Volume Manager Conceptual Overview

Why You Should Use Volume Manager

Volume Manager provides enhanced data storage service by separating the physical and logical aspects of data management. Volume Manager enhances data storage by controlling these aspects of storage:

Volume Manager Objects

Volume Manager is a storage management subsystem that allows you to manage physical disks as logical devices called volumes. The Volume Manager interfaces provide enhanced data access and storage management by using volumes. A volume is a logical device that appears to data management systems as a physical disk partition device. Volumes provide enhanced recovery, data availability, performance, and storage configuration options.

A Volume Manager volume is a logical object. Volume Manager creates other objects that you can operate, control, monitor, and query to optimize storage management. To configure and maintain a volume for use, Volume Manager places physical disks under its control and collects the disk space into disk groups. A disk group is a collection of claimed disks organized into logical volumes. Volume Manager then allocates the space on those disks to logical volumes. See Figure 1-20 “Volume Manager System Concepts”.

Figure 1-20 Volume Manager System Concepts

After installing Volume Manager on a host system, perform the following procedure before you can configure and use Volume Manager objects:

Volume Manager and the Operating System

Volume Manager operates as a subsystem between your operating system and your data management systems, such as file systems and database management systems.

Before a disk can be brought under Volume Manager control, the disk must be accessible through the operating system device interface. Volume Manager is a subsystem layered on top of the operating system interface services. Therefore, Volume Manager is dependent upon how the operating system accesses physical disks.

Volume Manager is dependent upon the operating system for the following.

Dynamic Multipathing (DMP)

	NOTE: You may need an additional license to use this feature.

A multipathing condition can exist when a physical disk can be accessed by more than one operating system device handle. Each multipath operating system device handle permits data access and control through alternate host-to-device pathways.

Volume Manager is configured with its own DMP system to organize access to multipathed devices. Volume Manager detects multipath systems by using the Universal World-Wide-Device Identifiers (WWD IDs). The physical disk must provide unambiguous identification through its WWD ID for DMP to access the device.

If DMP cannot identify the physical disk through its WWD ID, identification is left to the Volume Manager device detection methods. Device detection depends on Volume Manager recognizing on-disk metadata identifiers.

Volume Manager DMP creates metanodes representing metadevices for each multipath target that it has detected. Each metanode is mapped to a set of operating system device handles and configured with an appropriate multipathing policy. Volume Manager DMP creates metanodes for all attached physical disks accessible through an operating system device handle.

Volume Manager DMP manages multipath targets, such as disk arrays, which define policies for using more than one path. Some disk arrays permit more than one path to be concurrently active (Active / Active). Some disk arrays permit only one path to be active, holding an alternate path as a spare in case of failure on the existing path (Active / Passive). Some disk arrays have more elaborate policies.

In general, Volume Manager is designed so that the VM disk is mapped to one Volume Manager DMP metanode. To simplify VxVM logical operations, each VM disk is mapped to a unique Volume Manager DMP metanode. The mapping occurs whether or not the physical disk device is connected in a multipathing configuration.

When using Volume Manager DMP, you should be aware of the layering of device recognition:

• How does the operating system view the paths?

• How does Volume Manager DMP view the paths?

• How does the Multipathing target deal with their paths?

References

Additional information about DMP can be found in this document in the following sections:

• “Dynamic Multipathing (DMP)”

• “vxdctl Daemon”

• Chapter 4 “Disk Tasks”

• “Displaying Multipaths Under a VM Disk”

  Related information also appears in the following sections of the Volume Manager Reference Guide:

• "Volume Manager Commands"

• "DMP Error Messages"

• "Multipathed Disk Arrays"

Volume Manager Layouts

A Volume Manager virtual device is defined by a volume. A volume has a layout defined by the association of a volume to one or more plexes, which in turn, each map to subdisks. The volume then presents a virtual device interface exposed to Volume Manager clients for data access. These logical building blocks re-map the volume address space through which I/O is re-directed at run-time.

Different volume layouts each provide different levels of storage service. A volume layout can be configured and re-configured to match particular levels of desired storage service.

In previous releases of Volume Manager, the subdisk was restricted to mapping directly to a VM disk. This allowed the subdisk to define a contiguous extent of storage space backed by the public region of a VM disk. When active, the VM disk is associated with an underlying physical disk, this is how Volume Manager logical objects map to physical objects, and stores data on stable storage.

The combination of a volume layout and the physical disks which provide backing store, therefore determine the storage service available from a given virtual device.

In the 3.0 or higher release of Volume Manager "layered volumes" can be constructed by permitting the subdisk to map either to a VM disk as before, or, to a new logical object called a storage volume. A storage volume provides a recursive level of mapping with layouts similar to the top-level volume. Eventually, the "bottom" of the mapping requires an association to a VM disk, and hence to attached physical storage.

Layered volumes allow for more combinations of logical compositions, some of which may be desirable for configuring a virtual device. Because permitting free use of layered volumes throughout the command level would have resulted in unwieldy administration, some ready-made layered volume configurations have been designed into the 3.0 release of the Volume Manager.

These ready-made configurations operate with built-in rules to automatically match desired levels of service within specified constraints. The automatic configuration is done on a "best-effort" basis for the current command invocation working against the current configuration.

To achieve the desired storage service from a set of virtual devices, it may be necessary to include an appropriate set of VM disks into a disk group, and to execute multiple configuration commands.

To the extent that it can, Volume Manager handles initial configuration and on-line re-configuration with its set of layouts and administration interface to make this job easier.