Datasheet

Chapter 1: Overview of Virtualization

Technologies such as RAID (Redundant Array of Inexpensive Disks) and logical volumes as provided by

the Linux LVM, LVM2, and EVMS packages are usually limited to use on the system to which the actual

storage devices are physically attached. Some RAID controllers are dual - ported, allowing multiple

computers access to the same volumes and associated filesystems through that RAID controller,

although how well this works depends on the type of filesystem in use on the shared volume and how

that filesystem is mounted on both systems.

To use a logical volume manager, you must define the disk partitions that you want to use for

logical volumes, create logical volumes on that physical storage, and then create a filesystem on

the logical volumes. You can then mount and use these filesystems just as you would mount and use

filesystems that were created on physical disk partitions.

Like standard disk controllers, RAID controllers provide block - level access to the storage devices that are

attached to them, although the size of the storage available from any set of disks depends on the RAID

level that is being used. The devices attached to the RAID controller are then made available to the

system as though they were a single disk, which you can then partition as you wish, create filesystems

on those partitions, and mount and use them just as you would use single physical partitions.

Operating systems such as Linux also support software RAID, where no physical RAID controller need

be present. The software RAID system functions exactly as a hardware RAID controller would,

providing block - level access to available storage, but it enforces the characteristics of different RAID

levels in software rather than in hardware. Software RAID is very efficient and has only slightly lower

performance than many hardware RAID controllers. Many system administrators actually prefer

software RAID over hardware RAID because hardware RAID controllers are very different from

manufacturer to manufacturer and even controller to controller. The failure of a RAID controller

typically requires a replacement controller of the same type from the same manufacturer in order to

access the data on the storage device that was attached to the failed controller. On the other hand,

software RAID is completely portable across all Linux systems on which the software is installed as long

as they support the same physical disk drive interfaces (IDE, EIDE, SATA, and so on).

Distributed filesystem technologies such as AFS and GFS have their own internal logical-volume

creation and management mechanisms, and also make it possible to share the filesystems on these

logical volumes between multiple computer systems because AFS and GFS provide locking mechanisms

to synchronize simultaneous writes to shared filesystems over the network. NFS, the default Network

File System for most UNIX - like operating systems, also makes it possible to share logical storage across

multiple computer systems, although it does this by exporting a directory from a filesystem on the

logical storage rather than by directly mounting a system - specific volume or networked filesystem.

Distributed filesystems such as AFS and GFS provide filesystem - level access to logical volumes. In this,

they are conceptually similar to Network Attached Storage (NAS,

http://en.wikipedia.org/wiki/

Network-attached_storage

) devices, which provide filesystem - level access over a network to the

filesystems that they contain.

Storage virtualization has become much more accessible across multiple computer systems with the

advent of Storage Area Networks (SAN, http://en.wikipedia.org/wiki/Storage_area_network ),

which support block - level I/O and therefore enable multiple systems to share low - level access to various

types of storage devices over the network. Most SANs use expensive, high - power network technologies

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