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This document describes how to prepare your storage sub-system before you configure Automatic Storage Management (ASM). When preparing your storage to use ASM, first determine the storage option for your system and then prepare the disk storage for the specific operating system environment.
A) You can create an ASM diskgroup using one of the following storage resources:
1) Raw disk partition—A raw partition can be the entire disk drive or a section of a disk drive. However, the ASM disk cannot be in a partition that includes the partition table because the partition table can be overwritten.
2) Logical unit numbers (LUNs)—Using hardware RAID functionality to create LUNs is a recommended approach. Storage hardware RAID 0+1 or RAID5, and other RAID configurations, can be provided to ASM as ASM disks.
3) Raw logical volumes (LVM)—LVMs are supported in less complicated configurations where an LVM is mapped to a LUN, or an LVM uses disks or raw partitions. LVM configurations are not recommended by Oracle because they create a duplication of functionality. Oracle also does not recommended using LVMs for mirroring because ASM already provides mirroring.
4) NFS files—NFS files are suitable for testing, but are not a recommended configuration for production environments. Using NFS files with ASM duplicates ASM functionality. Though, NetApp as an NFS vendor certifies its product with ASM. So there are customers using NFS and ASM together.
B) The procedures for preparing storage resources for ASM are:
1) Identify or create the storage devices for ASM by identifying all of the storage resource device names that you can use to create an ASM disk group. For example, on Linux systems, device names are typically presented from the /dev directory with the /dev/device_name_identifier name syntax.
2) Change the ownership and the permissions on storage device resources. For example, the following steps are required on Linux systems:
2.1) Change the user and group ownership of devices to oracle:dba
2.2) Change the device permissions to read/write
2.3) On older Linux versions, you must configure raw device binding
After you have configured ASM, ensure that disk discovery has been configured correctly by setting the ASM_DISKSTRING initialization parameter.
Note: Setting the ownership to oracle:dba is just one example that corresponds to the default settings. A non-default 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 ASM instance, which is defined at installation.
C) Recommendations for Storage Preparation. The following are guidelines for preparing storage for use with ASM:
1) Configure two disk groups, one for the datafile and the other for the Flash Recovery Area. For availability purposes, one is used as a backup for the other.
2) Ensure that LUNs, which are disk drives of partitions, that ASM disk groups use have similar storage performance and availability characteristics. In storage configurations with mixed speed drives, such as 10K and 15K RPM, I/O distribution is constrained by the slowest speed drive.
3) Be aware that ASM data distribution policy is capacity-based. LUNs provided to ASM have the same capacity for each disk group to avoid an imbalance.
4) Use the storage array hardware RAID 1 mirroring protection when possible to reduce the mirroring overhead on the server. Use 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 ASM mirroring in configurations when mirroring between geographically-separated sites over a storage interface.
Hardware RAID 1 in some lower-cost storage products is inefficient and degrades the performance of the array. ASM redundancy delivers improved performance in lower-cost storage products.
5) Maximize the number of disks in a disk group for maximum data distribution and higher I/O bandwidth.
6) Create LUNs using the outside half of disk drives for higher performance. If possible, use small disks with the highest RPM.
7) Create large LUNs to reduce LUN management overhead.
8) Minimize I/O contention between ASM disks and other applications by dedicating disks to ASM disk groups for those disks that are not shared with other applications.
9) Choose a hardware RAID stripe size that is a power of 2 and less than or equal to the size of the ASM allocation unit.
10) Avoid using a Logical Volume Manager (LVM) because an LVM would be redundant. However, thereare situations where certain multipathing or third party cluster solutions require an LVM. In these situations, use the LVM to represent a single LUN without striping or mirroring to minimize the performance impact.
11) For Linux, when possible, use the Oracle ASMLIB feature to address device naming and permission persistency.
12) ASMLIB provides an alternative interface for the ASM-enabled kernel to discover and access block devices. ASMLIB provides storage and operating system vendors the opportunity to supply extended storage-related features. These features provide benefits such as improved performance and greater data integrity.
From Oracle
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