Working of ASM

Oracle ASM  

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.

ASM Cluster File System (ACFS) -

Prior to Oracle 11.2 ASM was used to store database related files, but was not suitable to act as a general purpose file system to store Oracle binaries, core dump files, parameter or trade files. With the introduction of grid infrastructure in Oracle 11.2, ACFS also got introduced as part of grid. ACFS is a POSIX compliant general purpose file system built on top of the ASM Dynamic Volume Manager (ADVM). ADVM provides so called volumes on which the ACFS can be created. An ADVM volume is created as part of an ASM disk group. Volumes are not partitioned which is equivalent to using an LVM in a Linux system.

In addition to using ACFS for shared Oracle Homes, ACFS is used to store application data that previously could not be stored in ASM. For instance, external tables refer to a directory file to load data from the file system to the database. If this directory file exists on the file system on the first node, a user trying to access this external table from the third node will not be able to do it. If we move this file to a directory on the ACFS mount, this problem is solved as the ACFS mounts are shared across all cluster nodes.

Quote for the day

If you want the rainbow, you got to put up with the rain

--Dolly Parton