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Backup Concepts

The basics are quite simple; save files for possible future recovery.
The devil is in the details.

As soon as you consider this topic, you are and need to think System Administration.
There are System backups, User backups and even device level backups.
The first thing you need to realize is that 'I will just backup everything' is naive and fruitless.
Consider: If we backup the System+all(Users), just how do we run the restore when the system disk fails?
The system will not boot, can't be connected to a network, and there's no software to access the backups!

First concept: Segregate System backups from User Backups.

When a system disk fails, the tasks to bring the system up are:

  • * Replace the HD and make it serviceable (ie partition and format it).
  • * Install a fresh copy of that OS.
  • * Be sure to get this new system 'online' and able to network on your LAN.
  • * Make sure your backup software is available and if not get it installed too.
  • * NOW you can perform a System Restore to bring back the original tools,
  • configuration files and programs.
  • * If there are User Files on your system disk (clearly a bad idea), then perform the User Recovery task.

This clearly defines the requisites for a system level backup. The IBM AIX version of Unix has a neat backup mksysb which creates a system level backup on a bootable media - -
boot the meda and let it run - - behold, you are done! Sadly other systems didn't adopt that technique
(unless you consisder Acronis True Image or Norton Ghost).

The System Backup
A well managed system will segregate the system from the users. PC and Mac users with only one HD obviously
will have difficulty here. Unix/Linux systems more frequently apply partitioning to control and isolate data locations.
But all systems provide partitioning and learning to use it can be an advantage in systems administration.
(hint: a partition will be seen as a Device and we could elect to perform Device backups).

The system level backup is managed just like user backups; it is just restricted to contain only system
level files (or saying the other way around, doesn't contain any user owned files).
Unix/Linux users should include the ROOT user however in the System level backup.

One last issue for systems; making the new target device bootable. Recall that the Master Boot Record(MBR)
is not within any partition, so a backup of a device/partition will not capture it.
The MBR is always at sector zero of the HD, even / regardless of partitioning.
Software like Acronis True Image or Norton Ghost clone the full HD and will therefore include the MBR.

Second Concept: Backup techniques

The choice of a specific technique will have consequents of time and media as noted below:

As the name suggests, 100% of the input (beit disk or directory) is placed on the backup media;
again, again and again, as many times as you invoke backup. Clearly this takes the longes possible
time to create and will waste media space copying unchanged files multiple times.

However, you should always have at least ONE FULL BACKUP available.

The incremental backup attempts to conserve media space by looking for files changed since the
last invocation of backup. This will be fast and use the least media storage. HOWEVER,
you need to keep all the media created by the last FULL backup and every INCREMENTAL backup
and you better label them carefully - - they will be called for during the Recovery process in the order
created - - and YOU are the only one that knows that!

The differential backup attempts to remove the media issue of the incremental technique by looking
for files modified since the last FULL backup. This will the create a recovery process of applying
the Full Backup + the Last instance of the Differential - - just two. Thus, it would be possible to
recycle three pieces of media; one for Full and alternate between the A and the B storage media.
The longer between FULL backups, the more files will be discovered to have been changed and
the more data gets copied to the differential media. At some point, all the data will no longer
fit on one chunk of media (ie one tape, one cd, one dvd). When that occurs, you would be best
off aborting this backup and starting fresh with a new Full Backup.

These are Unix/Linux tools to create archives with or without compression.
Tar options --newer=DATE (date of creation) or --newer-mtime (date of last change) are available.
Cpio has similar features.

Third Concept: Keep All User Data Isolated

PC users have the %userprofile% anchored at C:Documents and SettingsuserLoginName,
Mac users are anchored at HD:Users:userLoginName
and Linux users are located Home/userLoginName

Isolating user files to their userLoginName directory is a good policy. If you allow a user to create
files in the root of the drive, then you've opened pandoria's box as far as backups are concerned.
Instead of backup just one file hierarcy, you must now script the backup for every directory in the root
that belongs to any user. Linux tools would allow that to be discovered, but a better choice would be
for force per-user isolation.

Some users (ie programmers) need to have source code, programs and test files which might need to be
shared with other programmers. This should be isolated into its own partition and mounted as a separate device
(mount point or mapped network drive). This list will be very short, well known and easily addressed by the
User Backup.

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An alternative to Backups is simple Replication.

Replication is the idea that having one or two copies located on other media
and / or off site on a remote system is usually cost effective and sufficient.

Why? Studies have show that the major use of backup systems is to recover
just a few corrupt times at a time, not whole systems nor even whole HDs.

A good plan might be to make copy-1 from the live HD to some other media inhouse for
easy access and make copy-2 to a remote system. The remote copy-2 would then
be available if there were a major disaster (fire, flood, hurricane,...).

Clearly this would be applied only to User files. Some consideration should be
given to the impact of replicating multiple users data over your lan to some central file server.
Software like Allway Sync works well here as it copies only changed files and reduces the
network traffic accordingly.

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