2 Considerations for Oracle ASM Storage

This chapter discusses some points to consider about the storage subsystem before you configure Oracle Automatic Storage Management (Oracle ASM). When preparing your storage to use Oracle ASM, first determine the storage option for your system and then prepare the disk storage for your specific operating system environment.

When configuring your system's storage, you must consider the initial capacity of the system and your plans for future growth. Oracle ASM simplifies the task of accommodating growth; however, your growth plans can affect choices, such as the size of the Oracle ASM disks. You must also consider that I/O performance depends on the interconnect between the storage and host, not just the storage disks. As you scale up the number of nodes in a cluster, you must also scale up the storage subsystem.

This chapter contains the following topics:

Storage Resources for Disk Groups
Oracle ASM and Multipathing
Recommendations for Storage Preparation

Storage Resources for Disk Groups

You can create an Oracle ASM disk group using one of the following storage resources:

Disk Partition

A disk partition can be the entire disk drive or a section of a disk drive. However, the Oracle ASM disk cannot be in a partition that includes the partition table because the partition table would be overwritten.
Logical Unit Number (LUN)

A LUN is a disk presented to a computer system by a storage array. Oracle recommends that you user hardware RAID functionality to create LUNs. Storage hardware RAID 0+1 or RAID5, and other RAID configurations, can be provided to Oracle ASM as Oracle ASM disks.
Logical Volume

A logical volume is supported in less complicated configurations where a logical volume is mapped to a LUN, or a logical volume uses disks or raw partitions. Logical volume configurations are not recommended by Oracle because they create a duplication of functionality. Oracle also does not recommended using logical volume managers for mirroring because Oracle ASM provides mirroring.
Network File System (NFS)

Oracle ASM supports NFS files as Oracle ASM disks. Oracle Database has built-in support for the network file system (NFS) and does not depend on operating system (OS) support for NFS. Although NFS and Oracle ASM have overlapping functionality, Oracle ASM can load balance or mirror across NFS files. Oracle ASM also supports Oracle Direct NFS (dNFS) client that integrates the NFS client functionality directly in the Oracle Database software stack.

See Also:
Oracle Grid Infrastructure Installation Guide for your operating system for information about Oracle Direct NFS

Notes:

Oracle ASM Dynamic Volume Manager (Oracle ADVM) does not support NFS.
Using Database Configuration Assistant (DBCA) or the Oracle Universal Installer (OUI) to store Oracle Clusterware or Oracle Database files directly on block or raw devices is not supported. Performing a new installation using block or raw devices is not allowed.

The procedures for preparing storage resources for Oracle ASM are:

Identify or create the storage devices for Oracle ASM by identifying all of the storage resource device names that you can use to create an Oracle ASM disk group. For example, on Linux systems without ASMLIB, device names are typically presented from the /dev directory with the /dev/device_name_identifier name syntax.
Change the ownership and the permissions on storage device resources.

For example, the following steps are required on Linux systems:
- Change the user and group ownership of devices, such as grid:asmadmin
  
  For information about Oracle ASM privileges, see "About Privileges for Oracle ASM".
- Change the device permissions to read/write

After you have configured Oracle ASM, ensure that disk discovery has been configured correctly by setting the ASM_DISKSTRING initialization parameter. For information about the ASM_DISKSTRING parameter, see "ASM_DISKSTRING".

Note:

Setting the ownership to oracle:dba is one example that corresponds to the default settings. A nondefault installation may require different settings. In general, the owner of the disk devices should be the same as the owner of the Oracle binary. The group ownership should be OSDBA of the Oracle ASM instance, which is defined at installation. For information about Oracle ASM privileges, see "About Privileges for Oracle ASM".

For detailed information about preparing disks for an Oracle ASM installation, refer to your platform-specific installation guide for Oracle Database, Oracle Clusterware, and Oracle Real Application Clusters (Oracle RAC).

Oracle ASM and Multipathing

Multipathing solutions provide failover by using redundant physical path components. These components include adapters, cables, and switches that reside between the server and the storage subsystem. If one or more of these components fails, then applications can still access their data, eliminating a single point of failure with the Storage Area Network (SAN), Host Bus Adapter, interface cable, or host port on a multiported storage array.

Multipathing is a software technology implemented at the operating system device driver level. Multipathing creates a pseudo device to facilitate the sharing and balancing of I/O operations across all of the available I/O paths. Multipathing also improves system performance by distributing the I/O load across all available paths, providing a higher level of data availability through automatic failover and failback.

Although Oracle ASM is not designed with multipathing functionality, Oracle ASM does operate with multipathing technologies. Multipathing technologies are available from many sources. Storage vendors offer multipathing products to support their specific storage products, while software vendors usually develop multipathing products to support several server platforms and storage products.

Recommendations for Storage Preparation

The following are guidelines for preparing storage for use with Oracle ASM:

Configure two disk groups, one for the data file and the other for the fast recovery area.
See Also:
- Oracle Database Backup and Recovery User's Guide for information about configuring the fast recovery area
- Oracle Database Administrator's Guide for information about specifying a fast recovery area
A minimum of four LUNs (Oracle ASM disks) of equal size and performance are recommended for each disk group.
Ensure that all Oracle ASM disks in a disk group use have similar storage performance and availability characteristics. In storage configurations with mixed speed drives, such as 10K and 15K RPM, I/O performance is constrained by the slowest speed drive.
Oracle ASM data distribution policy is capacity-based. Ensure that Oracle ASM disks in a disk group have the same capacity to maintain balance.
Create external redundancy disk groups when using high-end storage arrays. High-end storage arrays generally provide hardware RAID protection. Use Oracle ASM mirroring redundancy in the absence of a hardware RAID, or when you need host-based volume management functionality, such as mirroring across storage systems. You can use Oracle ASM mirroring in configurations when mirroring between geographically-separated sites (extended clusters).
Minimize I/O contention between Oracle ASM disks and other applications by dedicating disks in Oracle ASM disk groups.
Choose a hardware RAID stripe size that is a power of 2 and less than or equal to the size of the Oracle ASM allocation unit.
For Linux, use the Oracle ASMLIB feature to provide consistent device naming and permission persistency.
See Also:
- The Oracle ASM page on the Oracle Technology Network Web site at http://www.oracle.com/technology/products/database/asm for more information about ASMLIB
- The Oracle ASMLIB page on the Oracle Technology Network Web site at http://www.oracle.com/technology/tech/linux/asmlib to download ASMLIB