What's for your Backup: Disk or Tape

Most of the time, you find tapes and disks stading opposite each other-fighting a battle to win over the complete backup pie. The question is whether only one will survive or if we can have the best of both worlds

Monday, September 25, 2006

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Backing up is a must for all enterprises, large and small. But backing up is only part of the story. There is also recovery, which, in the long run, is far more important. And then you have the need to archive what you've backed up, so that you can pull out data as and when you need it later. What technology do you use for performing all these functions? If you asked this question about two years ago, you would get only one answer-Tape. Today, however, things are looking different with disk emerging as a strong contender. It's no wonder that the question that plagues most enterprises is whether to choose disk or tape, or should a combination of the two be used. And all that hype created by vendors around both doesn't help much either. In this story, we try to remove the hype and analyze both disk and tape based technologies in detail to help you decide the best option.

So what is the reason for this ongoing rage between tapes and disks? Is it just that the tapes are as old as probably the concept of storage, is it cost, speed, or time? Well, on the cost front, disk is definitely becoming cheaper, but it's unlikely to become as cheap as tape, so that can't be the reason for tapes to fade away. In fact, that would be one reason why people will continue to stick with tape for times to come. On the speed front, disks definitely have an edge, which directly links to the time taken for backup and restore. So the time advantage is with disk. How does one answer this dichotomy?

Before choosing any technology, you need to first assess your own requirements. If you're backing up data, what kind of backup do you want to do? Today, you have several options in hand: online, offsite, near-line (near online), incremental...mirroring, archival and the list goes on. Then put down the advantages and disadvantages of disk and tape against the options that are applicable to your organization. The immediate answers are that disk is more suitable for online, incremental, and mirroring based backups, while tape is more suitable for offsite, archival and near-line backup.

Is it anti-incumbency?
Despite lots of talk, nobody seems to be taking that bold step to do away with any one of them. Is it because the tape has been the traditional medium of choice that has been around for ages and has a good record for longevity and reliability if handled correctly and managed properly? Or is it because of the anti-incumbency, the inertia that resists change, that has crept in?

Well not exactly. Faster performance is one reason for everyone to move from tape to disk for backups, but it's not the only reason. The magnetic tape is also fragile, can break easily, is vulnerable to environmental factors such as humidity and heat, and it loses tension. Tape drive heads get dirty and have to be cleaned, and data isn't always restored perfectly due to such reasons.

On the other hand, disks are more durable, last longer, withstand more overwriting and you don't need to clean any heads. When it comes to backing up using disks, they are easier to manage. Disk backup systems include management tools, often browser-based, for you to easily configure settings and check status from anywhere.

Not only these, tapes can only read and write data sequentially, making the whole process slow. So there was a need to bring in a technology that could write to them fast enough. Consequently, multiplexing (also called interleaving) was born. Many back-up software use multiplexing to send back-up streams from multiple file systems or multiple clients simultaneously to one tape drive. This allows a tape drive to 'stream' at its rated speed. Many tape drives today would never write anywhere near their rated speed, if they don't make use of interleaving. But, restoring data from a multiplexed tape can take much longer. As a result, most organizations use both backup devices (tape and disks) for better results. Once last night's backups are sent to disk, they can then be easily copied to tape and sent offsite. (These tapes would also not need to be multiplexed, as the copy is being made locally.)

Here lies a dilemma. When you use tapes, you need multiplexing to be able to finish backups in time, but multiplexing makes it difficult to restore data easily.

To avoid it, you need a media type that doesn't need to stream as with interleaving. But how could you do that? Well, “the answer is blowing in the wind”-disk.

Mixed approach
Disk arrays are used for regular storage, tape is relegated more to just archiving and batch backup is done using autoloaders. But there are other types of backup that are being used-D2D (disk to disk) and D2D2T (disk to disk to tape).

D2D: In this type of backup, data is first stored on a primary disk for a shorter period of time and online, and later taken to secondary storage.

D2D2T: In this type, there's a disk array for primary storage and secondary array or a VTL that emulates a tape library. The data is stored in the disk array for primary storage and is then sent to the secondary array. After 30-90 days the data is backed up to tape. This is a type of tiered storage when you can retrieve data online for a certain period of time and then it is offloaded to be archived at a remote location.

But in today's scenario, organizations need to keep more data online as there are new regulations like Sarbanes-Oxley in US and health regulations governed by HIPAA that require the data to be kept online and to be accessed quickly for longer periods of time without it getting changed. Here, the requirement for quick and hands on access can be met only when the data is kept at online or near-online locations and not stored on tapes offsite.

Size and backup window
Most backups are done overnight or at weekends. This is to get a larger backup window. Backup window is the time between the start of a backup process and when it ends. And then growing volume is the biggest danger that enterprises face today. Most organizations want to save almost everything for at least longer periods of time for the rainy day, thus, requiring more storage size.

This disk is so economical that you could actually keep all of your cyclical full and incremental backups on disk. Day-to-day restores can be fulfilled instantaneously by disk. In such cases, tapes can only be used for disaster recovery and archival restores.

So one thing to remember is that atleast in near future tape will not go away completely since there will always be applications where cost per bit is 90% of the buying criteria. And then, as back-up compression gets built into more products, the economic advantage of the tape will gradually decline. So if tape is to survive in this dynamic environment, there is a need for change-not only in terms of increasing capacities, but also a more resilient technological platform.

Need for change
Although the capacity per cartridge of linear devices is projected to increase with time, it is unlikely that it will pose an aggressive threat to the disks unless there are any major technology breakthroughs. In fact, there's a limit to the current linear tape formats, in terms of the areal recording
densities it can support. And at this rate, the capacity per cartridge cannot exceed an uncompressed capacity of a few TBs. This can be countered only when a new class of tape technology is developed; one that would leapfrog current technology limitations to deliver capacities and performance well above the capabilities of current tape formats. Some technology that would reduce the aerial density per GB per square inch furthermore like the perpendicular magnetic recording in disks. This new technology platform would then be able to keep pace with hard disk drive advances. In addition, a new tape technology must be a perfect blend of capacity, performance, and reliability with an achievable roadmap and multiple sources of supply. Only then it will be able to fully meet customer needs and industry acceptance and, thus, assure the future of tape within the storage hierarchy.

Expected turnarounds
The makers of blue-laser disk are eyeing the market for enterprise archiving applications. This space is currently dominated by magnetic tapes and to some extent disk arrays. The new generation media support 2x speed, which is a data transfer rate of 72 Mbit/sec, making the disks suitable for video recording, data storage and file backup.

Similar to magnetic tape, optical disks can be removed and stored offsite for decades though the only disadvantage is the optical disk capacities that don't come anywhere closer to the tape drive capacities. But unlike tapes, with optical disks, searches can be performed at random at sub-second speeds, which is why vendors are pitching their optical disk technologies as competitors to tape. There are already a few products in the market that are based on a new UDO (Ultra Density Optical) laser optical disk format that offers 30 GB per platter. The archival appliance from Plasmon is one such product with 960 GB to 19 TB capacities. As per the roadmap laid for the optical disks, companies plan to release a 60 GB optical disk by 2007, followed by 120 GB and 240 GB versions in 2009 and 2011, respectively. With all these coming up, you won't need tapes for going to individual records for restoring. This is because while the disks will be almost of the same capacity, they will also be near-line accessible. The blu-ray disks are also gearing up to be used for archiving data, first by entering the SOHO space and then big corporates. All this is slated to become reality soon.

Different organizations have different objectives when they have to back up data.

In fact, backup and retrieval are not the same as continuous availability of the data. So you should not lock yourself into one technology solution. Instead, go for what suits you best for one particular need and scenario. For instance, if you have data that is to be seldom used, tape is an ideal choice, but if all you want is a faster backup in a smaller time window, disk may have an edge.

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