Understanding tape backup technology
Magnetic tape remains a quintessential storage technology. Early on, a long strip of thin plastic coated with magnetic recording media was simply spooled onto reels. Later, the reels were packaged into cassettes or cartridges to ease loading/unloading, simplify storage, protect the media from accidental damage and improve reliability by blocking dust. Once a cartridge is inserted into a tape drive mechanism, the media is passed at a constant speed across a stationary read/write head. These basic principles have gone virtually unchanged in over 50 years. As time went on, tape drives diversified into unique architectures, such as Travan, DDS, DLT and more recently, LTO. These tape backup architectures are not interchangeable, so tape systems must be selected for their technological longevity and media costs. You can learn more details about tape technology in our All-In-One Backup Research Guide.
Tape backup technology proved appealing for several reasons. The cost per gigabyte is still very low compared to disk. An LTO-3 cartridge with 400 GB of native capacity costs less than $100 (about 25 cents per gigabyte) -- half that with 2 to 1 compression enabled. Tape backup also offers "endless" storage capacity. Where a disk storage system has a fixed maximum capacity, tape storage can be expanded simply by using more cartridges. "You can see the leverage you'd get with 10 cartridges [in a single tape drive] versus 10 disk drives," says Robert Abraham, analyst at Freeman Reports in Ojai, Calif. Further, tape offers a "write once" nature that is ideal for long-term archival storage.
However, tape backup suffers from some disadvantages that have been exacerbated by the ready availability of disk systems. The principle problem is performance. Even the 160 megabytes per second (MBps) compressed transfer rates in LTO-3 tapes pale in comparison to disk systems. "The [tape] access times are very slow -- 1,000 times slower," Abraham says, noting that it's much more difficult and costly to retrieve data from tape. This wasn't such an issue years ago, but with corporate data volumes increasing from 50% to 100% each year, tape backup windows and recovery time objectives (RTO) have simply become prohibitive for many organizations, especially businesses that need to operate 24/7. The cost of disk is still seen as an advantage for tape, but disk costs are falling quickly and technologies, like data deduplication, are dramatically improving disk efficiency, allowing data to remain on disk for longer than ever before. See our article Data deduplication explained for more details and user studies on deduplication technology.
Today's tape backup technology and performance
While tape technology has changed very little, storage and recovery needs have evolved dramatically -- yet users are reluctant to discard their existing tape backup systems. This has changed the role of tape from a primary backup vehicle to long-term disaster protection. "Tape is [now] the doomsday tier," Biggar says.
There have been numerous tape backup technologies implemented in the enterprise, but the objective has always been the same -- achieve maximum capacity and performance with minimum cost. The LTO family has gained tremendous traction over the last few years, bolstered by its open nature and support from industry giants like IBM, Hewlett-Packard Co. (HP), Quantum Corp. and Tandberg Data. Today, LTO-3 has emerged as one of the most important tape formats, offering 800 GB of compressed storage per cartridge and compressed data rates of 160 MBps. "Right now, it's all about LTO-3. It's very popular, almost ubiquitous. It has displaced DLT in the enterprise as the primary compact/small tape device," Abraham says. LTO-4 provides 1.6 TB of compressed storage per tape and should support compressed data transfers to 240 MBps. LTO-4 is expected around mid-2007 and will probably be available from all four vendors by the end of 2007. Encryption will be a common option in LTO-4 drives, allowing users to protect tape data without the need of encryption features in backup software.
Abraham segregates today's tape backup systems into three categories. A low-end tape system typically includes one tape drive and some form of basic autoloader that is normally limited to 10 cartridges or less. A midsized tape library is common in the enterprise, using four tape drives with autoloading space for up to several thousand tape cartridges. For example, a four-drive system with an 80 cartridge library is common, with prices in the $30,000 to $80,000 range. For the large enterprise, Abraham suggests a much larger library with up to 64 tape drives and the robotics necessary to swap tapes from a library of 5,000 cartridges or more.
Tape reliability and life expectancy
Tape has garnered a reputation for poor reliability, but this is a notion that experts are quick to debunk. "It's not unusual to see data reliability specified at one million hours right now," Abraham says, noting that some vendors call this "power-on hours" with 25% to 50% tape motion. Some tape cartridges have been certified to retain data for upwards of 30 years -- even up to 50 years on premium media.
So, why do tape backups seem to suffer from such poor recoverability? Experts say that recoverability problems are not in writing to the tape. "The tape drive records information when it's told to do a backup or an archive, and it verifies that the information is recorded properly at the same time its recording -- disk drives don't do that," Abraham says. Both Abraham and Biggar suggest that the problem isn't in the tape cartridge or drive, but rather in other tangible errors or oversights, such as operator errors, backup software bugs, changed/incompatible backup software versions, lost or mislabled tapes and poor tape storage conditions.
Implementing tape backup platforms
A tape system typically consists of four elements: the tape drive or library, the backup software, the backup server, which runs the backup software, and the tape cartridge(s). Although there is considerable flexibility in the choice of these four elements, experts note that the single greatest concern is backup server performance. It's crucial that the server be faster then the tape drive so that a steady stream of data is always available -- if the data buffer empties, the tape drive must stop and reposition the tape before more data can be written. This results in a repetitive back and forth motion of the tape called "shoeshining," which puts excessive wear on the tape and shortens its working life. Many LTO-3 drives can track to slower servers, but this is a waste of potential performance. Experts urge full-speed operation to get the most possible value from tape backup investments.
When deploying a tape backup system, it's important to plan for future growth. Since forklift upgrades can be extremely expensive and disruptive to the backup process, experts suggest selecting a tape platform that can accommodate growth with relatively inexpensive upgrades. For example, a user needing 40 cartridges might select a tape library with a 120 slot capacity, then incrementally add to the remaining 80 slots over time as needs change. Similarly, a user might select a tape library with bays for six tape drives, but only install two tape drives to start. The remaining four drives can be added over time to boost recording performance. Another way to increase both capacity and performance later on might be to upgrade the tape drive and cartridge types. "For example if you're using LTO-3 right now, at some point in the future you simply upgrade everything to LTO-4," Abraham says. "You don't have to replace the robotics."
Cartridge handling and management is another area where experts suggest added attention. With an increasing emphasis on security, data encryption should be used wherever possible. While encryption is primarily a software-based feature at the moment, Abraham notes that the LTO-4 standard will support native encryption at the drive itself, and DLT vendors will also be adding encryption on their high-end offerings. Tape backup cartridges can become difficult to manage, especially as the library size scales. RFID technology is also being introduced from vendors, like Imation Corp. to track and locate tape cartridges, or to identify tapes quickly within a library. Secure offsite storage should not be ignored.
The impact of tape backup
Experts and users all stress that tape backup does exactly the same job that it always did -- economically protect enterprise data over the long term.
For home furnishing retailer R.C. Willey Home Furnishings in Salt Lake City, the need to maintain accounting, inventory and other business data for auditors has been a challenge not met with disk storage. The company currently supports about 10 TB of data, though only about 2 TB is considered "production data" located on an EMC disk subsystem. The production data is backed up daily, with full tape backups performed on a weekly basis. "We don't believe in incremental backups," says Richard Sheridan, IT manager at R.C. Willey.
Sheridan uses an aggressive array of tape storage to address backup needs. One SpectraLogic Corp. Python T950 and two Python T120 tape libraries handle the local tape backups. Three SpectraLogic Python T50s support remote backups at remote distribution centers. To make the backup process as unobtrusive as possible, backups are staged to EMC business continuance volumes (BCV) and staggered to four SAIT drives in the T950. "We're taking backups throughout the day," Sheridan says. "At any given time, we could have three or four backups happening." The daily tape backups are also performed twice, so one copy is kept onsite, while the duplicate tape is carried to a fire safe nearby location. With four hours to backup 500 GB, Sheridan estimates the total daily backup window runs between eight and 10 hours. Security concerns are met by encrypting the backup data at the server.
This daily backup strategy yields a recovery point objective (RPO) of 24 hours, though the recovery time objective (RTO) is considerably shorter. "I can take my whole production environment and put it back on a BCV in four hours." Shreidan says, noting that tapes are available back to January 2000. Recovery testing is an ongoing process, and Sheridan typically restores one tape per week to locate lost files. Recovery is tested more formally on a monthly basis by restoring to the company's disaster recovery site 45 miles to the north.
The SAIT drives were a big improvement over older DLT technology, but Sheridan is eagerly anticipating future tape drive upgrades. "We're staying on tape with LTO-4," he says. "With SAIT, it's probably a dead end." The ultimate goal is capacity, and 800 GB (compressed) per LTO-4 tape makes the economics and management needs of tape media far more appealing.
Long-term backups are also important in the gaming industry, and American Casino and Entertainment Properties (ACEP) must be able to furnish gaming data demanded by regulators. "All gaming-pertinent data has to be stored for seven years," says Aaron Perez, ACEP IT network manager. "We have huge safes full of tapes." Complicating matters even further, recent company acquisitions have caused ACEP's data to rocket from 8 TB to 20 TB in the course of just a year -- nightly tape backups were just not adequate.
Today, 20 TB backup jobs are first staged to Dell Inc. PowerEdge 1950 servers and onto two Dell PowerVault MD1000 disk platforms before being sent to an Overland Storage Inc. NEO 8000 tape library with six LTO-3 tape drives, each running about 50% of the time. "It's not all backed up at the same time," Perez says. "We have three or four backup servers running concurrent backups and dragging everything to tape." There are currently 200 tapes in the NEO library, but Perez rotates about 10 to15 of those tapes to the library each month.
The ability to take tape backups off site is a key element of the ACEP disaster recovery strategy. Perez relies on a NEO 4100 tape system located in another casino for emergency recovery. Efficiency is also a major aspect of tape economy, and Perez points out that the move to 400 GB (uncompressed) LTO-3 from older 50 GB AIT-2 tapes lets ACEP save far more data per tape in less time.
RTOs vary depending on the system that needs to be restored, but Perez notes a documented RTO of eight hours for "minimum usability." Slot systems with 80 GB to 100 GB of data may take far less time to recover, while email systems with 400 GB to 500 GB will take considerably longer. Recovery testing, however, is usually an ongoing process. "We test regularly when we have to restore a lost user file -- that happens three or four times each week," Perez says, adding that formal biannual testing also helps to verify the recovery process.
Looking toward the future, Perez expects to expand the NEO 8000 with additional tape slots, drives and servers, though a migration to LTO-4 isn't in the immediate roadmap. "I'd consider it, if the cost is right. But then you'd have to do a full overhaul [of the tape system and process]." The big dilemma for ACEP now is tape encryption, though Perez isn't yet satisfied with the capabilities of encryption in backup software. "Some software manufacturers aren't stepping up to the plate."
The future of tape backup
The demise of tape backup has been predicted for years, especially now that disk technologies, like continuous data protection (CDP), snapshots, replication and virtual tape libraries (BTL) have emerged in the data center. Still, experts are quick to emphasize that many disk technologies are basically used for buffering tape -- removing the backup burden from primary disk. Experts also underscore the benefits of mobility and long-term retention, noting that tape's reputation for questionable reliability is undeserved. "It's very easy to verify recorded data as you write it, so the reliability is never affected as it might be with disk," Abraham says.
The current market is stable, due in part to the current investments made in tape technology, and experts see LTO vendors with sales growth in the high single digits. "The total market size on average seems to be at a steady state or a slight increase," Abraham says. "That overall trend will probably continue for about five years." Experts say that the future of tape backup lies with the open, performance-oriented technologies, such as LTO. The eventual release of LTO-4 should support an incremental increase in storage capacity and speed, and the inclusion of encryption in the drive itself should attract attention from storage organizations concerned about security and compliance issues. The entire LTO roadmap promises growth to 6.4 GB of compressed storage with transfer rates to 540 MBps. "LTO is not on its deathbed," Biggar says.