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Safety First!

Develop a backup strategy for your system

From "Special Report",  Access to Wang, August 1990
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Simply stated, backup consists of copies of critical system elements necessary to continue operation of the organization in the event that the normal processing channels are unavailable. This is typically interpreted to mean tape or disk copies of systems files that are made at predetermined times and set aside, but other interpretations are also valid - such as the disk mirroring options available to users of Local Area Networks or the complete system redundancy offered by high-end mainframes.

My purpose here is to outline some of the considerations involved in the backup process and the options available to implement that process. This necessarily involves issues of file retention, emergency processing, and the accessibility of the system to users - since the backup process typically requires exclusive access to the system. The goal of this analysis is the formulation of a backup plan.

In referring the frequency that these copies are made, I will mention days - the most common backup frequency - even though it is probable that more or less frequent backups are necessary for your conditions.

Goals of a Backup Plan

Before determining the backup process, the possible uses of that backup should be considered. Backup data can be used to provide data processing capability in the event of complete destruction of the computer facility, but many less severe conditions also require use of a backup. Production errors - such as an incorrect date entry in a purge process - are far more likely uses, as are temporary "simulations" of prior periods for reporting purposes. Routine disk initialization also requires complete and accurate backup copies.

While an exhaustive study of emergency processing issues is beyond the scope of this article, such considerations should be present when formulating a backup plan. How long will it take to restore the data? How long to resume normal processing after a crash? And how frequently should backup copies be made? How accessible are backup copies of the data? Are these copies complete and correct? All of these and other questions must be addressed before the backup plan is enacted.

The ideal backup plan should:

Backup Administration

Once the plan is created, administrative elements must be considered. These include scheduling, staffing requirements, the timing and scheduling of routine maintenance, and the subsequent impact to system availability and service levels of all of this activity.

System administrators are often caught between the need for user access to the system and the goals of adequate backup and maintenance. Usually this means that the backup process is squeezed between times of user activity - traditionally, late at night. This type of schedule makes it difficult to hire, monitor, and retain good personnel. Fortunately, the time required for the backup process has been decreased and large-capacity backup media are making operator intervention during backup less necessary.

Types of Backup

Approaches to the backup process include full, incremental, and cycle plans. Each has advantages and drawbacks that must be considered in the complete backup plan.

With the full backup approach, all files are copied every time, regardless of whether they have been modified. Restoring data from this type of backup is the simplest, since every necessary file is present on any copy. The length of backup time and need for many volumes to contain the data are two principal drawbacks - though the emerging popularity of high-volume cartridge tape systems has negated most of these objections.

With the incremental approach, all files are copied at set intervals, with files changed since the last full backup copied every day. This lessens the time required for daily backup but makes restoration more difficult. To restore from an incremental backup, the last full backup must be copied and all daily incrementals overlaid up to the last valid day; this process can considerably increase the time required to restore data.

The cycle backup approach is similar to an incremental backup, except that files that change infrequently are divided into groups and backed up in a regular cycle. It eliminates the need to perform full backups by covering these static files over a predetermined backup cycle. Restoration from this backup approach amounts to successively restoring files from the beginning of the last cycle until the last valid backup - a process that depends on the number of days in the cycle. This process is best used in installations where the amount of time required for a full backup would significantly hinder production.

Most installations use a combination of the approaches mentioned to meet their organization's coverage needs. Such combinations might include: an incremental approach for most times, with full backups at the end or beginning of the accounting calendar; frequent incremental backups during the week and a separate weekend full backup, or other strategies.

Backup Strategies

I use the term backup strategy to refer to the approach you take to the backup process based on the usage of your system. The coverage you need depends on how much data on your system changes, how frequently it changes, and likely reasons for access to earlier versions of the data. By understanding the ways your system is used and the possible reasons for restoring data to it, you can select the manner in which the data is gathered and the medium used to record it.

Consider a simple example: a batch-oriented Purchasing system. In this hypothetical system, users enter information into transaction files and update master files every few days. There are no ties to other applications on the system. The only files that change daily (and, therefore, need daily backup) are the transaction files; all other files change less and might be backed up less frequently. Naturally, unchanged files must still be backed up on a known, consistent schedule. In this case, a sequential backup system (e.g. tape) and an incremental backup approach would be adequate.

Restoring this system might amount to copying the last full backup and overlaying the most recent incremental backup for recently-changed files. Since the full backup would contain the bulk of the files, the time required to restore from successive incremental backups would be less significant.

Most installations are more complex than this example. Systems are often integrated, so data changes made in a Purchasing system are likely to update Inventory and Accounts Payable files as well. In other words, an incremental backup is likely to pick up most of the data files on the system every time - perhaps most of the storage requirements of the system. In this case, full backups might be more viable than incrementals, since the time saved by not copying unchanged files might be overcome by the time required to determine whether the file is to be included in the incremental backup. Restoring data from a full backup would also be simpler and probably take less time. And sequential backup media would be suitable, since you would usually need to restore all of the system rather than portions of it.

Large document-processing shops have a very different situation: a multitude of files that change little, with a handful that change every day. Full backups might cover the large number of documents that have not changed, but changed documents are more likely to need restoration to cover user mistakes or other situations. In such conditions, the number of files that change and the likelihood that these files might need to be restored both go up. The backup frequency and speed of access become concerns, since the files must be accessed randomly for restoration.

The critical need for large Word Processing shops is rapid access to those documents most recently changed. This indicates that some part of the backup approach must be timely and access must be random. In addition, the entire catalog must be accessible relatively quickly. For such situations, a random-access medium (disk, optical disk) might be best, supplemented by routine full backups performed by lower-cost sequential media. Depending on the access speed deemed important, it may also be necessary for additional coverage - such as a disk-based document backup and archiving system that detects editing changes immediately and retains all prior versions.

Unfortunately, few systems fit into either of these extremes. It is more often necessary to tailor the backup approach and even the medium to specific systems. Conflicting access and service level goals between classes of users may mean that the best alternative is a compromise.

Implementing the Backup Plan

Once the goals and approach are decided, the plan must be implemented. For many shops, this amounts to careful planning and scheduling, the use of a appropriate backup medium, and Wang's BACKUP utility. If your needs are complicated, the traditional backup setup - Wang's BACKUP utility and reel-to-reel tape - may not be sophisticated enough. In any case, it is worthwhile to consider the tools of backup in relation to the goals of the backup plan.

BACKUP is by far the most common program used for the backup process. BACKUP offers universal access on all VS systems, preservation of File Descriptor data such as modification date, owner ID, and special access rights, and the ability to reliably span multiple disks or tapes. It uses a file orientation, assembling disparate portions of files into contiguous wholes during output, and ambiguous file elements (wild cards) can be used for the input or output specification. BACKUP produces a log file and an optional report, and either can be stored on the output volume or any other volume specified by the user. File conflicts are noted on the operator's screen and the backup log, and the files bypassed.

Drawbacks to BACKUP include its non-standard tape output (non-labelled tapes), tape headers that are large and space-consuming, and little internal verification of the file's integrity. While a log is produced by BACKUP, its use is limited to the BACKUP program itself; it cannot provide rapid answers to queries about a file's location. The skipping of open files is a valid approach, but BACKUP will not return later and try to get these files, as some other products do. BACKUP will copy files within similarly-named libraries that lie across multiple volumes, but cannot automatically restore these files to multiple volumes. Finally, BACKUP is considerably slower than some third-party offerings.

In response to these limitations, a small crop of third-party backup software has sprouted. Most of the claims made by these products center around transfer speed, but some also offer other features - such as the ability to restore to multiple output volumes or automatic maintenance of file location files for query purposes.

Actually, you may not have a choice of backup software if you use any of the new-generation backup devices, such as 8mm VCR tape cartridge units (the Exebyte format) or Digital Audio Tape (DAT). At present, all of these units use proprietary interfaces that require their own software for operation. This can be an issue if your emergency plan calls for your files to be restored at a site with another brand of tape cartridge unit. Be sure to consider this before purchasing such units.

Backup Hardware Choices

The types of equipment used to perform backups has changed radically in the last few years. The use of high-density 9-track (reel-to-reel) tape systems has increased, due to cost reductions and vendor competition. At the same time, removable magnetic disk options have remained stagnant: the only new removable drive of significance introduced recently is the 76-megabyte RSD, and 288-megabyte SMD drives remain the only large-capacity removable disk. Optical disks have made an appearance, but failed to gain wide acceptance due to slower transfer rates, non-standard data format, and limited availability. Finally, the appearance of an assortment of high-capacity cartridge tape systems has overtaken the market, causing many to reevaluate their backup approach.

Each medium has individual advantages and constraints, and must be evaluated in terms of meeting the goals of the backup process. Some of the considerations in choosing a medium include:

Other Items for Critical Attention

The backup plan must address many elements and strike a balance between complete system access and data safety. Other elements of the backup process not covered here include the specifics of retention strategies, including identification of critical information; file location logging and inquiry; the physical health of the storage systems, including bad block detection and VTOC verification; elimination of disk fragmentation through disk compression or routine full restores; detection of errors in indexes or alternate index chains within files; the storage conditions of the backup volumes; and the security of backup copies from unauthorized theft or destruction. Finally, the backup plan must be thoroughly tested and subjected to regular review for relevancy and completeness.

Careful attention to the backup process can help ensure that, as guardians of data integrity, we are not surprised by events beyond our reckoning without possibility of recovery.


Figure 1: Other Software for Backup

Here are some Wang utilities and Useraids that have some application to the backup process or data verification:


Figure 2: Glossary of Backup Terms

Term Definition
Back up; backup The process (back up; verb) or products (BACKUP; noun) of a routine procedure to copy vital information for safekeeping.
Contention Access conflicts, as when two tasks attempt exclusive access to a file.
Disk and tape labels Standard means of identifying a medium, recorded in the same manner as the data itself.
Disk mirroring The use of redundant storage such as disks to simultaneously record information as it is changed on the primary storage system. Presently available in specific Local Area Network products, but the theory could be used on any system.
Generation A set of backup volumes that represent a point in time. Usually used to refer to the number of back copies preserved.
Logs; logging Reports or files constructed that show backup activity, usually including file statistics, input and output locations, date of processing, and any errors encountered.
Random access Direct access to specific items. In data storage, a medium that quickly retrieves selected files. Opposite of sequential access.
Restore The process of copying backup copies of data onto a system.
Retention The amount of time a file is to be retained, based on its perceived usefulness to the organization.
Sequential access Access to individual items along a specific path. In data storage, a medium that retrieves selected files by reading all files, bypassing all that do not apply. Opposite of random access.

Figure 3: Storage Media for Backup

Backup media must be portable and large enough to cover the data storage needs of the organization. Below are the major categories of backup storage systems.

  • Removable disks: Offers fast backups, easy verification of backup copies, and rapid restoration through random access. Drawbacks include high media cost, low capacity of available units, and high initial and maintenance costs.

  • Reel-to-reel tape: The traditional approach to data processing backups, enhanced by greater use of high-density tape units (e.g. 6250 bits per inch). Slower backup and restoration than disk, but offers industry-standard recording approach. Sequential access makes this a poor choice when files must be restored individually.

  • Cartridge tape: Offerings in this category range from older Wang four-track cartridges and streaming tape units to the new 8mm units based on video technology. A lessor-known medium - Digital Audio Tape (DAT) - may become a standard medium in other computer environments. Somewhat faster than reel-to-reel tape; other features are similar.

  • Optical disk: Until recently, offered only in WORM (Write Once - Read Many) forms; revisable forms slowly becoming practical. Offers random access, but sluggish transfer rates slow this considerably. Media cost is moderate and the media can withstand more physical abuse, making this a practical means for permanent record storage. The permanent status of backups produced by WORM devices can be an advantage where copy integrity must be preserved without possible modification.


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Copyright © 1990 Dennis S. Barnes
Reprints of this article are permitted without notification if the source of the information is clearly identified