ASM is a file system provided by Oracle which aims to simplify the file management of Oracle datafiles, logfiles and control files using a concept of datafile groups. It supports RAC and single instance databases. Within a disk group there are disk which contain the file system for storing data. This data is evenly balanced among the disks in a diskgroup to eliminate hotspots and provided uniform and enhanced performance across the disks. Disks can be added and removed from a diskgroup online while the database continues to access data from the files in the diskgroup. ASM automatically redistributes the contents of data within a diskgroup without the need of any downtime when a disk is added or removed from a diskgroup. Two additional background processes are provided for this. RBAL and ORBn. While ORBn processes to the actual task of moving the extents between disks for balancing the data, RBAL coordinates this activity.The OSMB background process communicates between the Oracle instance and the ASM instance. ASM normal redundancy disk group provides 2 mirroring and High redundancy disk group provides 3 way mirroring. Oracle uses Oracle File Management OMF features to manage files. OMF creates the files in pre defined locations. The file naming is also managed by OMF, thus reducing the administrative overhead of managing a large number of Oracle file. ASM can also coexist with other storage management options like raw disks and third party file systems thus simplifying the integrations of ASM with other systems. The Oracle Enterprise Manager also provides a tool to migrate non ASM file systems to ASM. When creating a new tablespace or a control file or redolog file u can specify the name of a diskgroup instead of an OS file.
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