Network Storage Backups
We are sometimes asked questions about how network backups are done. This document explains how the network backups work – including how you can unintentionally prevent a file from being backed up.
What is the purpose of network backups?
Network backups are performed primarily for disaster recovery. Large-scale backup solutions are not geared towards restoring a single file – in fact, a single file can be somewhat difficult to find, which will be explained later. Even though it takes time to copy everything off a tape, restoring an entire disk volume is actually easier. Indivdual users can retreive a relativly small number of recently-deleted files with the Netware "Salvage" utility. The odds of a disk array failure on a server are low, since we can lose any single physical drive on any of the Netware servers and no data will be lost. While this gives us time to replace the malfunctioning drive, the institutional cost of losing that much data makes a good network backup system worth the investment.
Note: it bears mentioning that the backup software can only back up files that exist at the time the backup software runs. If you create a file at 9am and delete it at 1pm, that file isn't going to be there to copy to the tape since the backup won't start until shortly after midnight . Also, if you leave WordPerfect, Word, Quattro Pro, Excel, etc. running with an open document overnight, that open document will not be backed up. Because of the way the file system handles file locking, the backup software cannot access an open file. If you do this, and then delete the file the next day, the file won't appear on any tape. Please exit your software and shut down your computer when you leave at the end of the business day.
The Mechanics
By "network backup," we're talking about the Netware servers in this case. We do have other systems that have their own backup devices. However, all of the primary Netware servers are backed up to a single device called a "tape library" or "tape autoloader." It's a tape drive in a case that includes a simple robot which can load and unload any of seven tapes inserted in a "magazine." The tape library is attached to Whoville.
Each tape can hold between 60 and 120 gigabytes, depending on the data. At this time, any one day's backups can fit on a single tape, so we're changing the magazines once a week. We currently have a rotating pool of five magazines, each with a week's worth of backups. As data storage use increases, we will have to increase the numbers of magazines in the rotation, and eventually replace the current tape library.
We don't back up all data on every volume every night – we'd need a massive backup unit for that. We work on a rotating schedule. Each night, some volumes (for example on Monday morning: DEPTS on Janus, on Tuesday morning: USERS on Narnia, etc.) are backed up entirely. The other volumes have all of the new or changed files since their last full backups saved to tape. The backup software we use (Veritas Backup Exec), this is called a differential backup. The amount of data changed on a volume since the last full backup is usually a small percentage of the total space in use on that volume.
The end result of this is that in the case of a catastrophic volume failure, the files that existed at the time of the backup just prior to the failure can be restored with two tapes -- the most recent full backup tape for that volume, and the previous night's differential backup. Files that were created after that backup would be lost.