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|The case for SAIT|
The AIT family of media is the only one of the five major tape media that puts data down on tape using a helical scan instead of linear placement. Helical scan places data on tape the same way a VCR stores data on a VHS tape: storing data across the entire width of the tape with a rotating head. The DLT and LTO linear process lays data down back and forth from one end of the tape to the other in multiple rows by streaming the tape through a stationary read/write head.
While the helical scan reduces backhitches, the case against AIT was that the spinning head knocked particles and debris off the tape, reducing its life span. To correct this problem, current AIT and SAIT cartridges ship with Advanced Metal Evaporated (AME) technology, which doesn't flake off as much as the older media did. Now AIT media has about the same longevity and reliability characteristics as linear tapes.
The other big change made with the release of SAIT was its shape. After AIT conquered its reliability problem, few users wanted to purchase new libraries to house AIT media. To address this, SAIT media has been retrofitted to the same external height, width and length characteristics as LTO tape cartridges, allowing them to fit into LTO slots in an LTO library. SAIT offers users immediate backup relief by eliminating or reducing backhitches and having the ability to use their existing LTO tape libraries without a significant capital investment.
The IBM 3592 and StorageTek 9940 are used predominantly in mainframe-centric shops. The IBM 3494 libraries, which host this tape type, are only managed under the mainframe operating system that manages data on tape just like it does on disk using HSM. The 3592 tape media is well-suited to the frequent start and stop nature of HSM environments, expediting small and selective file recalls as well as batch processing transactions predominantly found in the mainframe environment.
StorageTek's 9940 cartridge represents the other more mainframe-centric tape media solution. However, this media may also be considered for large Unix and Windows environments because StorageTek tape drives are supported by backup software that runs on these systems. The media offers benefits similar to 3592 media, but the 9940 offers a write once, read many (WORM) option not found on 3592 tape. StorageTek's latest 9940 VolSafe data cartridge provides non-erasable, non-rewritable WORM features for users who want to store data in a tamper-proof format. The knock on the 9940 as of late hasn't been the cartridge, but StorageTek's delayed mainframe FICON support for its tape libraries, resulting in some customers opting for IBM's 3494 tape libraries and the 3592 tape.
Of course, for a variety of reasons, LTO, SAIT and SDLT are the most commonly selected tape media families in Unix and Windows environments. First, all three are widely supported in many tape libraries from a number of vendors. ADIC, Exabyte, Hewlett-Packard (HP), IBM, Overland Storage, Plasmon, Qualstar, Quantum, Spectra Logic, StorageTek and others offer libraries that support these types of tape media. In the cases of ADIC, Quantum and StorageTek, they offer libraries that concurrently support media from the different families of products.
Unlike the 3592 and 9940 formats that are manufactured by a select number of vendors, LTO, SAIT and SDLT are manufactured by a variety of third-party sources. LTO best represents this ideal, with the standard cooperatively set by Certance, HP and IBM while the media itself is available from a variety of manufacturers. This standard enables IT shops to separate the buying decision for tape media and tape libraries and drives.
Every tape user asks: "How much will it hold and how fast will it go?" Yet paying more for faster, higher capacity cartridges may be pointless if users don't use the extra capacity during backups or there isn't enough time to store or recover the data. The more important question becomes, "How well can existing tape drives take advantage of these higher capacities?"
While faster tape speeds will correctly lead one to assume more data gets stored on tape more quickly, issues like backhitches--an interruption in the flow of data onto tape during a backup--have a big impact on the overall speed of the backup. However, tape drive vendors have taken steps to mask the backhitches by putting cache and more intelligence in the tape drive (see "Tape: alive and full of options," in the April 2003 issue of Storage).
This additional cache and intelligence in the tape drives becomes the key to maximizing both capacity and speed on newer tape cartridges. For instance, even though the latest 3592 tape offers up to 300GB of native capacity, it's only with advancements in the 3592 tape drive that the 3592 tape capacity and speed can be maximized. The 3592 tape drive enhances current backhitch countermeasures by incorporating a new technology called recursive accumulating backhitch flush into the tape drive. This technology allows the tape drive to respond to the random requests of backup software to verify that a tape write occurred without flushing its cache or necessitating a backhitch.
In addition to how well the tape and drive work together, drive prices also need to be factored into the overall tape backup solution. For example, expect to pay about $4,000 for an SDLT tape drive, $5,000 to $6,000 for an LTO tape drive and upwards of $30,000 for a 3592 tape drive. And then there's tape's so-called "infinite capacity." For years, mainframes have kept large amounts of older data accessible by managing it through their file system. But this isn't practical in most Unix and Windows environments. Their operating systems lack the sophistication to natively manage large amounts of tape.
This was first published in March 2004