Mapping Files to Physical Devices - Oracle 10g

In an environment where datafiles are simply file system files or are created directly on a raw device, it is relatively straight forward to see the association between a tablespace and the underlying device. Oracle Database provides views, such as DBA_TABLESPACES, DBA_DATA_FILES, and V$DATAFILE, that provide a mapping of files onto devices. These mappings, along with device statistics can be used to evaluate I/O performance.

However, with the introduction of host based Logical VolumeManagers (LVM), and sophisticated storage subsystems that provide RAID (Redundant Array of Inexpensive Disks) features, it is not easy to determine file to device mapping. This poses a problem because it becomes difficult to determine your "hottest" files when they are hidden behind a "black box". This section presents the Oracle Database approach to resolving this problem.

The following topics are contained in this section:

  • Overview of Oracle Database File Mapping Interface
  • How the Oracle Database File Mapping Interface Works
  • Using the Oracle Database File Mapping Interface
  • File Mapping Examples

Overview of Oracle Database File Mapping Interface

To acquire an understanding of I/O performance, one must have detailed knowledge of the storage hierarchy in which files reside. Oracle Database provides a mechanism to show a complete mapping of a file to intermediate layers of logical volumes to actual physical devices. This is accomplished though a set of dynamic performance views (V$ views). Using these views, you can locate the exact disk on which any block of a file resides.

To build these views, storage vendors must provide mapping libraries that are responsible for mapping their particular I/O stack elements. The database communicates with these libraries through an external non-Oracle Database process that is spawned by a background process called FMON. FMON is responsible for managing the mapping information. Oracle provides a PL/SQL package, DBMS_ STORAGE_MAP, that you use to invoke mapping operations that populate the mapping views.

How the Oracle Database File Mapping Interface Works

This section describes the components of the Oracle Database file mapping interface and how the interface works. It contains the following topics:

  • Components of File Mapping
  • Mapping Structures
  • Example of Mapping Structures
  • Configuration ID

Components of File Mapping

The following figure shows the components of the file mapping mechanism.

Components of File Mapping

Components of File Mapping

The following sections briefly describes these components and how they work together to populate the mapping views:

  • FMON
  • External Process (FMPUTL)
  • Mapping Libraries

FMON FMON is a background process started by the database whenever the FILE_MAPPING initialization parameter is set to TRUE. FMON is responsible for:

  • Building mapping information, which is stored in the SGA. This information is composed of the following structures:
  • Files
  • File system extents
  • Elements
  • Subelements
  • Refreshing mapping information when a change occurs because of:
  • Changes to datafiles (size)
  • Addition or deletion of datafiles
  • Changes to the storage configuration (not frequent)
  • Saving mapping information in the data dictionary to maintain a view of the information that is persistent across startup and shutdown operations
  • Restoring mapping information into the SGA at instance startup. This avoids the need for a potentially expensive complete rebuild of the mapping information on every instance startup.

You help control this mapping using procedures that are invoked with the DBMS_STORAGE_MAP package.

External Process (FMPUTL)

FMON spawns an external non-Oracle Database process called FMPUTL, that communicates directly with the vendor supplied mapping libraries. This process obtains the mapping information through all levels of the I/O stack, assuming that mapping libraries exist for all levels. On some platforms the external process requires that the SETUID bit is set to ON because root privileges are needed to map through all levels of the I/O mapping stack.

The external process is responsible for discovering the mapping libraries and dynamically loading them into its address space.

Mapping Libraries

Oracle Database uses mapping libraries to discover mapping information for the elements that are owned by a particular mapping library. Through these mapping libraries information about individual I/O stack elements is communicated. This information is used to populate dynamic performance views that can be queried by users.

Mapping libraries need to exist for all levels of the stack for the mapping to be complete, and different libraries may own their own parts of the I/O mapping stack. For example, a VERITAS VxVM library would own the stack elements related to the VERITAS Volume Manager, and an EMC library would own all EMC storage specific layers of the I/O mapping stack.

Mapping libraries are vendor supplied. However, Oracle currently supplies a mapping library for EMC storage. The mapping libraries available to a database server are identified in a special file named filemap.ora.

Mapping Structures

The mapping structures and the Oracle Database representation of these structures are described in this section. You will need to understand this information in order to interpret the information in the mapping views. The following are the primary structures that compose the mapping information:

  • Files

    A file mapping structure provides a set of attributes for a file, including file size, number of file system extents that the file is composed of, and the file type.

  • File system extents

    A file system extent mapping structure describes a contiguous chunk of blocks residing on one element. This includes the device offset, the extent size, the file offset, the type (data or parity), and the name of the element where the extent resides.

  • Elements

    An element mapping structure is the abstract mapping structure that describes a storage component within the I/O stack. Elements may be mirrors, stripes, partitions, RAID5, concatenated elements, and disks. These structures are the mapping building blocks.

  • Subelements

    A subelement mapping structure describes the link between an element and the next elements in the I/O mapping stack. This structure contains the subelement number, size, the element name where the subelement exists, and the element offset.

All of these mapping structures are illustrated in the following example.

Example of Mapping Structures

Consider an Oracle Database which is composed of two data files X and Y. Both files X and Y reside on a file system mounted on volume A. File X is composed of two extents while file Y is composed of only one extent.

The two extents of File X and the one extent of File Y both map to Element A. Element A is striped to Elements B and C. Element A maps to Elements B and C by way of Subelements B0 and C1, respectively.

Element B is a partition of Element D (a physical disk), and is mapped to Element D by way of subelement D0.

Element C is mirrored over Elements E and F (both physical disks), and is mirrored to those physical disks by way of Subelements E0 and F1, respectively.

Illustration of Mapping Structures

Illustration of Mapping Structures

Note that the mapping structures represented are sufficient to describe the entire mapping information for the Oracle Database instance and consequently to map every logical block within the file into a (element name, element offset) tuple (or more in case of mirroring) at each level within the I/O stack.

Configuration ID

The configuration ID captures the version information associated with elements or files. The vendor library provides the configuration ID and updates it whenever a change occurs. Without a configuration ID, there is no way for the database to tell whether the mapping has changed.

There are two kinds of configuration IDs:

  • Persistent

    These configuration IDs are persistent across instance shutdown

  • nonpersistent

    The configuration IDs are not persistent across instance shutdown. The database is only capable of refreshing the mapping information while the instance is up.

Using the Oracle Database File Mapping Interface

This section discusses how to use the Oracle Database file mapping interface. It contains the following topics:

  • Enabling File Mapping
  • Using the DBMS_STORAGE_MAP Package
  • Obtaining Information from the File Mapping Views

Enabling File Mapping

The following steps enable the file mapping feature:

  1. Ensure that a valid filemap.ora file exists in the $ORACLE_HOME/rdbms/filemap/etc directory.

    The filemap.ora file is the configuration file that describes all of the available mapping libraries. FMON requires that a filemap.ora file exists and that it points to a valid path to mapping libraries. Otherwise, it will not start successfully.

    The following row needs to be included in filemap.ora for each library :

    lib=vendor_name:mapping_library_path where:
    • vendor_name should be Oracle for the EMC Symmetric library
    • mapping_library_path is the full path of the mapping library

    Note that the ordering of the libraries in this file is extremely important. The libraries are queried based on their order in the configuration file.

    The file mapping service can be even started even if no mapping libraries are available. The filemap.ora file still needs to be present even though it is empty. In this case, the mapping service is constrained in the sense that new mapping information cannot be discovered. Only restore and drop operationsare allowed in such a configuration.

  2. Set the FILE_MAPPING initialization parameter to TRUE.

    The instance does not have to be shut down to set this parameter. It can be set using an ALTER SYSTEM statement.

  3. Invoke the appropriate DBMS_STORAGE_MAP mapping procedure. You have two options:
    • I a cold startup scenario, the Oracle Database is just started and no mapping operation has been invoked yet. You execute the DBMS_STORAGE_MAP.MAP_ALL procedure to build the mapping information for the entire I/O subsystem associated with the database.
    • In a warm start scenario where the mapping information is already built, you have the option to invoke the DBMS_STORAGE_MAP.MAP_SAVE procedure to save the mapping information in the data dictionary. (Note that this procedure is invoked in DBMS_STORAGE_MAP.MAP_ALL() by default.) This forces all of the mapping information in the SGA to be flushed to disk.

    Once you restart the database, use DBMS_STORAGE_MAP.RESTORE() to restore the mapping information into the SGA. If needed, DBMS_STORAGE_MAP.MAP_ALL() can be called to refresh the mapping information.

Using the DBMS_STORAGE_MAP Package

The DBMS_STORAGE_MAP package enables you to control the mapping operations. The various procedures available to you are described in the following table.

Using the DBMS_STORAGE_MAP Package

Obtaining Information from the File Mapping Views

Mapping information generated by DBMS_STORAGE_MAP package is captured in dynami performance views. Brief descriptions of these views are presented here.

Obtaining Information from the File Mapping Views

However, the information generated by the DBMS_STORAGE_MAP.MAP_OBJECT procedure is captured in a global temporary table named MAP_OBJECT. This table displays the hierarchical arrangement of storage containers for objects. Each row in the table represents a level in the hierarchy. A description of the MAP_OBJECT table follows.


File Mapping Examples

The following examples illustrates some of the powerful capabilities of the Oracle Database file mapping feature. This includes :

  • The ability to map all the database files that span a particular device
  • The ability to map a particular file into its corresponding devices
  • The ability to map a particular database object, including its block distribution at all levels within the I/O stack

Consider an Oracle Database instance which is composed of two datafiles:

  • t_db1.f
  • t_db2.f

These files are created on a Solaris UFS file system mounted on a VERITAS VxVM host based striped volume, /dev/vx/dsk/ipfdg/ipf-vol1, that consists of the following host devices as externalized from an EMC Symmetrix array:

  • /dev/vx/rdmp/c2t1d0s2
  • /dev/vx/rdmp/c2t1d1s2

Note that the following examples require the execution of a MAP_ALL() operation.

Example 1: Map All Database Files that Span a Device

The following query returns all Oracle Database files associated with the /dev/vx/rdmp/c2t1d1s2 host device:

Example 2: Map a File into Its Corresponding Devices

The following query displays a topological graph of the /oracle/dbs/t_db1.f datafile:

Example 3: Map a Database Object

This example displays the block distribution at all levels within the I/O stack for the scott.bonus table.

A MAP_OBJECT() operation must first be executed as follows:

The following is the result of the query. Note that the o_size column is expressed in KB. Table

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