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The straight skinny on disk vs. tape pricing; tip: you may be surprised.

For years, tape was considered to be less expensive than disk storage. But after years of disk-price erosion, the perception that disk costs less than tape became more widespread. Determining the price of various storage subsystems has been subject to considerable misinterpretation, and meaningful pricing comparisons for tape and disk storage are scarce. For example, look at typical published average selling prices for various disk and tape subsystem components:

* 73GB Ultra SCSI hard drive: $861 (unit price), $11.82 (ASP$/GB)

* 181.6GB Ultra SCSI3 hard drive: $1,671 (unit price), $9.20 (ASP$/GB)

* 80GB Ultra ATA 100: $250 (unit price), $1.56 (ASP$/GB)

* 40GB Ultra ATA 100: $68 (unit price), $1.70 (ASP$/GB)

* SDLT Ultra SCSI tape drive: $5,800 (unit price), $26.36 (ASP$/GB)

* LTO Ultra SCSI tape drive: $5,300 (unit price), $26.50 (ASP$/GB)

* SDLT cartridge (220GB compressed): $129 (unit price), 59 cents (ASP$/GB)

* LTO cartridge (200GB compressed): $115 (unit price), 58 cents (ASP$/GB)

* Tape Library *: $33,000 -- $67,500 (unit price), $4.50-$1 .25 (ASP$/GB)

Note: ASP levels as of mid-September 2002 from storage VAR and online vendors including U.S. and international figures. Pricing levels may vary. * Prices used are for a variety of tape libraries ranging from 80 to 680-cartridge slot capacity.

Disk Pricing Considerations

Unfortunately, lists like the one above appear frequently and are often misinterpreted. The price of tape storage per gigabyte appears higher than the price of disk storage because the price per gigabyte of disk drives is lower than the price per gigabyte of tape drives. Adding the price of tape media only adds more to the tape price. For example, the prices of disk drives are indeed below the prices of tape drives and media, but these are not valid comparisons and do not represent the price of a working subsystem for either disk or tape. All too often, pricing comparisons are incorrectly made between the price of a disk drive and a tape drive. These do not represent real-life examples, however. The disk drive cannot function without a controller and the controller price uplifts the ASP of disk storage considerably-- typically adding five to 10 times the hard drive per gigabyte price. Therefore, disk pricing must be determined for a working and functional disk subsystem that includes the controller and hard d isk drives rather than just a single component of the subsystem.

To compare tape and disk pricing accurately, the prices for working subsystems must be used. Pricing for a workable disk subsystem falls into three distinct categories:

* Enterprise disk market: FC, SCSI; $70-$90 (ASP $/GB range for working disk subsystem)

* Mid-range disk market: SCSI, FC; $30-$40 (ASP $/GB range for working disk subsystem)

* Low-end "Ghetto RAID": IDE; $l0-$15

The price per gigabyte of the disk drive is actually a relatively small component of a workable disk subsystem price per gigabyte.

Tape Pricing Considerations

For tape pricing, no business normally operates a tape subsystem with just one tape drive and one tape cartridge. The most common tape environments will have an automated tape library containing several tape drives and several tape cartridges. The price of the associated tape library, tape drives and media contained in the library must be used to give a comparable (apples-to-apples) price comparison to disk prices. This then provides a comparison of working disk subsystems and tape subsystems, not just comparisons of the individual components. Comparing the price of a tape drive to the price of a hard disk drive is not a meaningful comparison.

* Enterprise tape library market: FC, SCSI; $1.2525 cents (ASP $/GB range for working disk subsystem)

* Mid-range tape library market: SCSI, FC; $4.50$1.25 (ASP $/GB range for working disk subsystem)

* Low-end market: SCSI, IDE; $l0-$ 15; NA, manual mount and autoloader

The key factor used to determine the purchase price per gigabyte for a working tape subsystem is the ratio of the number of tape cartridges to the number of tape drives. The price of tape decreases as the ratio of cartridges to drives increases. In a recent study by Horison Information Strategies, relative pricing comparisons were made for working subsystems of enterprise-class disk, mid-range disk, and for lower-cost IDE/ATA or subsystems, and for a variety of vendors' automated tape subsystems. For a working and functional tape subsystem, point-in-time average selling prices for automated tape libraries, ultra-SCSI tape drives and 100GB native, 200GB compressed cartridges were used. These comparisons do not include any storage management software, personnel or facility costs for tape or disk support.

To illustrate this key factor for tape pricing, for example, consider the following configuration. The smallest capacity library used in this study contained 80 cartridge slots and two SDLT tape drives yielding a capacity of 16TB at 200GB compressed per cartridge. The cartridge-to-drive ratio is 40. For pricing, the 80-slot library was priced at $33,375, two tape SDLT drives at $5,800 each and 80 100GB native capacity cartridges at $120 each. Total: $53,975 or $3.37 per gigabyte. By comparison, 16TB of disk storage will cost approximately $200,000 for IDE/ATA; $560,000 for mid-range disk; and $1.28 million for enterprise disk using the midpoints of each price range. Subsequent analysis may vary by changing the number of drives and cartridges.

Non-automated (offline) tape storage is not included in the study. Any cartridge-per-drive ratio is meaningless and there is no actual way to determine a price per gigabyte other than spreading the capacity of all of the tape cartridges onsite and offsite across all of the tape drives. Please note: Obviously, because there is no tape library cost, a manual tape environment will have a lower hardware price than an automated tape environment. Without automation, however, the manual labor or human effort and/or the associated TCO (total cost of ownership) expenses to manage the tape environment go up considerably. Therefore, for the purposes of this article, only an automated tape environment is used.

Typical tape libraries used in non-mainframe systems can attach from four to 20 tape drives and contain from 80 to 700 tape cartridges. The price per gigabyte of tape diverges from the price per gigabyte of disk as the ratio of cartridges to drives increases. This sliding scale is the key to understanding tape pricing dynamics. To correctly determine tape pricing use:

* Price of a tape library.

* Price of the tape drives.

* Price of the media inside the library.

* Total capacity of the library based on 2x (tape compression factor) or twice the native capacity of each cartridge times the number of cartridges.

* Divide the total price of the library, drives and media by the capacity (in gigabytes) of the tape library to determine the price per gigabyte for the working tape subsystem.

The average ratio of cartridges to drives in an automated tape library can range from 20-to-1 to 80-to-1 with 15-to-1 to 40-to-1 being the most common range. Increasing the number of tape drives attached to a library without adding additional cartridge capacity adds to the price and raises the price per gigabyte. Increasing the capacity of a tape library without adding drives lowers the price per gigabyte. For typical Unix, Win2K, and other mid-range automated tape configurations, the price per gigabyte of an automated tape subsystem ranges from one-fifth to over one-twentieth the price of an equivalent amount of disk storage. In mainframe (OS/390 and z/OS) tape storage environments, the ratio of tape cartridges to tape drives in an automated library often exceeds 200 and pricing can be less than one-thirtieth the price per gigabyte of disk. Changing the storage subsystem configuration for disk and for tape can alter the pricing and performance levels. Adding more tape drives raises the price per gigabyte, bu t also increases the throughput capability of the automated tape subsystem. Adding more disk drives to a disk subsystem without adding more data transfer capability may increase capacity, but lower throughput and response times given the increased workload.

This is a complex and dynamic subject and every attempt has been made to create a meaningful and realistic comparison between disk subsystem and tape subsystem pricing. Therefore, for usable or working subsystems, tape pricing is less expensive than disk pricing and it becomes increasingly less expensive as the cartridge-to-drive ratio increases.
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Article Details
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Author:Moore, Fred
Publication:Computer Technology Review
Article Type:Buyers Guide
Geographic Code:1USA
Date:Nov 1, 2002
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