Tape storage is not a topic that sets hearts aflutter with excitement, although it's probably one of the biggest causes of sleepless nights for IT staff. Yet there are so many different tape formats and schemes for backup, it can be a nightmare to come up with a solution that's best for your situation.

We have two scenarios for this comparison and while the first has rather the conservative backup requirements the second is a tad more serious.

Even so, the second Scenario can generally get by with relatively inexpensive solutions; there is no need to bring in autoloaders the size of a fridge on a pallet lifter just yet.

OK, so what did we receive? For the first Scenario we received a VXA, a DLT, and two DDS-4 solutions. Although we asked for internal drives, the HP DAT40 was the only internal drive we received.

The remainder were external, but would certainly be available as an internal solution. The solutions for Scenario 2 were relatively broad with Mammoth-2, Ultrium (LTO), DLT, Super DLT, and two AIT drives (one was AIT-1 and the other AIT-2). The AIT-1 drive was internal and the remainder external, although most of these should be available in an internal form factor.

It is interesting to note that two of the drives supplied could accommodate more than one cartridge. The Sony AIT-2 drive has a four-stacker cartridge that can hold four tapes for a total native capacity of 200GB, while the amazing Overland DLT unit has a 10-tape magazine for a total native capacity of 400GB.

These two units are very different, however. The Sony is small with a rather simple but effective loader mechanism, while the Overland is a very large and heavy unit tipping the scales at over 26kg.

The Overland unit merits further discussion. The loader appears as a separate SCSI device to the internal DLT drive (a Quantum 7000). Your software must include loader functionality to correctly operate the unit.

It has a front control panel with a large LCD menu system that is very easy to navigate. The front door, which covers the tape cartridge, must be opened using the menu system to prevent damaging the tapes or the unit while the loader is in operation.

The 10-stacker cartridge is large and very robust and while very easy to remove and install, nevertheless sits very snugly in the Overland.

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How often is enough?

There are very few businesses that can afford to lose more than a day's worth of data. The next obvious question is when should you carry out a full backup and when an incremental?

And again there is a short answer that in itself involves a question. How much time can you afford to be down if you have a catastrophic data failure in your primary storage?

With a full backup you simply remedy your primary storage problem by running the most recent full backup tape. It can of course become quite expensive, not to mention time consuming, to carry out a full backup every night.

Some businesses run incremental backups every weeknight and then a full backup over the weekend. This saves money as the incremental backups through the week may fit on a single tape, or several much smaller tapes, and it saves time as the time-consuming full backup takes place over the weekend when hopefully your system is not as busy.

The downside of this system is that restoring your data will take longer in the case of a large-scale storage failure. You will need to restore the last full backup, followed by several incremental backups to get the system up to date again.

Oh, and just a bit of common sense--your precious backups are not much use if the server room burns down and that's where you store the tapes. It makes sense to store your backup tapes at a different site, and we don't mean in an adjacent room. That way, if your hardware does go up in smoke, or is stolen for example, it is simply a matter of buying new hardware and restoring your data.

Battle of the technologies

Broadly, the field can be split into two dominant tape recording technologies: helical scan and linear scan. There are staunch supporters of both technologies but the basic pros and cons are outlined below.

Amongst the linear-tape technologies there is quite a battle between LTO and DLT (including its sibling SuperDLT). Although there are technical merits to both technologies, the large back catalogue of DLT tapes in the archives of many organisations still gives SuperDLT an advantage over LTO because of its backward compatibility with DLT.

Helical scan tape (AIT, VXA, DAT 4 & 8mm)

A helical scan drive operates similarly to a VCR. A rotating tape head lies at an angle to the tape and leaves diagonal strips of data.

The drives have a relatively complex design, with many more motors and contact points than LT, and there are long tape paths because the tape is pulled from the cartridge around tension devices, guide pins, and rollers.

All this enables the format to store a high bit density on media. In theory, this format is less affected by print through and tape stretch than LT.

Linear tape (DLT, SLR, Travan, Accelis, Ultrium)

A linear tape is closer to an audio cassette recorder in operation. The tape simply runs past the tape head in a straight line.

Supporters claim LT was designed for data storage because the tape never leaves cartridge and is only touched by the recording head resulting in a simple design with few moving parts and a short tape path.

Compared to helical scan, linear offers a relatively low bit density over a larger media area. In theory, this format is more affected by print though and tape stretch than the diagonally encoded helical scan.

Tape technologies

Linear Tape-Open (LTO) is an open standard that defines two formats: one for fast data access and the other for high storage capacity. Hewlett-Packard, IBM, and Seagate developed LTO jointly.

One of the tape industry's biggest problems has been a lack of standards, with each vendor providing its own technology. The LTO standard was devised so that different manufacturers' tapes and tape drives would interoperate.

LTO uses a linear multi-channel bi-directional format. LTO also includes timing-based servo (automates error correction), and hardware data compression.

The two formats previously mentioned are Accelis and Ultrium. Accelis uses an 8mm wide tape in a two-reel cartridge and the format reads from the mid-point of the tape to speed up data access and searches.

This is of great benefit for online searches and time-critical data retrieval. Typically the capacity of Accelis products starts at 25GB. Ultrium on the other hand is a single-reel format that aims to maximise the amount of data you can squeeze onto the tape, and is specifically designed for archival and backup purposes.

The capacity starts at 100GB with a native transfer speed of up to 15MB/s.

More information can be found at the LTO web site www.lto-technology.com.

Digital Data Storage (DDS)

In 1989, Sony and Hewlett-Packard defined the DDS format for data storage using DAT tape cartridges.

DDS uses a 4mm tape and the drive uses helical scanning for recording; the same process used by a VCR.

There are two read heads and two write heads. The read heads verify the data as it is written, and naturally if errors are present, the write heads rewrite the data.

There are four types of DDS drives:

• DDS-1: Stores up to 2GB of uncompressed data on a 120-minute cartridge.

• DDS-2: Stores up to 8GB of data in compressed format on a 120-minute cartridge.

• DDS-3: Stores up to 24GB of data on a 125-minute cartridge. DDS-3 uses PRML (Partial Response Maximum Likelihood), which eliminates electronic "noise" for a cleaner data recording.

• DDS-4: The newest DDS format, DDS-4 stores up to 40GB of data on a 125-minute cartridge.

It is important to note that a DDS cartridge needs to be retired after 2000 passes or 100 full backups.

You should clean your DDS tape drive every 24 hours of operation with a cleaning cartridge and discard the cleaning cartridge after 30 cleanings. DDS tapes have an expected life of at least 10 years.

Travan

The tape has pre-recorded "servo" patterns at both ends of the tape that the read/write head uses to move into position and correctly align itself to the tape.

Once the head is correctly aligned it locks itself into position. The tape shell is designed to prevent any tape wander, so once the head is positioned for the tape the data channel passes reliably under the heads.

DLT

Digital Linear Tape (DLT) uses special compression algorithm, known as Digital Lempel Ziv 1 (DLZ1), which is utilised to improve data storage quantity and retrieval of data at high speeds.

Inside the DLT drive, data is written on the tape in dozens of linear tracks, usually 128 or 208. Some cartridges can hold in excess of 70GB of data when compression is used.

A variant of DLT technology, called SuperDLT, makes it possible to store upwards of 100GB on a single cartridge and the SuperDLT drive can transfer data at speeds of up to 10MB/s. DLT and SuperDLT technologies are in direct competition with LTO, AIT, and the Mammoth Drive.

AIT

Advanced Intelligent Tape (AIT) uses tapes that measure 8mm across and a helical scanning technique, similar to that used in Mammoth drives.

This technique optimises the data transfer rate and the storage capacity. There are currently three generations of AIT starting at AIT1 that offers 50GB native storage and transfer rates of 4MB/s uncompressed.

AIT utilises a compression algorithm known as Adaptive Lossless Data Compression (ALDC) with a maximum compression ratio of 2.1:1. For details, see www.aittape.com/technology.html.

Mammoth Tape Drive

A Mammoth cartridge's tape measures 8mm across and a helical scan head is utilised to increase data transfer rate and capacity.

Each cartridge is capable of storing 40GB of compressed data. The compression algorithm used is known as Improved Data Recording Capability (IDRC) with a maximum compression ratio of 2:1.

The Mammoth drive can transfer data at speeds of up to 3MB/s without compression, and 6Mbps with compression. Mammoth2 is an improvement over Mammoth in terms of both capacity and transfer speed-up to 60GB (native) and 12MB/s without compression.

VXA

Ecrix apparently started with a clean slate when it developed the VXA format.

The cartridges bear a striking resemblance to AIT cartridges for example, but externally the notable difference is the lack of data contacts on the back of the VXA cartridge.

While all the other formats described here can be called streaming tape formats, VXA is not. In fact, it's a Discrete Packet Format (DPF).

In simple terms the other formats take all the data from your server and lay it on the tape as a constant contiguous "chunk" while VXA breaks the data stream up into packets with each packet containing 64 bytes of data in addition to sync, addressing, ECC and CRC data.

The VXA format has a capacity of 33GB native and 66GB employing a 2:1 compression ratio while data transfer speeds are up to 3MB/s native and 6MB/s compressed.

Compression

Even though most vendors claim a 2:1 compression, up to 30:1 compression rations have been achieved with extremely compressible data.

It is not uncommon at some sites to find compression ratios ranging between 3:1 and 5:1 depending on the data.

Cartridge Life

Generally vendors rate the tape life as less than five percent loss in magnetisation at 20 degrees Celsius and 40 percent non-condensing humidity. It is interesting to note the difference between "passes" and "uses". For helical scan tapes they both effectively mean the same thing.

However, for linear technology tapes, the head may make up to 56 "passes" of the tape in one "use". To put this in perspective, Quantum claims a life of 1 million passes for its SDLT220 tapes, but this only equates to 17,850 uses.

Steven Turvey is lab manager at RMIT Test Labs. RMIT IT Test Labs are an independent testing institution based in Melbourne, Victoria, performing IT product testing for clients such as IBM, Coles-Myer, and a wide variety of government bodies.

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