Understanding Storage Mapping

Access to mapping information is important since it allows for a detailed understanding of the storage hierarchy in which files reside, information that is critical for effectively evaluating I/O performance.

Mapping files to their underlying device is straightforward when datafiles are created directly on a raw device. With the introduction of host-based volume managers and sophisticated storage subsystems that provide RAID features, however, mapping files to physical devices has become more difficult.

With the VERITAS Storage Foundation for Oracle Storage Mapping option, you can map datafiles to physical devices. VERITAS Storage Mapping relies on VERITAS Federated Mapping Service (VxMS), a library that assists in the development of distributed SAN applications that must share information about the physical location of files and volumes on a disk.

The VERITAS Storage Mapping option supports Oracle's set of storage APIs called Oracle Mapping ("ORAMAP" for short) that lets Oracle determine the mapping information for files and devices.

Oracle provides a set of dynamic performance views (V$ views) that shows the complete mapping of a file to intermediate layers of logical volumes and physical devices. These views enable you to locate the exact disk on which any specific block of a file resides. You can use these mappings, along with device statistics, to evaluate I/O performance.

The VERITAS Storage Foundation for Oracle Storage Mapping option supports a wide range of storage devices and allows for "deep mapping" into EMC arrays. Deep mapping information identifies the physical disks that comprise each LUN and the hardware RAID information for the LUNs.

You can view storage mapping topology information and I/O statistics using:

• The vxstorage_stats command. This command displays the complete I/O topology mapping of specific datafiles through intermediate layers like logical volumes down to actual physical devices.
• The dbed_analyzer command. This command retrieves tablespace-to-physical disk mapping information for all the datafiles in a specified database. It also provides information about the amount of disk space being used by a tablespace.
• The VERITAS Storage Foundation for Oracle GUI. The VERITAS Storage Foundation for Oracle GUI performs file mapping and displays both storage mapping topology information and I/O statistics.

In addition, you can also use the Oracle Enterprise Manager GUI to display storage mapping information after file mapping has occurred. Oracle Enterprise Manager does not display I/O statistics information. Unlike the information displayed in the VERITAS Storage Foundation for Oracle GUI, the information displayed in Oracle Enterprise Manager may be "stale," that is, it may not be the latest information.