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BACKUP
STRATEGIES |
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Tape backup systems have been in use for years on computer systems, from mainframes to minis to micros. With so many systems being backed up there have also been many different backup technologies which vary in both architecture and methodology. One of the areas that has differentiated backup systems is how tapes are rotated and what files get backed up to which tape. Each rotation method has different advantages that can apply to different systems. Some of the differences between methods are the time it takes to backup and restore, the number of copies and the number of different revisions of a file that get stored on tape. When looking into how you are going to develop your backup strategy and rotation method, it is important to look at what data is on your system, how critical it would be if some data were lost, and how fast a system would need to be operational if the system did become damaged or inoperative. You should also assign one individual who is responsible and accountable for administrating the backup to make sure it is done on a consistent basis. Your plan should include a specific time or times when backup is done. For more critical data, you may want to backup files several times during the day while backing up all changed files only once or twice a day. The easiest way of backup is where a full image of the system is put on tape every single day. A different tape is cycled through the system each day. This guarantees that a complete restore is able to be done with only one tape. The downside is that it takes many tapes, and, if you have a large amount of data, longer backup times each day. This type of backup is common for smaller servers with less than 1 gigabyte of data and that do not need to be up 24 hours a day for operation. You will also need a tape drive that can hold the entire amount of data on the server(s). A variation of the above method is where one tape that contains a full image of the system and subsequent tapes receive copies of the files which are different or were updated after the image backup. This type of backup is called Differential Backup. This allows the system to be restored with a maximum of just two tapes should a full system restore be needed. A variation of this method would be to copy only the files that have changed since the last backup to the tape. This type of backup is called a Modified Backup. This would take less time to backup each time and could be done several times during the day. It would, however, require more time to restore since several tapes may have to be inserted into the tape driver for a total restore. Some rotation methods allow for files to be stored multiple times on multiple tapes so that you can have different versions of the same file on multiple tapes. This allows a revision history of files to be stored on the tapes in case a past revision of a file should be needed. This allows a great flexibility in single file restoration, but can hinder the time to restore the system. To summarize the types of backups discussed above, we have three main types. Full Backup: Differential Backup: Modified Backup: After thinking about how you wish to place file on the tapes, you need to choose a rotation method for how tapes are going to be fed through the system. A few of the more popular systems that utilize the backup types above are described next. Grandfather, Father, Son The Grandfather, Father, Son (or for the politically correct Grandparent, Parent, Child) is a simple method that has been used for many years. In this manor, you label tapes by the day of the week with a different tape for each Friday in the month and a different tape for each month of the year. Using a tape for Saturday and Sunday is optional depending on whether you have files updated over the weekend. This would look like the following: MONDAY TUESDAY WEDNESDAY
THURSDAY FRIDAY1 MONDAY TUESDAY WEDNESDAY
THURSDAY FRIDAYI MONDAY TUESDAY WEDNESDAY
THURSDAY FRIDAY1 Since, some months have more than four weeks, it will take over 20 tapes for regular backups over one year. The A-B-A-C-A-B-A-D-A-B-A-C-A-B-A-E-A-B-A-C-A-B-A-D-A-B-A-C-A-B-A-F-
In the case of tape backup, we use the same order to rotate tapes through a tape drive, each day putting a complete image of the system on the tape. Although the theories behind why this works as a good rotation method are beyond our discussion, the benefit of this rotation is that you will always have an older version of a file on one tape. In the case above, you would have 8 tapes and considering you used one tape per day you could have a copy from as long as 128 days previous to the backup on tape. A further example is that since tape F will contain a full backup of every file on the system from every 128 days, if your system was infected with a virus, you could restore an executable file without a virus as long as you caught it within 128 days. There are several variations to this rotation also. If you did backups twice per day, you would be able to capture work in progress during the day, but you would also only have versions from as long as 64 days previous in the example above. It would also be possible to backup to a single tape for a week by doing a full backup and then only backing up updated files during the week. You could increase the number of copies this way but may run out of space on the tape. You could also risk losing up to a weeks worth of data if that tape has a problem or is damaged. Incremental Tape Method This method has a few names and is fairly simple to implement. It involves determining how long you wish to maintain a copy of your data and how many tapes you wish to use. It is based on labeling each tape with a number and then incrementing them through adding and removing one Backup Set each week. It can be configured to allow for either 5-or-7 day backup schemes. An incremental tape rotation method is set up as follows. The first week you use 1-2-3-4-5-6-7 The second week you use 2-3-4-5-6-7-8 The third week you use 3-4-5-6-7-8-9 The fourth week you use 4-5-6-7-8-9-10 The fifth week you use 5-6-7-8-9-10-11 Tape 1 would then be inserted again 6-7-8-9-10-11-1 You continue this as long as you have tapes and have one tape from every week that you perform a backup able to be stored for a certain period of time. It puts even usage on each tape making sure that a file gets copied to a multiple amount of tapes. The disadvantage is that the backup time can take a while if doing a full backup of multiple servers. It could be varied to do a full backup on the first of every week and then incremental of differential backups every day after that. An advantage of this system
is that tapes can be removed or added to the system at any time if an archive
tape or longer file histories are needed. The key is to keep a log of the
tape sequence and what date it was last used. This can be calculated months
at a time or even for an entire year if necessary. |
