A major change in the enterprise data storage landscape over the past decade started once the cost per megabyte...
of tape storage became more than the cost per megabyte for disk storage. But tape still has its place in data backup and recovery for many companies, especially in small- to medium-sized businesses (SMBs).
There are many advantages to using tape backup and tape libraries. For example, backup tapes can be removed and stored safely offsite. And tape cartridges can usually recover from drops and other damages that could make hard drives non-functional. Tape cartridges may even survive floods and other disasters that would wipe out systems based on disk storage. Also, in the case of a disaster, tape recovery services can be used to clean and dry tapes, and recover much of the data that was originally stored on them. In this tip, read about the pros and cons of tape storage, popular tape data storage technologies today, and how tape compares to other backup considerations in terms of reliability, cost and more.
Longevity of tape media storage
Tape cartridges may last for as long as 30 years if they are properly stored. In addition, many tape storage devices are backward compatible. For example, a tape written by a digital linear tape (DLT) drive, made nearly 20 years ago may be readable on recent DLT drives. Drives that can read nine-track tape from the 1980s are still available. And although many early tape formats are not well supported today, it is possible that tapes made today will be readable on future drives.
Hard drive interfaces may not be as long-lived. The original, tiny capacity drives of the early 1980s that used MFM or RLL interfaces probably can't be read today because it is probably extremely difficult to find an old PC or a drive interface card that could read it. Parallel ATA (also known as IDE) drives are also well on their way to becoming dinosaurs. But SCSI has been around for more than two decades, and Fibre Channel (FC) has been in use for more than a decade, and they may continue to be around for a while longer. Serial ATA (SATA) is the new kid on the block, and drives as large as 2 TB in capacity are currently available, and it will probably be around for at least another decade. In general, for long-term storage and successful recovery of data, tapes may be the best option.
Downsides of tape storage
There are many reasons why SMBs may choose to use disk-to-disk backup instead of tape storage. Tapes are not random access like hard drives. If you want to restore a file that is at the end of a tape, the drive must go through the entire tape to find the file you want. If the data is on another cartridge, you have to eject the cartridge (or, if it's in a library with more than one drive, you have to load it into the other drive), and have the drive search for the file. In other words, tape may be a hassle for some people because it is slower than disk backup. Plus, the cost per megabyte of a tape cartridge and drive is often higher than the comparable hard drive backup. Today, terabyte hard drives are available for less than $100, and 1.5 terabyte drives can cost as little as $150.
In contrast, LTO-3 tape cartridges, capable of storing 400 GB native (800 GB compressed) cost at least $25, when bought in large quantities. LTO-4 cartridges, with 800 GB native (about 1.6 TB compressed) cost around $40 or more, which in this case, may actually cost less than comparable hard drives. DAT72 tapes, with a raw capacity of 36 GB run around $10 each, and 20 of these cartridges, with a total capacity of 720 GB, would cost around $200. Depending on which technology you purchase, in most cases the cost per gigabyte of capacity for tape is usually higher than the cost for the SATA drives that form the basis of many disk-based data backup systems.
Tape storage capacity is often less than disk storage capacity
Tape cartridge capacities are smaller than the disk drives they'll be backing up. The latest LTO-4 tape drive only stores a total of 1.6 TB. The actual raw (uncompressed) capacity of this cartridge is 800 GB. For years, the tape industry has quoted capacities that reflect "compressed" storage. For text data and simple data files, the actual amount of storage may be more than double the raw capacity. However, if you're backing up already compressed files (i.e., ZIP, video, MP3, or JPEG images), these files won't be compressed.
In other words, if you want to back up more data from your storage system than your tape can handle, you've got a few choices. First, you can have someone wait near the tape drive and, when prompted, switch out a blank tape to replace one that's already been filled with data. Or, you can get a tape library that can automatically remove a full tape and retrieve a blank tape from a storage slot in its enclosure, and load the tape into the drive. Similarly, the robot can automatically switch tapes to the appropriate volume when restoring files from the tape system.
Popular tape data storage technologies today
Over the years, many tape storage technologies have fallen out of favor and the industry has focused on two: LTO and DAT. LTO tape drives have a higher capacity than DAT, and use half-inch tape inside the cartridge. LTO drives are larger than DAT and in most cases, the drives and libraries are more expensive than DAT. The latest version of LTO is LTO-4, and the cartridges are capable of storing up to 800 GB of raw data (and about 1.6 TB of compressed data). Raw data transfer rates of up to 120 MB/sec are possible in full-height LTO-4 drives, with half-height drives capable of transfers of up to 80 MB/sec – compressed data transfer rates may be roughly double those for raw data. LTO-5 and LTO-6 have been proposed, and will offer capacities that roughly double those of the prior versions.
A handful of vendors offer LTO libraries, and they include: Hewlett-Packard (HP) Co.'s StorageWorks, IBM Corp., Quantum Corp., Spectra Logic Corp., Sun Microsystems Inc.'s StorageTek, Tandberg Data and others. Small libraries can house as few as one drive, with capacity for two or more drives that can be purchased along with the library or added later. Libraries typically start in a 2-U form factor, and these smaller form factors are probably best suited for SMBs, which may not have the need, or budget, for the larger units.
In November 2009, a new DAT drive was announced, the DAT 320, which provides up to 320 GB of compressed storage capacity (160 native). Unlike the earlier DAT cartridges, which used 4mm tape, the cartridges used by DAT 320 use 8mm tape. The DAT 320 also has a transfer rate of 11.9 MBps .
Earlier versions of DAT, the DAT 72 and DAT 160, offer compressed capacities of 72 GB and 160 GB, respectively. Because the cartridge sizes are different, it is unlikely that library systems designed for the DAT 72 and DAT 160 drives will support the DAT 320 cartridges. Tandberg Data, HP StorageWorks, and other vendors offer DAT libraries supporting 4mm DAT drives and cartridges.
Tape storage and backup considerations for SMBs
Based on cost, the decision between DAT and LTO may be fairly simple for many SMBs because the DAT products have considerably lower price entry points. However, it may be appropriate to consider earlier generation LTO-2 and LTO-3 systems that may provide comparable or superior performance and capacities in relation to DAT drives. For example, DAT 160 drives, which have a native capacity of 80 GB and a compressed capacity of 160 GB cost about $800. By contrast, an LTO-2 drive, with a native capacity of 200 GB and a compressed capacity of 400 GB costs about $200 to $400 more. The price of the new DAT 320 will probably be higher than an LTO-2 with slightly higher capacity. The estimated cost of a DAT 320 drive is between $850 and $1,300, according to a search of specialty storage websites.
Interface choices are also an important consideration. LTO drives are often available with native Fibre Channel interfaces. Serial Attached SCSI (SAS) is also a frequent offering, as is the more standard SCSI. According to sites offering the DAT 320 drives for sale, these drives will initially be available with Serial Attached SCSI (SAS) or USB interfaces. DAT libraries may not offer Fibre Channel or SCSI. The move towards external SATA (e-SATA) may make connections to computers easier and less expensive, because many computers are now equipped with e-SATA ports, and e-SATA adapter cards are available at low prices.
When cost is a major consideration for an SMB, an eSATAII interface card that includes one eSATAII port, three internal SATA ports and RAID capabilities, can be purchased for less than $100. This controller can provide RAID for three internal SATA drives, and can connect to an external tape drive.
Other tape technologies that have been eclipsed by the rise of DAT and LTO include DLT, SuperDLT, Exabyte and VXA (although, a VXA library is still offered by Tandberg Data). For some companies that don't mind risking the move to technologies that may not be as well supported in the future than DAT and LTO, the move to these other, still useful technologies may save you money.
Moving to Fibre Channel may seem like a good idea for some SMBs. But keep in mind that Fibre Channel adapters can cost many hundreds of dollars. They are available in versions that support optical connections (using fiber optic cables and special adapters), and in copper FC adapters that provide lower performance but use copper wire instead of optical cables. The fiber optic cables can connect devices over many miles distance, while the copper cables support much less distance. High-end Fibre Channel can transfer up to 4 GBps. However, tape drives aren't able to read or write at these speeds, so moving to Fibre Channel is probably overkill and a waste of resources.
As for SAS and SCSI? This also isn't the cheapest route to go. Their controller cards frequently include RAID capabilities for internal or external drives. These also typically cost at least $200 or more.
For many SMBs, once a full backup is made, incremental backups only of changes to data will usually not take up nearly as much time. (For example, if you back up 3 TB of data, but only make a few hundred megabytes of changes each day, the incremental backup should take minutes to perform, rather than hours). For incremental backups of an SMB's drives, a high-speed interface like Fibre Channel is probably unnecessary. For your needs, a USB or eSATA should be more than adequate.
In the end, when an SMB is choosing between the various tape library options, they must ask themselves several questions. How much data will actually be stored on tape, and how frequently will this data be written to tape? How important is form factor -- is there enough room in the physical location to support the library? What will the cost of the tape drives, libraries and media be over a few years of use? Does an upgrade path matter? How long a time window will there be for data transfers, and will the tape drives support it? What interfaces are supported and, if you don't have systems that use the interface, how much will it cost to upgrade your systems? What data backup or data archiving software is provided by the manufacturer? How much will support cost, if needed? And, finally, is tape really the best option to meet my SMB's needs?
About this author:
Mark Brownstein is a technology journalist with experience editing computer storage publications. He also runs his own networks, is owner and operator of a test lab, has written books about computing topics, and is an MCSA.