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TidBITS readers have both offered useful additional information and raised a number of interesting questions concerning the issues that swirl around backup strategies, as discussed in the previous three parts of this series.
<http://db.tidbits.com/series/1041>
My Backup Strategy -- A number of people asked me to explain the specifics behind my backup strategy, hoping that they could apply my rationale to their situation. So, here are the details. I back up to DAT tape using Retrospect 4.0 and an old APS HyperDAT drive hooked to a Centris 660AV that currently does nothing else other than run Apple's LaserWriter Bridge software. Before the 660AV, I used an SE/30 that had a much slower SCSI bus, and also ran numerous other applications. I've found that Retrospect coexists nicely with most types of applications, but not mail servers, which can lose data when Retrospect monopolizes the CPU during backup.
We have two types of machines - machines that are available on the network most days and which are backed up every night by a Retrospect script, and machines that appear on the network on a sporadic basis like PowerBooks and the PC, which back up whenever they appear via a Retrospect Backup Server script. Automated nightly backups make for minimal intrusion and work, since they free me from having to monitor the process, or even notice it. At the same time, nightly backups provide a high level of protection should we lose anything ranging from a single file to an entire hard disk.
To spread out exposure to problems, I back up to three sets of tapes, named Hades, Poseidon, and Zeus (hey, I have a degree in Classics, I'm allowed). The current tape in each set lives in the drive for a week, and every Monday morning I eject the current set and replace it with the next one, moving up in the alphabet. I used to use default names like "Nightly Backup A," but that became confusing when I had multiple tapes in the set, resulting in tape names like "Nightly Backup A 2" and it became even worse when I archived a set with a New backup in Retrospect, thus ending up with "Nightly Backup A [001] 2." I think better with real names, so I switched to the names of the Greek gods.
Retrospect's EasyScript feature wanted to set me up with a rotation schedule that swapped tapes on Fridays. However, I'm most likely to be "at work" on Mondays, since that's when we publish TidBITS. So I changed the schedule to swap tapes on Mondays. Make sure rotation schedules work for you or you may find yourself not backing up because you didn't have the right tape inserted. So, every Monday morning when I start working, the first thing I do is swap the backup tape set. I'm working on a system of storing the previous set off-site and rotating the off-site backups as well.
The trickiest part of my backup strategy involved dealing with tapes filling up. Retrospect can script Full backups (which erase the contents of that set and start over) or New backups (which keep the contents of that set and start over), but I couldn't figure out a schedule for those that made sense. Plus, I hate wasting space, so I couldn't imagine starting a New backup if I had several gigabytes left on the current tape, just because the New backup script had kicked in. So, I came up with a strategy that requires a little more manual intervention but better fits my style of working.
The Hades set contains three tapes. When it fills up and Retrospect asks for a fourth tape, I cancel that request and manually do a Full backup to reset the contents of that backup set. The Poseidon tape set uses a similar strategy, but it contains four tapes and is reset when Retrospect asks for a fifth tape. Otherwise, Hades and Poseidon would probably run out of space at roughly the same time, which could force me to go back two weeks to the Zeus set to recover a file that had been deleted just before the reset action took place. Finally, the Zeus set is allowed to contain five tapes, but when Retrospect asks for the sixth tape, I cancel the request and manually perform a New backup to completely new tapes, archiving the previous five tapes in the Zeus set and giving it five more. The filled-up Zeus archival tapes live off-site as well.
If a Full backup of Hades or Poseidon would erase a few days of backups, I give them one more tape and wait to perform the Full backup until they come up in the rotation again. At that point, I get the extra tape back again - it's just temporary space.
This system works well for me, since I end up with nightly backups of all files, three different sets of tapes in case one (or even two) fails, a set of tapes that can be stored off-site, and archival sets of tapes that can be stored off-site. And, the beauty of resetting two of the three sets when they fill up is that I don't have to buy nearly as many DAT tapes as I would otherwise. Of course, I do have to swap in new tapes for the Hades and Poseidon sets every year or so, but that's a minor liability.
DAT Longevity -- Several people asked about the number of times one should use a DAT tape, since a variety of advice seems to float around in the ether. I forwarded the question to Craig Isaacs of Dantz Development and Paul McGraw of APS for their input. Craig said that Dantz recommends sticking with the manufacturers' recommendations, but on a more realistic note, commented that the decision runs along with the entire backup strategy decision - you must decide how much you wish to spend for differing levels of protection. For instance, some large companies never reuse media, preferring to do complete backups daily and taking the previous day's backup off-site forever. These organizations feel that millions of dollars of data is well worth a few thousand dollars of backup media. Obviously individuals would rarely use such a strategy but still shouldn't assume a DAT tape will work forever in constant use.
Paul McGraw's comments were more concrete. He felt that 30 to 50 sessions is a totally reasonable expectation, and he has used DAT tapes personally for hundreds of sessions without failure. Paul qualified his comments, saying that it's probably a good idea to retire a tape after it's been used for 90 days, no matter what. In addition, media vendors say that new tapes leave more residue on the tape drive's heads, so you should run a cleaning tape after using a new tape for the first time. Finally, Paul suggested that if you see frequent media failures, you should look into switching media suppliers, and if that doesn't help, switch mechanisms or even media types.
My advice, particularly for those trying to keep costs down, is to limit your exposure to the possibility of any given tape (or any other form of removable media) going bad. Do this by maintaining multiple backup sets that rotate on a relatively frequent basis. For instance, if you have three backup sets that rotate every day, even if one fails, the previous set is never more than a day old. In addition, I think retiring tapes periodically is a great way to create archival backups - although you might not trust a tape after hundreds of sessions of use, if you retire it in working order, it's likely to be accessible for years, should you need to recover a file from it.
DAT Tape Usage -- An interesting fact arose from one of the responses I received from a previous article. A reader wrote to say that he'd been encouraged to buy a 9 GB hard disk in addition to a 12 GB Sony DAT drive. The reason given was that his 10 Mbps Ethernet network probably wasn't fast enough for Retrospect to use the tapes fully, and by backing up first to the 9 GB hard disk and then backing that up to the DAT tape, there wouldn't be any wasted space on the DAT tapes. It seems that if Retrospect can't get a stream of data coming in fast enough to write to the tape, the DAT drive itself writes what are called "pad blocks" until more data is available. Otherwise, the backup would take far longer, because the DAT drive would have to stop, rewind, and locate the last write point after each time Retrospect's buffer empties out.
Although true, this design decision on the part of DAT drive engineers (a trade-off for speed versus tape capacity) probably doesn't affect you unless you back up to or from an incredibly slow computer, or using LocalTalk. In addition, DAT tapes are cheap and reusable, so if some space is wasted invisibly in this fashion, it's unlikely to be worth the cost and effort of putting a hard disk in the middle of the system.
DAT Drive Pricing -- The perennial complaint arose in relation to DAT drives: why are PC DAT drives cheaper than Mac DAT drives? According to Paul McGraw at APS, they're not - at least when you make a fair and accurate comparison. When you compare a PC DAT drive in an external enclosure, without software, to one of equal performance on the Mac (the same mechanism models mostly work on both platforms), the price is virtually the same. Macs generally don't support internal DAT drives, so there is a nearly $100 difference between an internal DAT for PCs and an external DAT for the Mac. The second factor in the cost is that of software bundled with the drive. Low-end software in the PC world is likely to be cheaper than the high-end Retrospect, which is the most common program in the Mac world for backing up to DAT. When high-end PC backup programs are bundled, costs are comparable. Finally, there are undoubtedly some economies of scale involved in selling more units into the market.
In short, the answer relates to difficult or impossible internal installation, and a substantial difference in the cost of what is considered "acceptable" backup software. When you compare apples to apples (in this case, high-performance SCSI DAT drives bundled with high-end software), the prices are generally quite comparable.
Many people confuse tape drives in general with DAT drives. There are numerous different tape technologies, some of which provide extremely low cost (and low performance) PC tape drives. Relatively few QIC (quarter-inch cartridge) tape drives are available for the Macintosh, and those that are use SCSI instead of the cheaper IDE or floppy interfaces used by the PC versions. In those cases, the PC tape drives are both more numerous and quite a bit cheaper.
8mm Tape -- Several readers wrote to chide me for claiming that 8mm tape drives were only expensive, high-end solutions. Although that does seem to be true of most 8mm drives, the Exabyte 8700LT stores up to 10 GB (compressed) on a single tape and costs about $650 (8mm drives don't seem to be as easy to find as DAT drives - I was directed to Computer Discount Warehouse). Reliability and speed are reportedly good, and media cost is as low as $6 per tape. It's worth a look.
<http://www.exabyte.com/products/8mm/8700lt/>
<http://www.cdw.com/product/default.asp? EDC=69081>
Redux Redux -- One of the more popular backup programs of yesteryear, Redux, is about to make a comeback. A new company called Redux Software has been created specifically to develop and support Redux. The company is working on Redux 2.6, which is an update to the current 2.5.1, and after that they will start version 3.0. We'll be sure to note when Redux 2.6 ships.
<http://www.reduxsw.com/>
Retrospect Express Availability and Features -- Craig Isaacs of Dantz Development has confirmed that Retrospect Express is available only through Dantz until 01-Jul-98. That's coming up soon, but some people have been confused by mail order vendors claiming the program hadn't shipped yet. It has shipped, just not to distributors yet. In addition, I've had a number of discussions with people who believe Retrospect Express is missing particular features, such as the capability to back up multiple volumes to a second hard disk. Although it does lack some of Retrospect's high-end features (like tape drive support, security, Backup Server capabilities, network backup capabilities, and most notably custom Selectors for flexible file selection criteria), in each case so far the requested features have been present and documented in the manual.
<http://www.dantz.com/dantz_products/ express.html>
<http://www.dantz.com/store.html>
Auto Backups -- Several people suggested using the trunk of your car as an off-site backup location that's easier to get to than a safe-deposit box. Although it's certainly easy, I would be extremely leery of storing magnetic media in a car. It depends on where you live, but here in Seattle, the inside of cars can get quite cold and damp in the winter, and extremely hot in the summer sun. It's possible that CD-Rs might not be as susceptible to environmental damage as tapes or removable cartridges, but frankly, I don't think it's worth the chance. Stick with storing off-site backups in a climate-controlled location like an office or, ideally, a safe-deposit box.
QuickBack -- I missed a freeware backup program in the last installment in this series. Jacques Cornell <jacques@white.plala.or.jp> recommends the freeware QuickBack 1.9.3 by PopChar author Gunther Blaschek. It appears to be a true backup program, though it has a rather annoyingly modal interface and hasn't been updated in over two years.
<http://infosoft.soft.uni-linz.ac.at/Info/ MacSoftware.html#QuickBack>
Backup Book -- Marc Shipman-Mueller <msmueller@arri.com> wrote to recommend The Complete Guide to Mac Backup Management, a $34.95 book by Tom Dell and Dorian J. Cougias.
<http://www.amazon.com/exec/obidos/ ISBN=0121925625/tidbitselectro00A/>
Lock that Door! Finally, Alastair Rankine <alastair@cia.com.au> comments that it's worth thinking about the physical security of your backup media and hardware. Although it's probably not a major concern for individuals, businesses should treat backups as valuable data and keep them in appropriately inaccessible places. In addition, at least Retrospect enables you to set security levels on your backups. You can choose Password Only (no encryption), SimpleCrypt (fast encryption), or DES (strong, slow encryption). Retrospect has other security options as well, such as password protecting access to the program and encrypting data while it transfers from a Retrospect Client to the backup server. If security is paramount for you, make sure your backup software and overall strategy support your security needs.
