I/O to remove "hot spots." It also supports direct and asynchronous I/O and uses the Oracle Data Manager API (Simplified I/O System Call Interface) introduced in Oracle9i.

ASM is implemented and deployed as a separate Oracle instance, and it only needs parameter files and does not require any other physical files to start the ASM instance. Only when it is running can it be used by other data access. On the Linux platform, ASM can only be accessed if the OCSSD service (installed by Oracle installer by default) is running.

2. Advantages of using ASM

Advantages:

(1) Connect I/ O is evenly distributed across all available disk drives to prevent hot spots and maximize performance. In ASM, the concept of DiskGroup is similar to the concept of VG in LVM volume group management. Multiple physical disks are also assembled into a disk group, and the data in a disk group is evenly distributed on each physical disk. When adding and deleting physical disks manually, the data will be automatically balanced (ReBalance);

(2) The configuration is simpler and maximizes the storage resource utilization of database merger; data files are stored in DiskGroup (referred to as DG) ), management is relatively simple.

(3) Built-in support for large files and BIGFILE files.

(4) Perform automatic contact redistribution after incrementally adding or removing storage capacity, that is, automatic ReBalance;

(5) Maintain redundant copies of data to improve availability;

(6) Supports 10g and 11g data storage and RAC shared storage management. Relatively speaking, 11g ASM is more stable, and there are some BUGs in 10g.

(7) Support third-party multi-path software; such as powerpath (aix), mutlipath (linux)

(8) Use OMF (Oracle Managed Files) method to manage files.

Disadvantages:

(1) Adding an ASM instance to the database requires ASM management for maintenance, which increases maintenance costs;

(2) ASM is relatively a black box. If something goes wrong, recovery will be difficult.

3. ASM redundancy method

ASM uses a unique mirroring algorithm: it does not mirror disks, but mirrors extents. As a result, to provide continuous protection in the event of a failure, only the space capacity in the disk group is required, rather than the need to prepare a hot spare disk. It is not recommended that users create failure groups of different sizes as this will cause problems when allocating secondary extents. When ASM assigns a file's primary extent to a disk in a disk group, it assigns a mirrored copy of that extent to another disk in the disk group. The primary extents on a given disk will have their own mirror extents on a partner disk in the disk group. ASM ensures that a primary extent and its mirror copy do not reside in the same failure group. Disk group redundancy can take the following forms: normal redundancy (default redundancy) using two-way mirror files (requiring at least two failure groups) and using three-way mirroring (requiring at least three failure groups) to provide a higher degree of protection High redundancy. Once a disk group is created, its redundancy level cannot be changed. In order to change the redundancy of a disk group, another disk group with appropriate redundancy must be created, and then the data files must be moved to this newly created disk group using RMAN restore or DBMS_FILE_TRANSFER.

Three different redundancy methods are as follows:

　 (1) External redundancy (external redundancy) --Commonly used

means that Oracle does not help you manage the image, and the function is implemented by an external storage system, such as through RAID technology; the effective disk space is the sum of the spaces of all disk devices.

　(2)Default redundancy (normal redundancy)

Indicates that Oracle provides 2 mirrors to protect data. The effective disk space is 1/2 of the sum of the sizes of all disk devices (most used)

　(3) High redundancy

Indicates that Oracle provides three mirrors to protect data to improve performance and data security. At least three disks (three failure groups) are required ); the effective disk space is 1/3 of the sum of the sizes of all disk devices. Although the redundancy level is high, the hardware cost is also the highest.

4. ASM process

In addition to the traditional DBWR, LGWR, CKPT, SMON, PMON and other processes, the ASM instance also includes the following four new processes: Background process:

　(1) RBAL: Responsible for coordinating the rebalancing activities of the disk group (responsible for disk group balancing)

　(2) ARB0-ARBn: Many such processes can exist at the same time , they are respectively named ARB0, ARB1, and so on, to perform the actual rebalancing allocation unit movement process.

　(3) GMON: used for ASM disk group monitoring

　(4) O0nn 01-10: This group of processes establishes connections to ASM instances, and some long-term operations such as creating data files ,RDBMS will send information to ASM through these processes

ASMB connects to the foreground process of the ASM instance and periodically checks the health status of the two instances. Each database instance can only be connected to one ASM instance at the same time, so the database will only have one ASMB background process. If there are multiple database instances on a node, they can only share one ASM instance.

RBAL is used to make global calls to open disks in a certain disk group. The ASMB process communicates with the node's CSS daemon and receives file interval mapping information from the ASM instance. ASMB is also responsible for providing I/O statistics for ASM instances

CSS cluster synchronization service. To use ASM, you must ensure that the CSS cluster synchronization service is running. CSS is responsible for synchronization between ASM instances and database instances.

Note:

The ASM instance must be started before the database instance, run synchronously with the database instance, and be closed later than the database instance. The relationship between ASM instances and database instances can be 1:1 or 1:n. If it is 1:n, it is better to install a separate ASM_HOME for ASM.

5. ASMSupported file types

　ASMSupports automatic database file management such as datafiles, logfiles, control files, archivelogs, RMAN backup sets, etc.

Generally speaking, a single-instance database using ASM consists of two DGs. One is datadg and the other is fradg. Datadg mainly stores data files, while fradg (flash recovery area) stores archivelog files, control files, backup files, etc.

6. Correspondence between ASM instance and database instance

7. Cluster ASM architecture

8. Related views

X$KFDSK_STAT,X$KFKIDList disks and Its usage metric informationX$KFFILListing ASM files also includes metadata informationX$KFALSLists ASM aliases, files and directoriesX$KFTMTAX$KFNCLX$KFGMGX$KFKLIBX$KFDPARTNERX$KFFXPX$KFDATX$KFBHX$KFCCEX$KFENV(new in 11g)##V$ASM_DISK_IOSTAT(new in 11g)X$KFNSDSKIOST(new in 11g)　X $KFDFS(new in 11g)　

View name	X$Base table name	Description
V$ASM_DISKGROUP	X $KFGRP	Implement disk discovery and list disk groups
V$ASM_DISKGROUP_STAT	X$KFGRP_STAT	Display disk group status
V$ASM_DISK	X$KFDSK,	V$ASM_DISK_STAT
		V$ASM_FILE
		V$ASM_ALIAS
		##V$ASM_TEMPLATE
List available templates and their properties		V$ASM_CLIENT
List DB instances linked to ASM		V$ASM_OPERATION
List rebalancing operations
Available ASMLIB paths
List Disk-partners relationships
extent map of all ASM files
extent list of all ASM Disk
Describe ASM cache
Linked list of ASM block		V$ASM_ATTRIBUTE(new in 11g)
Asm attribute, the X$ base The table also shows some hidden attributes
I/O statistics
	##	X $KFDDD(new in 11g)
		X $KFGBRB(new in 11g)
	##	X $KFMDGRP(new in 11g)
	X$KFCLLE(new in 11g)
	X$KFVOL(new in 11g)
	X$KFVOLSTAT(new in 11g)
	X$KFVOFS(new in 11g)
	X$KFVOFSV(new in 11g)

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