What is Veritas Volume Manager
VeritasTM Volume Manager (VxVM) is a storage management subsystem that allows you to manage physical disks and logical unit numbers (LUNs) as logical devices called volumes. A VxVM volume appears to applications and the operating system as a physical device on which file systems, databases and other managed data objects can be configured.
VxVM provides easy-to-use online disk storage management for computing environments and Storage Area Network (SAN) environments. By supporting the Redundant Array of Independent Disks (RAID) model, VxVM can be configured to protect against disk and hardware failure, and to increase I/O throughput. Additionally, VxVM provides features that enhance fault tolerance and fast recovery from disk failure or storage array failure.
VxVM overcomes restrictions imposed by hardware disk devices and by LUNs by providing a logical volume management layer. This allows volumes to span multiple disks and LUNs. VxVM provides the tools to improve performance and ensure data availability and integrity. You can also use VxVM to dynamically configure storage while the system is active.
VxVM and the operating system
VxVM operates as a subsystem between your operating system and your data management systems, such as file systems and database management systems. VxVM is tightly coupled with the operating system. Before a disk or LUN can be brought under VxVM control, the disk must be accessible through the operating system device interface. VxVM is layered on top of the operating system interface services, and is dependent upon how the operating system accesses physical disks.
VxVM is dependent upon the operating system for the following functionality:
- operating system (disk) devices
- device handles
- VxVM Dynamic Multi-Pathing (DMP) metadevice
How data is stored
Several methods are used to store data on physical disks. These methods organize data on the disk so the data can be stored and retrieved efficiently. The basic method of disk organization is called formatting. Formatting prepares the hard disk so that files can be written to and retrieved from the disk by using a prearranged storage pattern.
Two methods are used to store information on formatted hard disks: physical-storage layout and logical-storage layout. VxVM uses the logical-storage layout method.
How VxVM handles storage management
VxVM uses the following types of objects to handle storage management:
Physical disks, LUNs (virtual disks implemented in hardware), or other hardware with block and raw operating system device interfaces that are used to store data.
When one or more physical disks are brought under the control of VxVM, it creates virtual objects called volumes on those physical disks. Each volume records and retrieves data from one or more physical disks. Volumes are accessed by file systems, databases, or other applications in the same way that physical disks are accessed. Volumes are also composed of other virtual objects (plexes and subdisks) that are used in changing the volume configuration. Volumes and their virtual components are called virtual objects or VxVM objects.
A physical disk is the basic storage device (media) where the data is ultimately stored. You can access the data on a physical disk by using a device name to locate the disk. The physical disk device name varies with the computer system you use. Not all parameters are used on all systems.
Typical device names are of the form c#t#d#s#, where c# specifies the controller, t# specifies the target ID, d# specifies the disk, and s# specifies the partition or slice. For example, device name c0t0d0s2 is the entire hard disk connected to controller number 0 in the system, with a target ID of 0, and physical disk number 0.
VxVM uses multiple virtualization layers to provide distinct functionality and reduce physical limitations.
Virtual objects in VxVM include the following:
- Disk groups
- VM disks
The connection between physical objects and VxVM objects is made when you place a physical disk under VxVM control. After installing VxVM on a host system, you must bring the contents of physical disks under VxVM control by collecting the VM disks into disk groups and allocating the disk group space to create logical volumes.
Bringing the contents of physical disks under VxVM control is accomplished only if VxVM takes control of the physical disks and the disk is not under control of another storage manager such as Sun Microsystems Solaris Volume Manager software.