Vinum volume manager

Vinum, is a logical volume manager, also called Software RAID, allowing implementations of the RAID-0, RAID-1 and RAID-5 models, both individually and in combination
Vinum is part of the base distribution of the FreeBSD operating system. Versions exist for NetBSD, OpenBSD and DragonFly BSD. Vinum source code is currently maintained in the FreeBSD and NetBSD source trees. Vinum supports raid levels 0, 1, 5, and JBOD.
Note: vinum is invoked as gvinum on FreeBSD version 5.4 and up.
Software RAID vs. Hardware RAID
The distribution of data across multiple disks can be managed by either dedicated hardware or by software. Additionally, there are hybrid RAIDs that are partly software- and partly hardware-based solutions.
With a software implementation, the operating system manages the disks of the array through the normal drive controller (ATA, SATA, SCSI, Fibre Channel, etc.). With present CPU speeds, software RAID can be faster than hardware RAID.
A hardware implementation of RAID requires at a minimum a special-purpose RAID controller. On a desktop system, this may be a PCI expansion card, or might be a capability built in to the motherboard. In larger RAIDs, the controller and disks are usually housed in an external multi-bay enclosure. This controller handles the management of the disks, and performs parity calculations (needed for many RAID levels). This option tends to provide better performance, and makes operating system support easier.
Hardware implementations also typically support hot swapping, allowing failed drives to be replaced while the system is running. In rare cases hardware controllers have become faulty, which can result in data loss. Hybrid RAIDs have become very popular with the introduction of inexpensive hardware RAID controllers. The hardware is a normal disk controller that has no RAID features, but there is a boot-time application that allows users to set up RAIDs that are controlled via the BIOS. When any modern operating system is used, it will need specialized RAID drivers that will make the array look like a single block device.
Since these controllers actually do all calculations in software, not hardware, they are often called "fakeraids". Unlike software RAID, these "fakeraids" typically cannot span multiple controllers.
Example configuration
A simple example to mirror drive enterprise to drive excelsior (RAID1)

drive enterprise device /dev/da1s1d
drive excelsior device /dev/da2s1d
volume mirror
 plex org concat
   sd length 512m drive enterprise
 plex org concat
   sd length 512m drive excelsior