Data Backups: Advanced Topics

Continuing from our introduction to Data Backups, we will now investigate the most efficient and economical methods of storing data on backup media. Below is an excerpt from Prepare for the Worst, Plan for the Best: Disaster Preparedness and Recovery for Small Businesses by Donna R. Childs, which discusses the multitude of media and services on which businesses can store their backups:

The backup strategy for equipment failure will differ from the strategy to deal with human errors. With respect to contingency for human errors, the priority was immediate accessibility of data, preferably by the users themselves, and brief retrieval and restoration times for individual files. For equipment failure contingencies, the goal shifts to minimize the total time needed to restore the full functionality of the affected system.

Creating a system backup will help to protect critical and important business systems. Again, we must select the appropriate backup media for this purpose. In this case, we no longer seek quick retrieval of data, preferably by the user, but a complete backup of a computer system. Depending on the number and average system size of computers, tape can be beneficial because of its low unit cost per megabyte of stored data. If we compare the costs between the different storage media and assume that the cost of storing 1MB on a CD is normalized to one, storing the same megabyte on a DVD drive is about 50% less expensive, on hard disks about 99% less expensive, and on tapes about 99.9% less expensive. However, this picture changes when you take into account the cost of the actual hardware for the media drive. Fast and reliable tape drives are expensive, and unless you measure your data storage requirements in terabytes, which would be unusual for small businesses, high-capacity hard disks are close to an optimum between cost-effectiveness and quick data retrieval.

We often use a backup system with large disks to store complete images of the system partition. The advantage of doing so is the instant availability of this image anywhere in the network should the system ever fail. There are a variety of software tools in the market that provide this disk to image functionality. Note that disk images are only dependent on the file system being used, which means that the data format is not proprietary to specific software.

Since your backup system itself becomes a critical part of your operation, hosting two different backup data sets, you should consider installing a Redundant Array of Inexpensive Disks (RAID) system. You simply add a second hard disk and connect both the old and the new drive to a RAID card that is available at low cost. Configured properly, the operating system will recognize the two drives as one logical drive and any data written to that drive are written to both disks at the same time, effectively mirroring all data. Actually, if you use one of the free UNIX operating systems, like Linux, on your backup system, you can use the software RAID solution that is included for free as it is done on a commercial network attached storage drives. It is more than sufficient for a backup system.

The mirroring functionality gives you protection against hardware failures. But remember that a mirroring RAID system does not protect you from data lost due to human error or electrical shock failures. If you delete a file, it is deleted on both mirrored hard disks and in case of an electrical shock; both hard drives would be affected. This also applies to externally connected hard drives, so for protection you should always use two alternating external hard drives. If you are concerned about human error as well, you should simply partition the logical hard disk into three areas, one for the operating system, and two partitions of equal size, one for user data and the other as backup of the user data partition. Periodically, or on demand, you can use a file synchronization tool to update the backup data partition. Now you are storing each user file four times on two disks.

It is a low-cost solution that should give you more confidence about storing data on your desktop computer. Of course, the computer could be stolen or destroyed in a severe disaster. We will come back to this topic later in this book when we examine more severe forms of disaster, as in the section for "Fires." For now, you could make the backup hard drive a removable drive and take it home with you each night to provide a readily available backup in the event of severe disaster. It will not be necessary to do anything other than to reinstall a disk should one of the hard disk drives fail as replacing the defective disk with a new one will automatically trigger a copy from the old disk to the new disk until the data on both disks are the same again. This process is easy, but can take hours to complete if the disks are really large.

If this RAID method appears to be overkill with regard to your contingency requirements, another method is to use a single additional disk and copy data from the main disk to that additional backup disk using the same file synchronization or disk-to-disk backup tools as previously described. The advantage to this method is that you would initiate the backup yourself after you are sure that the changes on your main hard disk reflect the changes you wish to make. You can even use these tools to create a backup directly on an external backup medium, such as CDs or DVDs. With these solutions, however, you lose the on-the-spot safety of a RAID strategy. If the main disk fails, you will have lost all changes from the time you ran your last backup to the moment that main disk failed.

Don't forget to periodically back up your system partition. You might think that if it fails it can be easily reloaded from the installation CDs. Think again. Within one year, you probably install numerous patches and updates, software specifically for use by only that particular computer and customized settings with regards to the actual computer usage. Recreating this environment when the goal is to minimize system downtime is simply counter productive. Reloading the system from scratch could easily become a daunting and time-consuming task.

Always remember, the additional costs of a backup system will pay off handsomely if you have to make use of your backup system only once in its lifetime. Trying to repair parts or recover data from defective hard disks rarely makes economic sense. Replace the faulty part and rely for restoration of data on your backups instead. Interestingly though, manufacturers report that about 70% of the hard disks returned for repair under warranty are still in perfect working order, but their data structure is corrupted. So before you return a hard disk for replacement under warranty, try to reformat the disk first and run a thorough hard disk check.

If you send your disk to a data recovery service, make sure you understand the costs involved. The service typically charges a low fee to take an initial look at your hard disk, but data recovery work on your disk can easily cost hundreds of dollars, even thousands, if data forensic experts have to reconstruct files that have been deleted. Consider not only the cost, but also the inconvenience of not having your data for up to several weeks. Of course, there is also a potential security concern when you send your hard disk to others.

One last piece of advice on this subject: If you have to re-create a system disk from the stored image on the file server, you will need to buy a new disk. Try to purchase the exact same model disk from the same manufacturer. If you choose a hard disk from another manufacturer, you need to be aware that hard disks vary slightly from manufacturer to manufacturer in size, even if the overall given size is the same. Copying the disk image back to a disk that is too small, even if by only a couple of megabytes, will not guarantee success.


From Prepare for the Worst, Plan for the Best: Disaster Preparedness and Recovery for Small Businesses by Donna R. Childs. Copyright 2008 John Wiley & Sons, Inc. All Rights Reserved. Used by arrangement with John Wiley & Sons, Inc.