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#802: RoTW: The Last Visible Kernel Panic, Time Machine Follow-Up, Tip of the Week


Happy Tuesday,

I continue to doubt what time of year it is every time I go outside. The weather here in southern New Hampshire has been much warmer than normal lately, and I haven’t seen a snowflake for at least a month or so. Glancing at the forecast now, there seems to be a slight chance of snow showers later on in the week. I certainly hope the snow comes soon; I’ve been aching to get some use out of the many sleds and snow tubes piled high in my basement.

I’m greatly looking forward to the upcoming release of many different thunderbolt storage options. I’ve seen many product confirmations, as well as some awesome rumors, that really get me excited for the general direction of new technology. I’ll give you the pleasure of researching these unreleased (some unconfirmed) gems on your own. The general price range for these items does seem quite high, but with most new technology, the price comes down with patience.

Thanks for reading, and here’s to the possibility of enough snow to cover my favorite sledding hills!


  RoTW: The Last Visible Kernel Panic  

I had a Mac mini come in that was experiencing random crashes and kernel panics upon booting. The story was that it was running Leopard and started crashing randomly, so the customer upgraded the mini to Snow Leopard hoping that would fix the problem. Now, instead of random crashes, the system shows a kernel panic when it boots, rendering it completely unusable.

The Mac OS is written to be fault tolerant, with the idea that if an application crashes it will not affect any other application, and it cannot affect the operating system itself. Every now and then an application can develop a corrupted preferences file, causing the application to close unexpectedly, but other applications stay running. (Compare this to Windows, where an application that stops responding can cause the entire session to hang.)

If the operating system support files somehow get corrupted, the system can start to do weird things. In extreme cases, it may be necessary to reinstall the OS. The problem is that by default, Snow Leopard will preserve any settings from the previous installation, so if you have any corrupted settings, these corrupted settings are migrated to the new OS install. Now you have an up-to-date OS that crashes faster than the old one did. The only way to truly fix the system is to back up your data, wipe the hard drive, and do a clean installation.

I did verify that her system was crashing while loading some applications, but I wasn’t seeing the kernel panic. I backed up her data, reloaded Snow Leopard from scratch, and put all her stuff back on the system. It was now stable again, and she took it home. Next day I got a call that it was crashing again—she hooked it all up, then plugged in her wireless mouse USB module and it crashed with a kernel panic. Suspecting that the USB transceiver was at fault, I asked her to bring it in with the mini so I could test it.

A kernel panic is a major system failure, where the operating system cannot continue. You have no choice but to restart the system and hope that whatever caused the panic will not happen again. On a Windows system, this is known as the infamous “Blue Screen of Death.” It’s also occasionally called a “core dump” because everything that was running at the time is “dumped” to a log file in case debugging is necessary.

On a Mac, the screen you see telling you to shut down your computer in 7 different languages is a sign that a kernel panic occurred. In most cases, a kernel panic is caused by something external to the OS itself, such as bad RAM—if the data being read and written to a bad RAM chip is found to be unreliable, the system may shut down to prevent corruption to your data. If you’re driving nails into a wall and someone comes up behind you and covers your eyes, this places your fingers in danger, so naturally you stop hammering. Once the problem (the person behind you) is removed, you can restart what you were doing.

A kernel panic can also be caused when an incompatible hardware driver is installed, or in this case works fine with the previous OS but doesn’t work with the new one. The driver to support the USB mouse worked fine under Leopard, but didn’t play well with Snow Leopard. Hardware drivers are not loaded if the hardware they support is not present, so the crash never happened in the shop. Once the mouse was connected, the driver attempted to load, the OS said “I don’t know what to do with this!” and the entire house of cards came crashing down.

The solution was simple—download the correct driver for Snow Leopard. Things ran just fine from there. One could wonder why this happens. Had someone attempted to install the Leopard driver into Snow Leopard, the OS would have rejected it as incompatible. In this case, the driver was already there, but the OS couldn’t use it. (In the case of a data restore, no version checking is done, so Snow Leopard never got a chance to say “I don’t know what to do with that!” until it was too late.)

One way to avoid this is to check the web sites for not only your applications, but also for any devices or peripherals you use like printers, scanners, and mice. You want to make sure there is a compatible driver before you upgrade to make sure that your OS is stable afterward.

  Readers Respond: Time Machine Follow-Up  

I received many similar emails from you following last week’s article describing Time machine setup. I’d like to respond to those questions here just in case more of you had the same questions but didn’t write in.

The most common question was about partitioning the drive. If you have been using a drive for Time Machine, you don’t need to go and partition it now; that is already done. You also don’t need to partition the drive if it comes pre-formatted for the Mac. Most external drives are set up for PCs and will need to be formatted, and I also recommend formatting any LaCie drive to get rid of their proprietary backup software that doesn’t work with Lion. Other hard drive manufacturers bundle their own proprietary software, and in all cases this software can’t begin to touch Time Machine’s powerful elegance. Partitioning (read: erasing) the drive gets around all this.

Another question was about backup frequency. If Time Machine is set up properly, and the drive is plugged into the computer, you’ll get a backup every hour. If you are backing up over a network this can slow your network down while the backup is being performed. You can find a utility program called Time Machine Editor that allows you to schedule your backups. I do this for mine so it backs up at 3 am every day rather than every hour. This will not do a full system backup every time; after the initial backup, only incremental changes to your drive will be backed up. So if it does a backup at noon and between Noon and 1:00 you’ve edited a document that document will get backed up and Time Machine will keep the noon version and the 1:00 version.

The third most common question was about the amount of backups and drives getting full. Time Machine will get you those hourly backups till there isn’t enough space left on the drive. At that point it will delete the oldest backups until it has enough space. You can set it up to warn you or just delete depending on your preference. You can periodically archive your drive either by cloning it to a second external drive using Disk Utility or a program like SuperDuper. If you’re running a Time Capsule you can connect a second drive to that device and archive it through the setup utility.

Fourth in the list was about viewing and using files. Apple did not intend the backup to be used in Finder. You should be entering Time Machine through the menu item on the top of your screen. This will bring you into the backups for whatever the front most application is. So if you need to restore a picture from iPhoto you’d launch iPhoto, then enter Time Machine, then navigate back in time till you find what you’d need then click the restore button. It’s as easy as that. It also makes more sense once you’ve tried it. If you navigate through the file structure of the drive you run the risk of grabbing incomplete files depending on how those files changed over the course of Time Machine backing them up.

The last question was about using the Time Machine drive for storing other information. I would strongly urge you not to do this. In an ideal situation a backup drive is used for nothing other than backups. This would also be a drive that is not moved much. The act of copying and reading data are putting stress on the drive and slowly decreasing its lifespan. With a backup drive you want to minimize this as much as possible so that drive outlives your main drive. Just because it’s a backup drive doesn’t mean that it will never fail.

I hope this answers all your questions.

  Tip of the Week: Browser Spoofing  

Today we had a customer in the store who had just switched to Apple by buying a new MacBook. He was running into trouble with his company’s web-based calendaring and collaboration system, as the site required Internet Explorer 6 or greater to run. He’d browse to it using Safari or Firefox, and the site would throw an error and refuse access, presumably to make it easier for novice users to know which browsers work and which don’t. Turns out, it was compatible with Safari after all. We just had to tell Safari to masquerade as Explorer.

Unfortunately, not all web developers keep up with the times. It’s likely that this site’s administrators hadn’t conducted compliance testing since at least the release of Internet Explorer 7. It’s getting rarer and rarer, but plenty of sites out there still require Explorer. This trick won’t work on all of them, but it will for a good chunk of them.

While in Safari, select Preferences from the Safari menu. Now, click the Advanced button at the top right of the Preferences window, and check “Show Develop menu in menu bar.” Close the Preferences window, and notice the new Develop menu on top of your screen. In the Develop menu is the User Agent sub-menu. From the sub-menu, select Internet Explorer 7.0. Once this is done, the web sites you visit will think you’re using Explorer, and some of the especially restrictive sites will work for you!

Be mindful of the setting you choose, as Safari will not render some web sites correctly when pretending to be Explorer.

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