Tape vs disk backup: Pros and cons of tape backup and disk-based backup

Tape is an established backup target, but disk prices are falling. Factors to consider when switching from tape to disk-based backup are performance and the overall DR strategy.

Tape vs disk backup: What are the pros and cons? Tape is still the cheapest backup media and during actual backup is the quickest method, as long as you have everything set up correctly. Meanwhile, disk-based backup offers the promise of speedy restores because of its ability to keep a lot of recent data at hand. And disk or...

tape can fit well as part of a disaster recovery strategy, but which is best for you is entirely dependent on your IT infrastructure.

In this interview, SearchStorage.co.UK Bureau Chief Antony Adshead speaks with David Boyd, principal consultant at GlassHouse Technologies (UK), about the pros and cons of tape backup vs disk backup in terms of cost, performance and manageability. Boyd also outlines some key decision points that will help you decide if it's time to switch to disk or stick with tape.

You can read the transcript of the podcast below or listen to the MP3.

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David Boyd on tape backup vs. disk backup
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SearchStorage.co.UK: What are the pros and cons for backup of tape vs disk?

David Boyd: There are three key differentiators for choosing between using disk as a target for backups or using tape: cost, performance and manageability.

Let's take cost first because it is always high on an IT director's priority list. A drop in the price of disk storage over the past several years has led to a lot of organisations adding disk as a backup target. However, when priced alongside the cheapest disk technologies, tape regularly keeps its cost-competitive edge, particularly for long-term storage.

With increasing data volumes and ever-reducing backup window sizes, throughput is also a key differentiator in the decision whether to use tape or disk as a backup target.

Here, pitching one over the other is not simple. Tape drives have evolved to become very fast; for example, the recently released LTO-5 is capable of writing at 280 MBps (assuming 2:1 compression) which will outperform disk, particularly as cheaper lower-tier disk such as SATA is normally chosen for backup.

Pushing one or several tape drives to write at 280 MBps requires a considerable upstream infrastructure, including very fast servers, SANs, LANs, clients and disk, and a backup product that is capable of writing multiple backup jobs to a single target. Where conditions are not optimal for backup -- I'm thinking slower infrastructure, small data chunks or workgroup-class backup products -- running a tape drive at these speeds is not going to be possible. While they do step down their performance, the general rule of thumb is that tape drives do not perform well in sub-optimal conditions. Here, because of its non-linear architecture, disk is likely to outperform tape.

When it comes to restores there are two elements to consider: read time and seek time. Seek time on a disk can be a matter of a few milliseconds whereas in a tape system a robotic arm has to complete any activity already scheduled or underway, locate the correct physical tape and load it into a drive. This is assuming the tapes are in the library. The drive then has to forward through the tape to find the relevant marker and then read the data from that point to find the data to restore. This process can take several minutes. If the tape isn't in the library, seek times can extend to several hours while it is recalled from off-site storage.

The third area is manageability. Tape is very portable. You can remove it from the drive or the tape library and take it home, send it to a third party for storage or ship it to another site for data migration purposes. While this action increases your recovery times, it makes adherence to your disaster recovery strategy very easy. That tape can be stored in a geographically distinct location providing a low-cost option for protecting your data from site-wide disasters.

You do not have the same flexibility with disk. However, if you do have a disaster recovery site and sufficient network links between it and your primary, then automatic replication of data to your remote site will be considerably easier and less risky than manually moving tapes -- that is you only have to set up the replication once and there is no chance of tapes getting lost in transit.

SearchStorage.co.UK: When is the right time to switch from tape to disk for backup?

Boyd: While there has been plenty of movement away from tape and toward disk over recent years, tape is far from being dead and I would not recommend switching from one to the other unless there is a change in requirements or if your infrastructure has reached end of life.

Even then there may not be a need to make a wholesale switch from tape to disk. Most organisations use tape and disk together. For example, it is now common to see backups of very fast databases get sent to tape, whereas images of file and print data are often stored on disk and then cloned to physical tape at a later time.

When making changes or upgrades to your backup environment it is always important to look at your recovery requirements. These should drive your design decisions. For example, if your backup images must be stored off-site but you only have a single data centre and you are not yet willing to jump into the cloud, then physical tape with off-site storage is your best option. If you have the same requirements, but you have two sufficiently disparate sites separated with a reasonable network link, then perhaps writing backup images to replicated disk is a better option. Several enterprise-class backup products can integrate with storage arrays supporting array-based replication of backup images to a remote site, and data deduplication can further enhance your ability to ship data between sites.

While disk continues to take market share from tape as storage for backup images, it is also worth noting that improvements in snapshot technologies are allowing storage administrators to take control of backup and recovery without the need for network-based backup products. If your recovery requirements demand very small recovery time objectives [RTOs] that can't be satisfied by traditional backup and recovery, disk snapshots of the primary volumes could be a good alternative. Furthermore, evolving technologies like continuous data protection are also increasing the value of disk by offering not only exceptionally low RTOs but also very low recovery point objectives.

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