Oracle ASM Kickoff

Interestingly enough, Oracle's default stripe size is one megabyte. This differs from most disk storage management systems, which often utilize 32K or 64K stripe sizes. Oracle found that one-megabyte stripes on disks provided a very high level of data transfer and best met the needs of disk intensive applications. One can only assume that advancements in disk storage technology have allowed Oracle to access the data in one-megabyte chunks and not drive disk utilization to unacceptable levels.

One of the reasons why we are interested in ASM is its ability to reduce hot spots. A hot spot occurs when one particular area of disk (or disks) are accessed more heavily than others. The heavy I/O concentration acts like a funnel and slows the data transfer to a point where performance degradation occurs.

Oracle documentation states that ASM is able to identify hotspots and move data from one disk to another during off-peak times to prevent them from affecting Oracle application performance. Rest assured that we intend to perform thorough testing on this feature to ensure it works "as advertised." We are interested in when rebalancing occurs, rebalancing on demand, the ability to prevent rebalancing from occurring during specific time-periods and finally, rebalancing's affect on performance.

Administrators provide disk mirroring by creating failure groups. Failure groups define ASM disks that share a common potential failure mechanism. Multiple disks that use the same SCSI controller would be one example. The DBA creates the appropriate number of failure groups to accommodate the data requiring disk fault tolerance. ASM's mirroring capability ranges from the mirroring of individual datafiles to entire disk arrays, providing administrators with a high level of flexibility when creating fault-tolerant disk subsystems.

Administrators can choose from the following mirroring options in ASM:

• External - No mirroring.
• Normal - Data is mirrored on two separate disks. This is the default setting.
• High Redundancy - Data is mirrored on there separate disks providing three-way mirroring capabilities.

ASM requires its own instance, which identifies the various disk groups and files during instance startup. The ASM instance then mounts the disks under its control and creates an extent map, which is passed to the database instances. ASM does not perform the I/O for the database instances; it is only used to manage the various disk groups under its control. ASM is only activated when individual datafiles are created or dropped or disks are added and removed from the disk groups.

We use EMC as our storage system of choice. One of the benefits that ASM provides is the ability for administrators to add disks to diskgroups and remove disks from diskgroups without taking them offline. When disks are added or removed from the disk group, ASM automatically rebalances the files contained in the disk group while the database is open and functioning. In our current configuration, EMC requires us to shut the databases down (for somewhat lengthy time periods) when we need to add or remove disks. Being forced to shut down a database to perform an administrative function is certainly not desirable. Considering that our UNIX, DBA and Disk Administration groups are all involved when we add or remove disk, you can understand our desire to perform these functions while the database is up and functioning. If ASM allows us to do this without forcing us to take an outage, we are all for it.

Oracle Enterprise Manager (OEM) for Oracle10G and the Database Configuration Assisstant (DBCA) have been updated to allow administrators to configure and manage databases using ASM.

Summary
With Oracle data in double-digit terabytes, we are very interested in ASM and are currently kicking off an evaluation project to determine its viability. The ability to have the database pro-actively tune I/O on its own certainly warrants our upcoming evaluation. Does ASM worry me? You bet it does.. Any time I add another product into the mix that has the capability of preventing my databases from running worries me. That's what this upcoming evaluation is all about.