Network Data: Backup Retention
Now that you are backing up your data in the most appropriate and timely way — how long and how many of these backups do you keep? Not having a good backup retention plan can be as disastrous as having no backups at all.
Don’t try to cut costs by purchasing one backup tape and reusing it every day. What happens on Thursday when you find out an important file was accidentally deleted on Tuesday ? Because the file didn’t exist on Wednesday, it won’t be on Wednesday’s backup tape and there is no longer a Monday backup.
The safest scheme is to use a new backup tape every day and keep all your old tapes in a vault. Pretty soon, though, your tape vault can start looking like the warehouse where they stored the Ark of the Covenant at the end of Raiders of the Lost Ark.
As a compromise between these two extremes, most users purchase several tapes and rotate them. That way, you always have several backup tapes to fall back on, just in case the file you need isn’t on the most recent backup tape. This technique is tape rotation, and several variations are commonly used:
The simplest approach is to purchase three tapes and label them A, B, and C. You use the tapes on a daily basis in sequence: A the first day, B the second day, C the third day; then A the fourth day, B the fifth day, C the sixth day, and so on.
On any given day, you have three generations of backups: today’s, yesterday’s, and the day-before-yesterday’s. Computer geeks like to call these the grandfather, father, and son tapes.
Another simple approach is to purchase five tapes and use one each day of the workweek.
A variation of the preceding bullet is to buy eight tapes. Take four of them and write Tuesday on one label, Wednesday on the second, Thursday on the third, and Friday on the fourth label.
On the other four tapes, write Monday 1, Monday 2, Monday 3, and Monday 4. Now, tack up a calendar on the wall near the computer and number all the Mondays in the year: 1, 2, 3, 4, 1, 2, 3, 4, and so on.
On Tuesday through Friday, you use the appropriate daily backup tape. When you run a full backup on Monday, consult the calendar to decide which Monday tape to use. With this scheme, you always have four weeks’ worth of Monday backup tapes, plus individual backup tapes for the rest of the week.
If bookkeeping data lives on the network, make a backup copy of all your files (or at least all your accounting files) immediately before closing the books each month; then retain those backups for each month of the year. This doesn’t necessarily mean that you should purchase 12 additional tapes. If you back up just your accounting files, you can probably fit all 12 months on a single tape.
Just make sure that you back up with the “append to tape” option rather than the “erase tape” option so that the previous contents of the tape aren’t destroyed. Also, treat this accounting backup as completely separate from your normal daily backup routine.
Keep at least one recent full backup at another location. That way, if your office should fall victim to a disaster, you can re-create your data from the backup copy that you stored offsite. Make sure the person entrusted with the task of taking the backups to this off-site location is trustworthy.