Time Machine vs. Deja Vu

Question

What is the functional or practical difference between Time Machine and Deja Vu?

Answer 1

Hi, and welcome to the forums.
I don't know anything about it except what's on their web page, and don't think it's been mentioned in this forum.
It appears that it was first released for TIger, prior to the release of Leopard and Time Machine. So it may be an improvement over what was available from Apple for Tiger, but perhaps not better than Time Machine.
There aren't many details on that site, and I don't see any user forums (not a good sign, in my opinion).
It seems you can set it up as either a traditional archive type backup (with separate archives), or a bootable "clone" (without any archives). In most archive type systems, locating the version of a file you want to restore can be tedious, and deleting old ones must be done manually, thus you risk deleting the only backups of current items.
It's probably much more than you want, but here's a general comparison of the main types of backup systems:
There are three basic types of backup applications: Bootable Clone, Archive, and Time Machine.
This is a general explanation and comparison. Many variations exist, of course, and some combine features of others.
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_*BOOTABLE "CLONE"*_
These make a complete, "bootable" copy of your entire system on an external disk/partition, a second internal disk/partition, or a partition of your internal disk.
Advantages
When your internal HD fails, you can boot and run from the clone immediately. Your Mac may run a bit slower, but it will run, and contain everything that was on your internal HD at the time the clone was made or last updated.
You can test whether it will run, just by booting-up from it (but of course you can't be positive that everything is ok without running everything).
If it's on an external drive, you can easily take it off-site.
Disadvantages
Making an entire clone takes quite a while. Most of the cloning apps have an update feature, but even that takes quite a while, as they must examine everything on your system to see what's changed and needs to be backed-up. Since this takes lots of time and CPU, it's usually not practical to do this more than once or twice a day.
Normally, it only contains a copy of what was on your internal HD when the clone was made or last updated.
Some do have a feature that allows it to retain the previous copy of items that have been changed or deleted, in the fashion of an archive, but of course that has the same disadvantages as an archive.
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_*TRADITIONAL "ARCHIVE" BACKUPS*_
These copy specific files and folders, or your entire system. With many, the first backup is a full copy of everything; subsequently, they're "incremental," copying only what's changed.
Most of these will copy to an external disk or network locations; some to CDs/DVDs, or even tape.
Advantages
They're usually fairly simple and reliable. If the increments are on separate media, they can be taken off-site easily.
Disadvantages
Most have to examine everything to determine what's changed and needs to be backed-up. This takes considerable time and lots of CPU. If an entire system is being backed-up, it's usually not practical to do this more than once, or perhaps twice, a day.
Restoring an individual item means you have to find the media and/or file it's on. You may have to dig through many incremental backups to find what you're looking for.
Restoring an entire system (or large folder) usually means you have to restore the most recent Full backup, then each of the increments, in the proper order. This can get very tedious and error-prone.
You have to manage the backups yourself. If they're on an external disk, sooner or later it will get full, and you have to do something, like figure out what to delete. If they're on removable media, you have to store them somewhere appropriate and keep track of them.
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_*TIME MACHINE*_
Advantages
Similar to an archive, TM keeps copies of everything currently on your system, plus changed/deleted items, on an external disk or Time Capsule.
Like many Archive apps, it first copies everything on your system, then does incremental backups of additions and changes. But TM's magic is, each backup appears to be a full one: a complete copy of everything on your system at the time of the backup.
It uses an internal OSX log of what's changed to quickly determine what to copy, so most users can let it do it's hourly incremental backups without much effect on system performance. This means you have a much better chance to recover an item that was changed or deleted in error, or corrupted.
Recovery of individual items is quite easy, via the TM interface. You don't have to find and mount media, or dig through many files to find what you're looking for.
You can also recover your entire system to the exact state it was in at the time of any backup, even it that's a previous version of OSX.
TM manages it's space for you, automatically. When your backup disk gets near full, TM will delete your oldest backup(s) to make room for new ones. But it will never delete it's copy of anything that's still on your internal HD, or was there at the time of any remaining backup. So all that's actually deleted are copies of items that were changed or deleted long ago.
Disadvantages
It's not bootable. If your internal HD fails, you can't boot directly from your TM backups. You must restore them, either to your repaired/replaced internal HD or an external disk. This is a fairly simple, but of course lengthy, procedure.
TM doesn't keep it's copies of changed/deleted items forever, and you're usually not notified when it deletes them.
It is fairly complex, and somewhat new, so may be a bit less reliable than some others.
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RECOMMENDATION
For most non-professional users, TM is simple, workable, and maintenance-free. But it does have it's disadvantages.
That's why many folks use both Time Machine and a bootable clone, to have two, independent backups, with the advantages of both. If one fails, the other remains. If there's room, these can be in separate partitions of the same external drive, but it's a bit safer to have them on separate drives.
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_*OFF-SITE BACKUPS*_
As great as external drives are, they may not protect you from fire, flood, theft, or direct lightning strike on your power lines. So it's an excellent idea to get something off-site, to your safe deposit box, workplace, relative's house, etc.
There are many ways to do that, depending on how much data you have, how often it changes, how valuable it is, and your level of paranoia.
One of the the best strategies is to follow the above recommendation, but with a pair of portable externals, each 4 or more times the size of your data. Each has one partition the same size as your internal HD for a "bootable clone" and another with the remainder for TM.
Use one drive for a week or so, then take it off-site and swap with the other. You do have to tell TM when you swap drives, via TM Preferences > Change Disk; and you shouldn't go more than about 10 days between swaps.
There are other options, instead of the dual drives, or in addition to them. Your off-site backups don't necessarily have to be full backups, but can be just copies of critical information.
If you have a MobileMe account, you can use Apple's Backup app to get relatively-small amounts of data (such as Address book, preferences, settings, etc.) off to iDisk daily. If not, you can use a 3rd-party service such as Mozy.
You can also copy data to CDs or DVDs and take them off-site. Re-copy them every year or two, as their longevity is questionable.
Backup strategies are not a "One Size Fits All" sort of thing. What's best varies by situation and preference.
Just as an example, I use TM plus a CarbonCopyCloner clone (updated daily, while I'm snoozing) locally, plus small daily Backups to iDisk, plus some other things to DVD/RWs in my safe deposit box. Probably overkill, but as many of us have learned over the years, backups are one area where +Paranoia is Prudent!+