Testing memory

I wrote some days ago about badblocks for testing a hard drive surface. Now, the same for memory.

As I said, I bought a second-hand PowerMac G5 to replace my old G4. When I got the new machine, I run Apple Hardware Test (AHT), using the Extended Test. AHT tested my hardware, including the 2.5 GB of RAM, taking more than two hours (and making a hell of a noise, because during tests, the G5 ventilation system works in failsafe mode, which means, full power). Everything seemed to be fine. Until I installed Retrospect. I use Retrospect to make all my backups at home, and despite all it’s quirks, it always worked fine on the G4. Since I installed it on the G5, I got strange errors (the famous “internal consistentcy check”) and even crashes.

After nailing down all the possibilities (trashing preferences and existing backup sets, reinstalling Retrospect, etc) I suspected it could be an hardware problem, because I was told that the “internal consistentcy check” appears when the backup set contents are corrupted. So, I thought, my hard drive is corrupting data. I duplicated one backup set with about 80 GB, and surprise – after duplicating and running an md5 checksum on it (and diff), the files were different! This was NOT supposed to happen, naturally. So I tried the same thing on my boot drive – same problem. Ops… it’s not the drives. So, if it’s not the drives, and supposing (more exactly, praying) that it was not a motherboard issue, it must be the memory.

All my collegues at IST System Adminstration team use memtest on PCs to test the memory. This great distribution of memtest has a really nice touch: you can burn this on a CD, and boot the PC from it. It takes less that 200K of RAM, so all the other memory will be tested. Unfortunately, it’s not possible to boot it in Macs (not even Intel Macs – I tried it!). So, you must get the Mac OS X version of memtest, and boot the OS in Single User mode (using command-S during the boot sequence). The OS will take about 50 MB of RAM, which is, of course, much worse than the 200K used by the PC version, because those 50 MB will simply not be tested. But it’s better than nothing.

The official site for the Mac OS X version of memtest is here, but unfortunately, the author requires you to pay a small ammount for the download. I don’t like the approach very much because I don’t really know what I’m buying. the author says that, after paying, he sends a password for the encrypted DMG you downloaded. But I cannot download without paying, because the link is no-where. So… what happens when a new version comes out? Do I have to pay it again? Well, anyway, someone else is distributing memtest for OS X for free. Yes, it’s legal, because the software is under GNU license. So, if you don’t want to pay, just click here and grap your own free copy. Happy testing!

By the way, some tests take a lot of time. Let all of them run. Don’t assume the fact that all the “quick” tests passed means your memory is OK. Some problems may only be found with the more complex and slower tests – that’s why they are there. So, let it run. And if you have a G5, get the hell out of there, or use ear-plugs. It won’t be a nice office to work during testing, trust me.

memtest will detetct lots of common problems in memories, and will probably identify more than 99% of the defective memory modules arround. But never forget: it’s impossible to be entirely sure that a memory module is OK, simply because it’s not possible, in a reasonable time frame, to test all the possible combinations of data. Also, memory may pass all the tests in a day, and fail the next day. There are many factors that may trigger a hidden problem in memory modules: temperature, electrical flutuations, the data it contains, age, etc. If you suspect you have a bad memory module, and if you have time, run memtest for several days in a row, using the option to do many passes.

6 thoughts on “Testing memory

  1. Peter Parker

    There is a hardware / memory tester on the installation disks for each Mac. Get the hardware test disk, Boot from the DVD by holding the ‘c’ key down, and you will boot into it.

    I don’t know how much base memory it consumes, but it’s very little, as it doesn’t load but a very basic – (text only no graphics interface).

    I had some memory questions about my (now) old G5, and used it to exonerate the memory.

    Peter Parker
    Photographer and Science Geek (by day, I also have a night job)
    Mac User (because only bad guys use PC’s!!!)

  2. Miguel Arroz Post author

    Straight to the point: don’t trust the Apple Hardware Test. I don’t know what does it do for so many time, but I’m sure it’s not doing all the tests memtest does. My G5 memory passed all the tests using Apple Hardware Test, but failed with memtest, and it was defenetly damaged, because I had some memory-related problems during the G5 usage.

  3. Ken Anderson

    I had an xserve that passed Apple’s hardware test with flying colors, but memtest found bad memory. Same exact situation for a macbook pro – I trust memtest.

  4. Maccelerate

    I have just identified the opposite scenario. I used memtest and tested through 5 full tests overnight and found nothing. However running AHT, which I suspect has a smaller memory footprint—and this may be why it catches the problem—it identifies the bad RAM on each pass. Move the module to another slot, and it is found each time. AHT doesn’t run as many different types of tests, but it does seem to be useful.

  5. Pingback: Memtest update « Terminal.app

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