A VTL (Virtual Tape Library) is a high capacity (terabytes to petabytes) disk-based backup storage system. To the backup software, it looks like a standard networked physical tape library or autoloader (tape robot, autochanger or jukebox). This allows them to be deployed as drop-in replacements for mechanical tape drives, with minimal disruption.
One alternative to a VTL for disk-based backup is a NAS (Network Attached Storage) or file server. Some storage arrays, such as the HP StorageWorks, can be configured as both VTL and NAS at the same time.
VTLs are made practical because of the large capacities (terabytes) of new hard disks, and high compression ratios (more than 10:1) provided by deduplication technology. The hard disks are usually combined into RAID 5 or RAID 6 arrays to create a large and reliable (redundant) logical hard disk.
High data rates to meet backup windows, means that SANs (Storage Area Network) are usually used to connect a VTL to the backup server: Fibre Channel, or iSCSI over Gigabit Ethernet.
VTLs can be turnkey appliances or software-only solutions (the IT department needs to buy a server and disk array separately). Examples of VTL brands include Falconstor, Sun (StorageTek), HP (StorageWorks), NetApp, Sepaton, StarWind and EMC (Data Domain).
Why Use a VTL: Advantages Compared to Tape
The combination of high disk capacities and deduplication, means that it is possible to store most or all of an enterprise's server backup data in a VTL. The advantages are:
- Reduced manpower, due to reduced handling of backup tape cartridges.
- Higher backup speeds as multiple servers can be backed up concurrently, to fully utilize network and VTL bandwidth ("ingest rate"). This is done by creating multiple virtual tape drives that can run at the same time. Speeds of a few terabytes per hour per VTL node, are possible.
- Tape library consolidation. A VTL can replace multiple physical tape libraries, supporting different backup software by mimicking different tape libraries and tape drives.
- Reduced restore times. Files can be restored from online storage instead of having to manually load tape cartridges.
- Data replication. Backup data can be replicated off-site over the network for disaster recovery. No need to courier tapes.
- Expandability. Future increases in backup storage requirements can be met by adding disk arrays to an existing VTL node, or by adding a new VTL node.
VTL Disadvantages and Limitations
Physical tape does have a few advantages over VTLs:
- Magnetic tape is still required for long term archival purposes, and sometimes for regulatory compliance.
- Tape drives use minimal electrical power when not in use. The VTL disk arrays will draw power all day unless powered down, which usually isn't practical. The exception is a MAID VTL (Massive Array of Idle Disks) which spins down idle disks. Examples of MAID VTL vendors are Copan, Nexsan and Itiso.
VTL Features
Some useful features in a VTL:
- Disk-to-disk-to-tape (D2D2T) support, writing virtual tapes to physical tapes. This includes tracking and management of each tape cartridge. Mainly for archiving, off-site disaster recovery or regulatory compliance.
- Replication of data over the network, for off-site disaster recovery.
- Encryption of data when writing to tape, or when replicating data over the network.
- Multi-site integration, for example in a Remote Office/Branch Office (ROBO) situation. A ROBO can use a small VTL that replicates to a large VTL at the head office, where archiving to tape is centralized.
- Tape consolidation. Writing multiple (short) virtual tapes to one physical tape, to reduce the number of physical tapes.
- Tape shredding. Overwriting deleted virtual tapes to ensure that the data cannot be recovered.
- Scripting to automate operations.
- High availability (redundant power supplies, cluster fail-over).
- Allocating disk space on write, not tape creation. This makes full use of available disk space, instead of allocating empty disk space that might not be used.
- Global data deduplication support, for higher compression ratios.
How to Choose a VTL Solution
Data deduplication performance has a large impact on VTL sizing and storage capacity. To evaluate VTL products, it is important to understand the basics of deduplication.
More information on VTL (case studies, PDF product brochures, whitepapers) is available from VTL vendors such as HP, EMC Data Domain, FalconStor, NetApp, Itiso (MAID VTL) and Sepaton.
Yuen Kit Mun - Kit Mun is a self-confessed information junkie, reading an average of a book a week over the past two decades. His growing Internet ...
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