Oracle Database 10g introduced ASM as an alternative to the file system for storing database files.ASM challenges file system management as we know it today by providing an integrated file system and volume manager in the data base kernel.Many DBAs ask the same question: What does ASM buy me? There are many reasons why DBAs, SAs,and SAN administrators should consider ASM. Here are the most importantreasons:
Misconceptions of ASM
Many DBAs have numerous misconceptions about ASM. By revealing “the truth” about ASM,DBAs,SAs, and SAN administrators can make cognitive decisions about implementing ASM. First, ASM does not automatically rebalance the IO.
The rebalance activity of an ASM diskgroup happens only when adding or dropping disks from the diskgroup. Second,ASMLIB is available only for Linux and Windows. ASMLIB is a support library for ASM that simplifies the management and discovery of ASM disks. ASMLIB is not a requirement to implement ASM but is highly recommended.
Third, you can implement ASM without RAC.Although ASM tends to be imple mented with RAC, it performs well in stand-alone non-RACenvironments too.With ASM, Oracle installs and configures a lightweight cluster agent called Cluster Synchronization Services (CSS).More and more companies are adopting ASM in a single instance for ease ofmanageability and performance.
On the same note, DBAs, SAs, and SAN administrators may be unaware of some hidden ASM features.You can leverage ASM mirroring technologies to simulate protection equivalence to mirroring and triple-mirroring technologiesusing normal and failure redundancy.
RMAN and ASM
Knowing RMAN is a must when considering ASM.DBAs who are not intimate with RMAN do not have a choice but to learn and become experts with RMAN.There are numerous ways to transfer files from and to ASM, but believe it or not,nothing compares in performance and ease of maintenance than RMAN.ASM has the option to utilize technologies such as WebDav or FTP in conjunction with XML DB, but this option complicates the environment and supportability.
sysasm Privilege and osasm Group
Oracle Database 11g defines a new privilege called sysasm for the ASM instance.The primary purpose of the sysasm privilege is to manage the gray area of roles and responsibility introduced by ASM.The sysasm privilege separates the roles between the storage admini strator, system administrator, and database administrator.
Eventually the sysdba privilege will be deprecated from the ASM instance. There will be a clear delineation between the RDBMS and ASM credentials.In Oracle Database 11g, the security credentials of sysdba will behave as they did with Oracle Database 10g. In future releases of Oracle, sysdba will be restricted from ASM. Separating the sysasm privilege from the sysdba privilege improves the security model since you are separating the role of the ASM administration from the role of data base administration.
The following message is generated in the alert log file if the sysdba role is used to perform ASM maintenance:
The following code example illustrates the process necessary to grant the sysasm privilege and configure a new user for the sysasm privilege. Let’s first create a user named ckim and grant that user the sysasm privilege:SQL> create user ckim identified by oracle123
You can clearly see that the ckim user has the sysasm privilege granted in the V$PWFILE_USERS view:SQL> select * from v$pwfile_users;
Now, let’s connect as the ckim user created in the earlier example.The user ckim now has privileges to perform sysasm-related tasks.In the following code, user ckim will unmount and remount all the diskgroups in the ASM instance:[oracle@rac11]$ sqlplus ckim/oracle123 as sysasm
In Oracle Database 10g ASM,the sysoper privilege has similar privileges as the sysdba role;however, in Oracle Database 11g ASM, the sysoper privilege has similar restrictions and privilegesenforced on the database side.The following are commands granted to the sysoper privilege:
sysoper is restricted from all the privileges required to create or drop diskgroups or disks such as create diskgroup,add/drop/resize disk, and so on. These privileges still require the sysdba or sysasm privilege.sysoper is restricted from privileges that create or destroy objects in the ASM instance.
Oracle Database 11g Enter prise Manager Database Console provides full support for user management of the ASM instance on the Users tab.This screen lists all the users who have administrative privileges against the ASM instance, as depicted in Figure
Figure ASM user maintenance
In the upper-right corner of the screen, you will notice a button to create users.You will be prompted to enter the username, password, and list of roles to assign, as shown in Figure
Figure ASM Create User screen
The osasm Unix group is a new privileged operating system group available in Oracle Database 11g that complements the osdba and osoper groups.Prior to the installation process, the osasm group can be added to the /etc/group file. Unix accounts that are members of the osasm group can log in as sysasm using OS authentication.
When you add new storage or remove disks from an existing diskgroup, you have to perform a rebalance operation.In a RAC configuration,all the ASM instances must communicate with each other during the rebalance activities to send lock and unlock extent map messages.This communication between the ASM instances degrades the performance of the rebalance activity.
Effective in Oracle Database 11g,what is known as fast rebalance can be performed to eliminate ASM messaging with other ASM instances.Fast rebalance improves the throughput of the rebalance activity.This feature is enabled by using the startup restrict or alter diskgroup …mount restrict command. When the diskgroup is in restricted mode, databases are not allowed to access the datafiles.
In the previous release, one way that DBAs disabled this inter-ASM instance messaging was to shut down the remaining ASM instances during the rebalance activity.
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