Feature

Tape libraries automate backup

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Unfortunately, the interchange capability of different tape technologies is nonexistent or extremely limited. There is interchange between vendors of the same type of devices. For practical reasons and safety, make sure the interchange is demonstrated and certified.

Comparing the capacity of different formats can be tricky. Most vendors offer data compression for tape systems built into the hardware. Also, independent software vendors (ISVs) provide compression. A typical loss-less data compression rate to use for estimation is two to three times, although the algorithms vary and the compressibility of data varies widely. Generally, business data compresses well, scientific data compresses poorly and systems data varies. To compare different tape technologies, always use native, uncompressed specifications. But you should also consider how cartridge construction, recording technology, software support and other tape characteristics meet your needs.

Cartridges.Tape cartridges come in two flavors, single reel and dual reel. Your choice will affect both the capacity and load time characteristics of the system.

The single reel tape cartridge has all the tape on one reel - the take-up reel is inside the tape drive unit. That saves space and cost in the cartridge, providing better alignment of the head assembly. The drive is larger - since the take-up reel is there - but the cartridge has more capacity per cubic inch because there

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is no empty space in the cartridge.

On the other hand, dual reel has a faster load time, which is the time from when a cartridge is inserted until the tape is ready for read or write of data. Single reel takes about one minute. Dual reel midpoint load is faster and can load in four seconds. In midpoint load, the cartridge has the take-up reel included and is always half empty. Put simply, choose single reel for capacity and dual reel for performance.


A guide to tape formats

Choosing the right combination of tape formats helps calculate a tape library's TCO.
TECHNOLOGY VENDOR EXAMPLE TAPE CAPACITY (NATIVE) SUSTAINED TRANSFER RATE (NATIVE) INTERFACES SUPPORTED FORM FACTOR MEDIA AUXILIARY MEMORY IN CASSETTE
QIC HP 4GB 0.6MB/s IDE 3.5 MP No
4mm DAT Helical HP SureStore 12,20,33GB 3MB/s SCSI 3.5 MP No
4mm Linear Packet Exabyte/Ecrix VXA 12,20,33GB 3MB/s SCSI,1394,IDE 3.5 AME No
8mm DAT Helical Exabyte
MammothL
Mammoth2
14GB
60GB
2MB/s
12MB/s
SCSI,
Fibre
3.5
3.5
AME
AME
No
No
8mm Helical AIT1 AIT2 Sony
SDX3/400
SDX500
25,30GB
36,50GB
3MB/s
6MB/s
SCSI
SCSI
3.5
3.5
AME
AME
64k MIC
64k MIC
DLT Linear Quantum
DLT7000
DLT8000

34GB
40GB

5MB/s
6MB/s

SCSI
SCSI

5.25
5.25

MP

No
No
DLTI Linear Benchmark
VST 80

40GB

3MB/s

SCSI

5.25 half height

MP

No
LTO Linear HP
Ultrium215
Ultrium230
10,30,50GB
10GB
10GB

7.5MB/s
15MB/s
Fibre-IBM
Ultra 2
SCSI

3.5
5.25

MP
MP

32k CM
32k CM
SDLT Quantum 110GB 11MB/s SCSI 5.25 AMP No
3490E Linear STK 9490
Timberline
.8GB
1.6GB
6MB/s ESCON
SCSI
5.25+ MP No
3590 Linear IBM
Magstar
10,20,40 GB 9MB/s or 14MB/s ES/FICON,
SCSI, Fibre
5.25+ MP No
9840 STK
Eagle
20GB 10MB/s
19MB/s
ESCON
SCSI, Fibre
5.25+ MP No
Half-inch Helical STK SD3
Redwood
10,25,50 GB 11MB/s ESCON
SCSI
5.25+ MP No
Source: Evaluator Group

Recording technology. There are many ways that data is actually written onto the tape. The different formats have different characteristics in density, speed and reliability. Linear parallel recording writes multiple tracks simultaneously. Linear serpentine writes data in one channel along the tape and reverse and writes in the other direction on another channel. The cost of writing in serpentine is less expensive than linear parallel due to not having to have an expensive, parallel tape head. A third technology - helical scan recording - is similar to that used in consumer VCRs. Data is actually recorded in stripes at an angle across the tape by the rotating head.

Software support. Tape manufacturers maintain a close relationship with backup/recovery ISVs to ensure compatibility with their new products. A new tape product or library is not ready for use in the market until the ISVs port compatibility with their software and the popular operating systems.

Footprint. Tapes drives follow the computer industry trend demanded for a smaller footprint to reduce the real estate required for the device. Small form factor is key in lower end systems. Tape drives for midrange have been mostly in the 5.25-inch format, while enterprise class drives are larger. The 3.5-inch format is becoming popular in new midrange models to increase the number of drives in a library or to fit in an internal system bay.

Interconnects. There are many ways to connect tape drives to the system platform or host library. SCSI was the popular standard, but native Fibre Channel (FC) connections are increasing in importance, due to the popularity of network-attached storage (NAS) and SANs. Enterprise systems still use ESCON and the newer, faster FICON.

Auxiliary memory. Newer generation tape cartridges have non-contact, non-volatile memory chips that can be read by an RF reader. Metadata of where and what is stored and key diagnostics on these chips can provide vital information prior to loading the tape, increasing the degree of automation libraries can perform. Unfortunately, there is no auxiliary memory standard, which will frustrate automation providers and users until vendors get together and do the right thing.

This was first published in June 2002

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