Tape Rotation Schedule
Rotating backup tapes is the most practical way to manage a tape backup scheme, since the costs of using a new tape each day are prohibitive. Although tapes are the cheapest form of storage compared with other media, that does not mean that tapes are cheap. A single DLT cartridge can cost $40. If you use a tape for every day of the month, plus one for every month, you will be spending more than $1600 per month, not counting taxes, for just one server. To go back to any day in the year on a single tape would require a tape per day. When you use a tape for every day of the year, the cost is more than $14,000. And the cost can increase even more, because most companies have more then one server and advanced backup programs can remotely back up users’ workstations as well.
The solution is tape rotation. Do not use a different tape each day. Instead, reuse tapes from previous months and weeks. We will look at some simple rotations, such as weekly, along with some rather complicated schemes.
Weekly Rotation
In a weekly rotation, you use a different backup tape or tapes for each day of the week. Weekly rotations are the simplest to understand and set up. You first assign a tape to each weekday and label the tape with the name of the day. You have five tapes, and you overwrite each tape as the day of the week comes again. The furthest you can go back to do a restore is one business week. On Friday, before the backup, you can go back to any day for one week, but no further.
Monthly Rotation
Rotating tapes on a monthly basis allows you to restore data for an entire month. Managing this type of backup scheme is more complicated because you must keep track of many more tapes. A straightforward solution is to assign 31 tapes and do a full backup each day. This becomes unwieldy if a full backup takes many tapes. For example, a thousand-user corporation’s e-mail, file, and print servers can take multiple high-capacity DLT tapes per session.
Most of your restore requests will be reported shortly after the file is accidentally deleted or corrupted. Take your typical user who accidentally deletes his home directory. Using a GUI interface, this is as easy as rightclicking a folder and then left-clicking Delete. The user will immediately call network support and plead for quick rescue. In this case, you only have to go back to the previous day’s tape. To plan for this scenario, have daily backups that go back a week. Supplement this with a weekly backup for an entire month.
In this configuration, you would use no more than nine tapes. You will use one tape for each day of the week, Monday through Thursday (four tapes) and one tape for each Friday of the month (four or five tapes, depending on how many Fridays there are in a month). A maximum of nine tapes will give you daily backups for a week and weekly backups for a month. Label the tapes Monday through Thursday, and Friday Week 1 through Friday Week 5.
Yearly Rotation
You can build a yearly backup on top of the monthly system. You’ll need 12 tapes, one for each month, labeled with the names of the months. Rename the last weekly, full backup of each month to the corresponding month. You go from nine tapes to 21 tapes and gain the capability of going back a year to restore data. Only one day out of each month is available after you go back further than your current month.
Grandfather-Father-Son Rotation
A standard rotation scheme for tapes is the Grandfather-Father-Son (GFS) method. With this method, daily backups are differential, incremental, or full. Full backups are done once a week. The daily backups are known as the Son. The last full backup of the week is known as the Father. Because the daily tapes are reused after a week, they age only five days. The weekly tapes stay around for a month and are reused during the next month. The last full backup of the month is known as the monthly backup, or the Grandfather. The Grandfather tapes become the oldest, and you retain them for a year before reusing them.
Long-Term Configurations
In addition to daily, weekly, and yearly backups, some companies, for archival purposes, do an end-of-year backup, which is then kept offsite in long-term storage. They do this to keep a record of the year’s financial and transactional data so that they can refer to it in case of tax problems. (The IRS may require businesses to keep transactional data for seven years.)
Some companies do two end-of-year backups—one before closing out the fiscal year, and another after closing out. They do this in case they mess up the closing and need to start over. When the closing out is finished, they back up the closed-out system and place the tape in long-term storage.
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