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    Introduction to ASM

    The article explains about what is the ASM in Oracle database. ASM stands for Automatic storage management.

    ASM is that the acronym for Automatic Storage Management , which is an Oracle product that acts as a logical volume manager and / or filing system to deal with the files of the databases of an equivalent company. during this way, when creating an Oracle database you’ll prefer to have the storage for your files be ASM rather than a traditional filing system .

    But what’s actually ASM? ASM is an Oracle instance, with its SGA and its background processes, that interact with the instances of the “client” database to supply it with the required information in order that it can read and write to the info files ( datafiles, control files, redo logs, archived logs, flashback logs, spfiles, RMAN backups, Incremental tracking bitmaps and datapumps dumpfiles ).

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    It’s important to notice that ASM doesn’t serve the info blocks to the “client” instances, but they’re those that access the disks to read or write. All ASM does is provide you with meta information about where to read or write the info .

    The ASM instance, as an instance that it’s , rather than mounting a database, mounts disk groups , which are groups of ASM disks that are used transparently together . to ascertain it more easily, it’s as if the disk groups were tablespaces and therefore the ASM disks , datafiles.

    The ASM disks are often LUNs, physical disks, partitions physical disks or logical volumes albeit Oracle doesn’t recommend this because there would be overlapping of functions between the quantity manager installed and therefore the ASM itself.

    ASM can serve both one database and a RAC (Real Application Cluster), which actually is where it’s most ordinarily installed. In a RAC, ASM runs on each node and communicates with the remainder of the RAC’s ASMs through the interconnection, but let’s not stray within the RAC and ASM configuration.

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    Returning to the term disk groupAs we’ve said, a enter the “client” databases is assigned to one disk group (a file is merely related to a disk group , but a disk group has many files). The file extensions are homogeneously distributed among the ASM disks that structure the disk group, thus balancing the workload of the disks and improving the general performance of the system. Typically, instances use only 2 disk groups to host all their files, one for data and one for fast recovery area .

    Another feature of ASM is that it allows you to hot add or remove ASM disks by redistributing the contents of the disk group among the prevailing ASM disks . If you would like to delete an ASM disk , first ASM has got to redistribute the info on the disk to be deleted among the remainder . If an ASM disk is added , ASM redistributes the contents of the disk group among all the ASM disks , since there’s now another . the foremost interesting thing is that this operation is completed by hot ASM allowing the instances to access their files while the rebalancing is happening .

    In addition to having the ability to try to to redistribution (striping), ASM can do redundancy (mirroring) of the info . to try to to this, when creating a disk group , the associated failure group is indicated and ASM distributes each copy to a special fail group . the thought of the fail group is to require the ASM disks from the disk group and make different groups, the so-called fail groups , in order that each group is shielded from the failures that the opposite may have. Now, counting on the sort of contingency that we would like to guard ourselves, we’ll have one group or another. for instance , a disk group with two ASM disks we will define it with two fail groups , each with one disk.

    Thus, the failure of a disk wouldn’t leave me without data (I have another replica of the affected one). But if there’s a failure of the control , which is common for both, we might be left without access to the info . Then you’d need to choose different disks on different controllers. What if the hardware where both controllers reside breaks down? Okay, so we use absolutely different storage devices. What if the CPD catches fire? …. then we will still infinity.

    Regarding redundancy, data also can be replicated by doubling or tripling them. When the disk group is defined, it’s indicated if the replication are going to be EXTERNAL (without ASM replication), NORMAL (the data is duplicated) or HIGH (the data is tripled). during a NORMAL configuration, two fail groups are going to be necessary and in HIGH, three fail groups .

    Finally and to end this brief introduction, just say ASM are often fully integrated with OMF (Oracle Managed Files) by writing only the name of the disk group where the complete name of the file should be put.

    I will continue writing things about ASM, this is often only a quick introduction to urge some notions of what it’s and what it’s for.

    When you want to make a strong Oracle DBA career then you should be aware of database services and other database technology. Without having knowledge of Oracle internals, Oracle performance tuning, and skill of Oracle database troubleshooting you can’t be an Oracle DBA expert.

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