Drive Speed and Capacity Comparisons

The table below summarises the capacity and throughput of some tape drives. Click on the link to see more detail for a given drive. Capacity figures are native, that is, uncompressed. Mainframe tapes will generally hold three times as much data with compression, and Open Systems data two times as much. Throughput figures are for sustained speed without compression, and assumes that all connectivity can be driven that fast. Throughput Density is the ratio of tape capacity to transfer speed, and is an indication of the tape drive's performance. The lower the figure, the better the performance. However, it does not tell the whole story. A tape drive which supports fast positioning can have a high throughput density, but can still perform well.
I'm quoting drive speeds in Gigabytes per hour, rather than the more traditional Megabytes per second. This is because tapes are usually used for backups and we measure backup requirements in Gigabytes. It seems to me that a gigabytes per hour measure is more useful.

Drive details

DAT

DAT stands for Digital Audio Tape and was originally intended for audio recording. Sony and HP defined the DDS (Digital Data Storage) standard for computer data storage. DAT technology is based on helical scan, and uses 4mm tapes, similar in size to an audio cassette cartridge.

My own experience, from about 9 years ago, classified DAT as cheap, cheerful, but don't rely on it for critical recoveries. No doubt DAT reliability has improved since then, MTBF is now quoted at 300,000 hours. Other sources quote DAT can be used for 2,000 reads or 100 writes before the cartridge should be replaced. Modern HP StorageWorks DAT drives are now available with capacities up to 72 GB and SCSI or USB 2.0 interfaces
DAT comes in different formats and lengths as shown in the table below

# (Very early DD1 models will only handle 60m tapes).

A DDS2 drive will write in DDS1 format to 60m and 90m tapes but will use DDS2 format when writing to 120m tapes. The latter can't be read on a normal DDS1 drive.

Capacity quoted above is all uncompressed. You will at least double capacity if your DAT drive supports compression, but you will not be able to read it back on a drive which does not support compression. Also, some operating systems do not support the SCSI commands to switch compression on/off. Drives generally compress by default.

If you want to ensure your DAT tapes are always compatible then consider the following

• Drives which support compression should be hardware switched with compression off, and not software switchable.
• Don't use 120m tapes
• use the 'noswap' device on IRIX when moving data to a non-IRIX system
• use a 128 blocking factor on IRIX when moving data to a non-IRIX system ... when reading a 'foreign' DAT use 'mt blksize' to determine the blocksize and use that figure in the tar command.

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DLT-S4

Quantum own the DLT technology, but have licensed it out to lots of companies, including Compaq, IBM, Hewlett-Packard, Dell, and SUN.

Digital Linear Tapes are general purpose half-inch cartridge tape drives and have evolved through several different models, the current ones are DLT-S4 . Some characteristics of these are -

As server capacities grew, it became more difficult to back them up onto a single cartridge. Options were either employ someone to swap tapes during the backup, or buy an automatic tape loader. The DLTx000 series got a reputation for being reliable, while having sufficient capacity to back up a large server. The MTBF for a SDLT600 is quoted at 250,000 hours, which translated to about 28 years of continuous running.

DLT is a 128-track serpentine longitudinal recording device. The price of a drive is in the region of £1,500 to £4,500. The cartridges are 1/2", based on Digital's CompacTape III design (similar to a TK cartridge).

DLT-S4 is a quantum leap above the DLTx000 series. It uses a number of innovations, including -

Laser Guided Magnetic Recording LGMR uses both sides of the tape. Data is recorded magnetically on one side of the tape, but it is tracked optically on the other side of the tape. These are standard, proven technologies, but combined, they dramatically increase the recording capacity of the tape, while retaining reliability.

Pivoting Optical Servo POS is the optical half of LGMR. It eliminates the need to pre-format tape for magnetic tracking. It also eliminates magnetic interference between data storage and tracking bands, and eliminates the need for magnetic tracking bands altogether.

Magneto-Resistive Cluster (MRC) Heads MRCs are built using advanced thin-film processing technology. This means they are smaller traditional MR heads, and so can deliver higher data-transfer rates and capacity.

New High-Efficiency PRML Channel Advanced Partial Response Maximum Likelihood (PRML) channel technology provides high encoding efficiency recording densities for greater capacity and performance, which enables Super DLTtape systems to substantially increase cartridge storage capacity and transfer rates. It basically samples the electrical signal coming from the magnetic read heads and uses that to determine the absence or presence of data, rather than just reading peak signals.

Advanced Metal Powder Media The Super DLTtape system uses state-of-the-art Advanced Metal Powder (AMP) media technology for storing very high densities of data. AMP media consists of four layers including the coated back layer for receiving optical servo tracks.

Positive Engagement A highly robust tape leader-buckling mechanism for high duty cycle automated environments.

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AIT

AIT - Advanced Intelligent Tape

AIT (Advanced Intelligent Tape) is the latest 8mm drive technology from Sony. The current top range product offers a capacity of 400GB with a data transfer rate of 108 GB/hour. It has a 72Mb 'Memory in Cassette' (MIC) which can store index information for faster access.

The AIT format is designed to double in capacity and transfer rates every two years. AIT partly achieves this by using helical scan recording, which packs more data on tape for a given number of tracks.

There are two variants of AIT, AIT and SAIT. AIT-1 introduced the use of Advanced Metal Evaporated (AME) media technology. The differences between the two are summarised below.

Features

• A solid state on tape storage area, or Memory In Cassette (MIC) is used to speed up data access. This is a 64Kb flash memory chip, which contains volume map information for fast positioning. You don't need to read through an entire tape to find a file anymore, the MIC chip knows where your file is, and can locate selected data in less than 20 seconds. The MIC chip also contains a log of tape usage, and has space for user-defined data.
• Typical for helical scan, AIT uses a Read-after-write, four-head design.
• High-performance 4-Mbyte buffer
• Adaptive loss less data compression (ALDC) for high efficiency. This is based on the Lempel-Ziv algorithm and can handle many different data types. This algorithm processes a sequence of bytes by keeping a recent history of the bytes processed and pointing to matching sequences within the history. Compression is performed by replacing matched byte sequences with a copy pointer and length code that together are smaller in size than the replaced byte sequence.
• High durability AME (Advanced Metal Evaporated) tape technology. This employs a diamond-like carbon (DLC) coating, which is smooth and hard wearing. The buffering technology reduces tape overshoot and consequent rewind, which means the tape spends less time passing over the heads.

The SDX400 is a WORM tape and is intended for long term records storage. Previous versions have re-write capability

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LTO

Liner Tape Open (LTO) gets its high capacity by writing up and down the tape several times, the LTO format will record up to 384 tracks across a 1/2 inch tape. It can write in either direction, so it writes tracks down the tape, reverses, and writes back up again. It also has embedded 'servo tracks' to allow fast location of specific data on a tape. Also, LTO has few moving parts. This makes the product reliable and easy to maintain. LTO now supports WORM media for compliance purposes.

LTO uses a single-reel, and is typically for very high capacity backup. LTO-4 capacities and speeds are :-

• data access in under 10 seconds,
• capacities of up to 800 GBytes native
• transfer rates 420 GB/hour uncompressed

High capacity is achieved by using 384 data tracks over the half inch tape. The 384 tracks are split into four bands of 96 tracks each. Data is written to the innermost bands first, to provide protection to the data recorded earliest in the process. On pass one of a round trip down the length of the tape and back, eight tracks are read, or written, concurrently. At the end of the tape, pass two of the round trip starts. The read/write heads are indexed and positioned over eight new tracks, and the tape reverses direction back toward the beginning of the tape to complete the round trip. For the next round trip, the heads are again indexed to a new position over a new group of 8 tracks.

When the LTO standard was originally developed, it was envisaged that there would be two types of LTO, Ultrium; large capacity and slower and Accelis; small capacity but fast. The Accelis format was designed for applications that require exceptionally fast access times, such as online data inquiry and retrieval. No products were ever produced as there was no market for them. The Ultrium name has now been dropped and the technology is just called LTO.

The Linear Tape section explains the LTO technology in detail

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TS1120

The 3592 drive was a complete change in track format, and is not compatible with earlier 3590 Magstar models. The TS1120 is the renamed, large capacity 3292. The following table compares the TS1120 with the older 3590 models. Both the TS1120 and 3592 support WORM media for Sarbanes-Oxley compliance. The TS1120 supports 4Gb Fibre Channel and 4Gb FICON, as well as standard ESCON.

The 3592 will support a burst data transfer rate of 2Gb/s per second with FICON connectivity, and the native capacity can be reduced to 60GB for sets of small files which need a high retrieve performance. It uses a bi-directional longitudinal serpentine recording technique, writing to 512 tracks (The TS1120 uses 896 tracks). The 3592 drive is smaller than the older 3590, and so its possible to replace a 3590 drive in a 3494 tape library with two 3592 drives. You can also fit twenty 3592 drives onto a single STK powderhorn silo panel.

The 3592 eliminates some tape back-hitching. Most tape drives store data in a buffer which is then flushed out to tape when the application completes sending a block of data. The tape drive overruns then back-hitches to the correct point on the tape, ready to write the next block of data. The 3592 will write this block of data to a temporary space on the tape, and retain the data in the buffer. It can then write out several data blocks when the buffer is nearly full. This reduces tape wear, as well as improving performance.

3592 compatibility between IBM and STK silos

The information below is supplied as presented in an IBM redbook,but I have no personal experience in trying to swap 3592 cartridges between IBM and STK silos. I have swapped 3590 cartridges without problems.

It is possible to install 3592 drives in both StorageTek and IBM tape silos, but a 3592 cartridge cannot be moved into a 3584 from an StorageTek (STK) ACS without being relabeled. The ACS labels will be unreadable in the 3584, as they consist of two labels pasted together.

Also, a 3592 cartridge cannot be moved into or from a STK ACS or 3494 without changing the exterior labels.
The 3592 standard media identification characters are:

• JA - Enterprise Tape Cartridge
• JJ - Enterprise Economy Tape Cartridge
• JR - Enterprise Economy WORM Tape Cartridge
• JW - Enterprise WORM Tape Cartridge

When cartridges labeled with the standard media identification characters (JA, JJ, JR, or JW) or new cartridges are requested for temporary or permanent entry in an ACS, a new volser label must be applied.

For 3592 cartridges to be associated with the different emulated drives types within an ACS, the standard media identification characters (JA, JJ, JR, or JW) need to be replaced with one of the following.

IBM 3592 cartridges without the standard media identification characters (old style six character labels) are usable in IBM 3494 tape libraries.
The volser range table is used by the 3494 to determine the media type.
A seventh media type character on these cartridges will not work for the 3592 cartridge in a 3494, as the only 7th characters recognized by the 3494 are 1, E, J, or K, which equate to 3490 and 3590 media. Any other 7th characters will cause the cartridge to be ejected with an invalid media type.
If a media type is to be included in the barcode label for the 3592 in a 3494, it must be two characters long, be integral with the volser barcode, be code 3 of 9, and be JA, JJ, JR, or JW.

IBM labeled LTO cartridges can be moved from an ACS to a 3584 without being relabeled. HP labeled LTO cartridges, while similar to the IBM label, are different enough so that re labeling is necessary.

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9840 / 9940

SUN inherited the StorageTek range of tape drives when they bought them out in 2005. SUN drives are all a proprietary format, which basically means that you have to source your cartridges from SUN.

SUN's mainframe flagship drive is the T10000, which uses a 32 channel linear architecture. It was released at the end of 2005 as a competitor to IBM's 3592. The TS10000 has a 256 MB buffer and supports encryption, WORM, 2 or 4 Gb Fibre Channel and FICON and emulates an IBM 3592.

The older 9840 drives use an 8 MB data buffer, allowing 10MB/s head to tape transfer rates. Overall it is capable of driving both 18MB/s ESCON, and 40MB/s ultra SCSI. The 9840C supports FICON at 30MB/s without compression. SUN claims the tape will also drive a 100MB/s fibre, burst rate. The 9840 uses and enhanced LZ-1 compression engine, which can compress up to 4:1, depending on the data. As a rule of thumb, OS/390 data will generally get 4:1, while open systems data will get about 2.5:1. You might get less, you might get a lot more.

The 9840 uses its own tape media technology. The dual drum tapes are 288 track advanced metal particle, with an uncompressed capacity of up to 40GB. SUN claim a compressed capacity of 160GB.

The 9940 is the high capacity version. The spec. is basically the same as the 9840, but the capacity is larger, at 60Gb uncompressed or 200GB with compression. The SUN tape feeds and speeds are summarised below.

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Exabyte

Exabyte is now part of Tandberg. The latest offering from Exabyte is the VXA-320, which offers 160GB uncompressed capacity of data storage with a data transfer rate of 43.2 MB per second. Its facilities include Smartclean (self-cleaning tape cartridge), AME (Advanced Metal Evaporation) data cartridges, a 32MB adaptive data buffer.
Exabyte claim that their drives are much more reliable that DLT, due to wider head tolerances. They support both Firewire and USB interfaces.

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