12.1 Backup Strategies
12.1.2 Backup methods
There are several different backup types or methods that can be compared based on how long the backup takes and how easy it is to restore data. The three most common types of backups are: Full, Incremental and Differential. All of the methods discussed here assume backups of critical user data to a tape drive occur daily. Alternatives to tape backup media will be discussed later in the chapter. The operating system and applications software are not typically backed up since they can be reinstalled from the original media.

It is common to have a set of 5 to 7 daily tapes, 4 or 5 weekly tapes and 12 monthly tapes. The daily tapes can be overwritten each week. The weekly tapes can be overwritten each month and the monthly tapes are usually kept and are not overwritten. Weekly and monthly tapes should be kept offsite. If the volume of data to be backed up requires multiple tapes, it may be necessary to install a larger capacity tape drive or a tape changing system. If not, it will be necessary for someone to be there when the first tape finishes and put the next tape in. Backup methods performed by organizations vary. However, all organizations take a number of factors into account when determining which method to utilize. These factors include:

  • the criticality of the data,
  • the amount of data to be backed up,
  • the time window available to do backups, and
  • funds available for a backup system.

Loss of data can be just as damaging for a small business as for a large corporation. The backup system is not the place to cut costs. Figure compares the three different backup methods and Figure shows the relationships between quantities of data backed up each day.

Full Backup
With the full backup, all critical data is backed up daily. Every night after close of business, all data is backed up whether it has changed or not. This method takes the longest and may require multiple tapes depending on the amount of data. A larger capacity drive or tape changing system may be required if data volumes get too large. If an organization has a large volume of data and/or a short window of time to back it up, the full backup method may not work for them. However, it is the surest method because all data is backed up every day to one or more tapes. A full backup can also make it easier to find and restore data since it is usually all on one tape or a set of tapes for a given day. For purposes of comparison to the other methods, we will assume that we can do a full backup on one tape.

Incremental Backup
With the incremental backup, data is backed up in increments. All critical data is backup up once a week on a particular day (e.g., Sunday in the evening). Each day after that, only the changes are backed up. The next backup on Monday will only backup new files or those which have changed since Sunday night. Tuesday's backup will only back up the new and changed files since Monday's backup. Wednesday's backup will only get the new and changed file since Tuesday's and so on until we get back to Sunday and we then do another full backup. The Sunday night full backup creates the baseline backup and may take a significant amount of time, but the successive incremental backups on the weekdays will take much less time. The amount of data and the time it takes to back up on each weekday will vary depending on the number of new and changed files but will be relatively small compared to the full backup.

With the incremental backup, each tape will have a piece of the whole backup. As a result, the overall time required to back up the data is significantly less than doing a full backup each day. The disadvantage of the incremental backup is the number of tapes it may require to rebuild the data. If a hard drive fails on Monday morning, the Sunday night backup tape will be the only tape needed to restore the data. However, if it fails on Friday morning, it will require Sunday's full backup tape plus Monday through Thursday's incremental backup tapes to fully restore the data. This means that we must successfully read and restore data from five tapes, any of which could have a problem. This results in a more complicated and time-consuming restoration process. Since incremental backups only back up a portion of the data each day to multiple tapes, a failure of any of the tapes will result in a corrupt backup. In other words, if you need five tapes to do the restore, if any one of them cannot be read, the entire backup is bad.

Differential Backup
With the differential backup, we still do a full backup once a week on Sunday. On Monday, we back up anything that has changed since Sunday. On Tuesday, we also back up anything that has changed since Sunday. On Wednesday, again we back up anything that has changed since Sunday and so on. Monday's backup may not take much time since we are only backing up one day's worth of data (same as incremental) but every day after that the backup takes longer. On Saturday night, we are backing up everything that has changed since last Sunday which is 6 days worth of data. The differential backup is a compromise between a full backup and an incremental backup. It takes less time than a full backup since we are not backing up all the data every night but it takes more time than an incremental. If a hard drive fails and we need to restore all of the data, it will only take two tapes. As an example, if the hard drive fails on Friday morning, we will only need the Sunday baseline tape and the Thursday night tape since the one from Thursday night's backup includes everything from Sunday night through Thursday night. This makes restoration a little easier than the incremental backup but not as simple as the full backup.