Published: 10 Mar 2007
Removable disk cartridges are faster than tapes and easier to manage--but don't dump that tape library just yet.
New removable disk drives represent the next step in data protection: the best features of disk and tape. Combining the speed and reliability of disk with tape's portability, removable disk drives foreshadow an impending change in the way companies will back up and restore data in the not-too-distant future.
Some of the benefits removable disk drives offer vs. tape include the following:
- Drives appear to back up software as a disk, file system or virtual tape image
- Disk packs come with different RAID options
- Faster backup and restore times
- Disk drives maintain persistent drive letters in operating systems
Most small- to medium-sized business (SMB) backup software products support removable disk drives. But widespread adoption of portable drives may be inhibited until some problems are solved. For example, removable disk drives cost more per gigabyte than tape cartridges, and they aren't compatible with some major backup software products and automated tape libraries.
|Transporting disk cartridges|
Protecting disk cartridges during transit requires different procedures than when moving tape cartridges.
Vibration protection. Removable disk cartridges have more rugged casings and are less susceptible to damage than normal disk drives, but they're still disk drives. Use cases that absorb shock and keep them away from areas where there might be excessive vibration.
Buy carrying cases with locks. Thieves can remove the internal disk drive, insert it into a standard ATA or IDE interface, and access data. Storage cases that can be locked can serve as a deterrent.
Protect against liquids. Liquids can penetrate a case unless it's designed to shed liquids (drain water away from the center of the case) or is waterproof.
Use multimedia cases. Some cases can accommodate multiple media types. Be cautious about exposing tape to the foam often used as cushioning in cases. Over time, the debris created by foam can damage tapes and cause data loss. Disk drives, however, are sealed and aren't susceptible to damage from floating debris.
Removable disk vendors, trying to overcome user skepticism about changing from tape to disk, are emphasizing product durability, ease of use and price. Removable disk cartridges measure durability in many of the same ways that tape cartridges do. For instance, a common measurement of tape cartridge durability is the ability to withstand the force of a drop and remain usable. While tapes are generally rated on their ability to withstand a 36-inch drop to a carpeted surface, removable disk cartridges such as the Imation Corp. Odyssey Removable Hard Disk Storage System and Dell Inc. PowerVault RD1000/Tandberg Data RDX QuikStor products are rated to a higher standard and can withstand a one meter (approximately 39 inches) drop to concrete and remain usable.
Tape drive insertion is another common measurement of tape cartridge durability that most removable disk cartridges now far exceed. The life of a tape cartridge is often determined by the magnetic head of the tape drive, which makes contact with the media in the tape cartridge as it reads and writes data. The disk drive bays into which removable disk drives cartridges are inserted don't make this type of direct contact, which gives the media a longer life. Curt Krempin, product marketing manager, Dell enterprise product group, says the life of the simple connector interface found on most removable hard disk drives has "an expected reliability rating 10 times that of typical low-end tape drives like 4 mm DAT or Travan."
|Removable disk drives and cartridges|
|Click here for a comparison of removable disk drives and cartridges (PDF).|
While each removable disk vendor uses connectors to interface with the disk drive bay, the biggest differentiator is how they insert into the disk drive bay. RDX removable disk drive cartridges use a card-edge connector that produces friction when inserted into an RDX disk drive bay. This results in connector wear over time. Imation and Spectra Logic Corp. chose cartridge constructions that are more resistant to wear over time, according to the vendors. By using a pogo pin--a spring-loaded mechanism in their respective Ulysses and RAID eXchangeable TeraPack (RXT) Media--the removable disk cartridges are less susceptible to wear when inserted into the drive bay.
Users may also find that the actual number of times they can insert removable disk cartridges will run much higher than the guaranteed minimums to which they're rated. "Our life testing has demonstrated reliability for the RDX disk drive to about 10 times the minimum guaranteed specification of 5,000 insertions," says Steve Georgis, president and chief executive officer at ProStor Systems Inc., "and, in use, the RDX disk drives' specifications are 550,000 hours mean time between failure [MTBF]." ProStor's removable disk drive design is used by Dell and Tandberg Data.
Of course, the two biggest risks are data loss and disk drive failure. Data loss is a naturally occurring phenomenon on a disk drive that happens as the drive ages. Most disk drive manufacturers only rate the data as reliable for up to one year after the disk drive is powered off. While most disk drives include error-correction code in their firmware to deal with this eventuality, this firmware works only when the disk cartridge is powered on.
Most disk cartridges lack a mechanism to protect against data loss when the disk cartridge is powered off, but some, like Pro-Stor's RDX cartridge, will eventually include adaptive archival technology. This technology will act like RAID and spread the data across the disk. In the event data loss occurs in certain sectors of the disk while the disk cartridge sits idle, the removable disk drive recognizes that data loss has occurred and can recover data on the disk cartridge when it's brought back online. Because there's only one drive in each RDX cartridge, if a drive fails, all data is lost. Only Spectra Logic's RXT Media can recover from a catastrophic disk failure because RXT Media consists of many disks with factory-configured RAID 0, 1 or 5.
Ease of use
Companies will measure removable disk media's ease of use in two ways: how it works with their backup software and how easy the removable disk cartridge is to manage. EMC Corp. and Symantec Corp. don't support removable disk drives with their respective NetWorker and Veritas NetBackup products. However, EMC's Retrospect and Symantec's Backup Exec backup products, which are designed specifically for SMBs, do support removable disk drive products.
Because of the limited backup app support, most removable disk drives are bundled with a backup app. For example, Imation's Odyssey Removable Hard Disk Storage System includes EMC's Retrospect backup software. Other vendors also bundle backup apps such as CA BrightStor ARCserve, Idealstor's own Ibac, Symantec Backup Exec QS and Yosemite Backup.
Disk is easier to manage than tape, and there are more options to store and retrieve data with removable disk cartridges. For example, users can treat a removable disk cartridge like an external drive and write and read data from it. And if data is backed up in a native format, it can be recovered without having to go through the backup software interface.
To support removable disk cartridges, tape library vendors must retrofit their products in three ways: the library's bay must accommodate the removable disk cartridge drive; the library's firmware must be upgraded to support the removable disks; and the tape library may need to support partitioning to maintain LTO tape and disk cartridges in the same tape library.
Spectra Logic's RXT Media is the only removable disk cartridge currently available for use in a tape library. However, it operates only in Spectra Logic's Spectra T950 tape library and, with a list price per cartridge of $1,600 to $7,000 (depending on the number of disks), it's at the high end of the acceptable price range for most companies.
Despite the ability of removable disk cartridges to match or exceed tape in terms of durability and ease of use, for most companies it will still come down to the price delta between tape and disk cartridges. Disk is approximately the same price as tape, or just slightly more, depending on how much data there is to back up. Pricing for internal tape drives (cartridge and tape) like Travan and LTO start at $360 and $1,000, respectively, while pricing for a disk drive for Imation Odyssey Removable Hard Disk Storage System, Iomega Corp. REV and RDX disk cartridges ranges between $250 and $600. Assuming the data to be backed up is small (fewer than 100GB), and only one disk cartridge is needed daily for a complete backup, disk and tape work out to roughly the same price.
One key feature disk cartridges offer over tape cartridges is their longer life and five-year warranties. But despite their lower per-cartridge price, tape cartridges wear out faster than disk cartridges. When counting the upfront cost of tape cartridges plus the cost of replacement due to wear over a three- to five-year period, disk cartridges may be the less-costly alternative.
Another factor is the backward and forward compatibility of most removable disk drives and cartridges. Most new tape drives can only read and write to tape cartridges that match the current tape generation, and only read older tape formats going back a couple of generations. Conversely, most disk drives can read and write to any disk cartridge of the same technology--past, present or future. This allows companies to immediately take advantage of new disk cartridges as they become available without upgrading disk drives, and while still using previous-generation disk cartridges. One exception to this rule is Iomega's REV disk docking stations, which can read but can't write to older REV disk cartridges.
Normally there's lag time between when higher capacity disk drives are released and when they're available in disk cartridges. When a new, higher capacity disk drive is released, it usually carries a premium price and is available only in a vendor's proprietary disk cartridge casing. To circumvent this, Idealstor provides a casing into which off-the-shelf SATA disk drives can be installed before they're inserted into their backup appliance. This allows users to immediately purchase and implement new, higher capacity SATA disk drives at existing retail prices.
ProStor Systems also recognizes the inherent delays associated with releasing higher capacity disk cartridges. Rather than manufacturing its own RDX disk drives, it stole a page from the LTO playbook and licensed the RDX technology to companies like Imation and Tandberg Data. ProStor anticipates that competition between manufacturers over time will result in faster time to market for new, higher capacity disk cartridges as well as lower prices.
Removable disk drives are emerging as a viable alternative to tape. Though acceptance at the enterprise level remains at least a few years away, most removable disk drives are suitable for small businesses.