Linux System Administration Recipes A Problem-Solution Approach

CHAPTER 4 ■ TAKING BACKUPS AND MANAGING DATA• How much data do you need to store? See recipe 4-1 for a script to calculate yourcurrent total disk size and actual current usage.• How often do your files change? Recipe 4-2 has a script to establish this.• What is the purpose of your backups? Do you want a long-term archive of filehistory or just a backup so that if someone accidentally types rm -rf . in theirhome directory, you can restore their files for them?• Related to the previous point, how long do you want to archive your data for, andhow long do you want to give users to realize that they’ve accidentally deletedtheir Important Directory? Often this is going to be dictated either by companypolicy or by finances.• How important is it that you are able to restore fast? You can usually choose, whenbacking up, either to do a full backup dump or to do a diff backup. The formertakes up more space and takes longer to do but is quicker to restore from. Chancesare you want somewhere between the two.Once you’ve considered all this information, you can start pricing various options. For example, ifyou want a full backup weekly and then a diff backup nightly, use the information from the first twobullets to calculate how much space you want (and how much space you might want if all your disks fillup) for the full backup and then for each day’s incremental change. That’s your weekly datarequirement. Based on that, how many weeks can you afford tapes for before you have to rotate? Whatabout adding an extra archive copy per month?You also need to consider how long each backup will take—how fast does data move over your localnetwork, and how fast can your preferred backup media be written to? (Remember that tapes are fasterthan disk.) As a rule, you probably don’t want backups running during the day if you can avoid it, so youneed to think about how much you can back up every night.Finally, if you’re pricing a new backup solution, think about the future. You should allow for at leasta 100 percent increase in the quantity of data you need to back up over the life of the system, andpreferably more. Many systems now are modular, so you can buy more drives or libraries to extend yoursetup in the future for less than the cost of a whole new system.■ Note Your backups are only as good as your last restore check. Confirm that restores are working OK (andtherefore that the backups are working OK) weekly by running a test restore from your backup software.4-1. Calculating Your Network’s Total Disk Size andCurrent UsageTo establish what backup policy you need, one of the things to find out is the total potential size of yourdisks and what your current usage is. The following script can calculate this for all the machines in yournetwork:96Download at WoweBook.Com