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Last updated on January 8, 1999
Please send any comments to Jean-Claude Chetrit
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This is an extract from DELTA's manual (chapter 3). (C) 1996-1997 OPENetwork ``One must imagine Sisyphus happy!'' Albert Camus, in The Myth of Sisyphus You have heard ``Backup!'' many times, but now that you have DELTA, you can do it intelligently. The strategy we propose is nothing more than common sense. Many full backups?Let us assume that you can afford to backup everything frequently. Should you do it? Please think about it before looking at the answer below. On the upside, you will never lose anything; the downside is lots of backups to manage. Well, the answer is no, you should not! What you must do instead is backup your ``DATA'' and make sure that your ``PROGRAMS'' have not changed! We're going to explain these two words in detail.Two kinds of files:Your hard disk contains 2 kinds of files. The first kind (``PROGRAMS'') are the programs and their setup. Since you spent time and effort making the programs work well, you want to prevent them from changing without your permission and understanding. But if you lose these files, it is not catastrophic because you can always reinstall the programs. We call the other kind of files the ``DATA.'' They are the result of your working with the programs: your writings, your spreadsheets, your source code and even your compiled programs if you are a programmer, etc... It is critical to preserve these files because they are not replaceable. If your computer is stolen, and you have no backup, no amount of money in the world can ever recover what is lost.Separate the two kinds of files:So, an important priority is keeping ``DATA'' in subdirectories separate from the subdirectories where the ``PROGRAMS'' are installed. We even recommend keeping the two kinds of files on different physical drives because besides simplifying backup issues, it will also increase the performance of many large programs! If you tend to work at the DOS prompt and want a way to make that task easier, we recommend you try our shareware program called Mi-Shell (it resembles Norton Commander).The initial full backup:There are many kinds of backups, but we'll simplify here and concern ourselves only with full backups (also called complete backups) and incremental backups which only save the changes since the last backup. There was a time when a full backup meant 4 to 10 hours and more than 100 floppies. Now, a tape backup is inexpensive (less than $200) and will probably do a great job while you go out to lunch. But a SyQuest or Bernoulli drive is even better because it looks and behaves like a hard disk drive (random access rather than linear access). They are fast and cheap: between 7 and 20 cents a megabyte. Finally, a frequently overlooked backup method consists of keeping several PCs always in sync., an extremely desirable strategy which is described below. For simplicity, we will assume that your backup is a simple and complete copy of your hard disk.``PROGRAM'' changes:How do you know that a program has changed? MS-DOS provides part of the answer, called the archive bit, but it is not enough for many reasons (some viruses and some software will change files without changing the archive bit). First, let us assume that you have followed our advice and that, for instance, the WordPerfect program is under \wp, that its backup is under \wp.bak and that the data is under \writing. Now, type on the command line:C:\UTIL>delta -r \wp \wp.bak The -r means turn on the recursive option, i.e., compare not just these two subdirectories, but also all the ones below them, and below those, etc... If there has been no change, you are all set. If there is a change, DELTA will show you which directories and which files were changed. It may be serious: a virus attached to wp.com, or innocuous: you installed a fax, but either way, two things are now urgent:
``DATA'' changes:When you work on your PC, you fully expect your ``DATA'' to change. It is nice to have a recent backup while you work. Two months ago, I deleted everything on my home directory because I thought I was on a: but had typed a; instead of a: failing to get there. &@!!&$@@!! No problem, I thought, I'll undelete. Alas, my drive was networked and refused. In order to recover, I had to stop the network, undelete, and then reconnect the network: it was a pain and a waste of time. Anyway, backing up your ``DATA'' should be done more or less frequently depending on the importance of said ``DATA.'' We keep our list of clients and financial records on a database. We back it up every day because we can't afford to lose it. But we promised you details, so we'll give you some: we keep a mirror image of the directory on a server; we have 5 diskettes for the last 5 days; every day, we backup the database over the oldest backup of the 5. In addition, we keep a permanent diskette at the end of each month. This is not perfect, but better than the usual no backup at all.How I keep several PCs in sync.Rather than tell you what to do, I'll describe what I do and you can adapt it to your situation.When I am not consulting:I do all my work at home. Since my children come every week-end to spend time with me and play their favorite computer games, I do a complete backup before they show up. In my basement is hidden a backup system which contains last week's backup. The PCs are linked via ethernet cards and a LANTASTIC network. I type on the command line:delta -r d:\july96.jc c:\ then methodically explore all the differences (all my week's work and mistakes), and eliminate them, hence fixing my mistakes and making the backup current. When my children leave on Sunday night, I type the same command and I can see if they have done any damage. When I am consulting at a client's office:I go to work every day, spend all day at my work PC, then come home and frequently spend many hours working at my home PC. Finally, I frequently spend the night at my girlfriend's where I sometimes end up working on her PC. It is not imperative nor even desirable for these PCs to be identical, what I need is to always have my latest work with me. So, I try hard to keep all my ``DATA'' in various subdirectories below c:\home and I carry a ZIP drive with a copy of c:\home on a ZIP disk. Whenever I sit down at a different PC, I compare c:\home with the ZIP disk version D:\home and bring them into sync. Whenever I am about to leave a PC, I compare them and again bring them into sync so that I always travel with the latest version.This process may seem a bit time consuming, but I will now never wonder how old my backup was when my PC crashed (or worse, was stolen). Maybe even more important for me is that I will no longer have the frequent frustration "I wanted to work on xxx, but forgot it at home." Technical Details:A JAZ drive or a SyQuest EZ-135 or EZ-230, or an optical drive, or one of the new gigabyte removable media will do fine too. It is possible to operate with a tape backup, but it is so time consuming that we strongly advise you to get one of these new removable hard disks instead.Comparing computersIf the objective is to keep two PCs completely in sync., there are two cases: when they are close to each other, they should be networked (read the first section of the next chapter if they are not); then use DELTA to compare the two hard disks. When they are in different locations, use a removable hard disk and the technique outlined above.A harder situation is when the two PCs are similar (but not identical) and you want to solve a mystery. We once wondered in our office why ``ghostscript'' worked well on one PC, but not on the other.
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Last updated on January 8, 1999
Please send any comments to Jean-Claude Chetrit