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I have a software RAID 5 partition on LVM in Ubuntu (desktop, actually, but I'm using it as a server). I have been rsyncing a ton of data to it, and the computer was hard freezing, as in I needed to press "Reset".

So I thought it was rsync. But I decided I'd try a dd if=/dev/zero of=/path/to/raid5 and sure enough, the computer locked up. Did an identical dd to a JBOD partition on the same machine, and it didn't crash.

Assuming a clean RAID5 partition, tri-core processor 2GB of ram, 6GB swap, what could be causing this?

Edit: I've ruled out memory; I ran an 8 hour memtest without a crash.
04/26/2011 Edit: I've ruled out Ubuntu alone; the error occurred in Debian 6 stable. It's either hardware or an upstream bug.

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you did rule out bad memory sticks? –  Hubert Kario Jan 9 '11 at 20:44
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3 Answers

Yeah, test your RAM. Try testing plain IO more heavily. Other than that, try to get a repeatable scenario and open a bug on launchpad.net

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Assuming you're using software RAID5 through LVM (you don't say what's providing the R5) this could be a sign of a kernel bug. R5 requires parity calculation, which consumes CPU resources. If that goes high enough, the kernel might run into some unresolvable contention issues. This is just a guess, though.

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Is the RAID array everything in the server (including / and so forth) or seperate? If separate, can you see anything in the logs just before the hard hang? Also, could you confirm that it was a complete hang - could you ping the machine over the network at all and so forth?

The differences between writing out to a JBOD array and a RAID5 array are that the drives are accessed more evenly in the case of RAID5 and more CPU time will be used (for the parity calcs). If it were a problem with one of the drives I would expect it to fall out of the array rather than the machine hanging though, unless the problem is such that the drive controller fell over and took the machine's I/O controller with it. The first thing I'd do here is a full memory test, and make sure the CPU cooling is working as it should (the parity calcs for RAID5 will not impose any significant load on a modern CPU on its own, but may tip it over the edge if it is running close to trouble already).

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I keep an eye on htop as its doing the calculation and the CPU doesn't go high enough to cause issues. And yes, the root is on the raid 5, which may be why the whole system is crashing (and why I can't get logs). So you're thinking a possibly bad disk or kernel bug? –  Luke has no name Jan 9 '11 at 20:27
    
Or possibly dodgy RAM. You would expect to see symptoms elsewhere too, but the effects of bad RAM can be very random so it is always a suspect when something odd happens. It could be an iffy I/O controller to, upset by the particular load pattern of RAID5 activity - so make sure you have any firmware updates that might be available. –  David Spillett Jan 10 '11 at 0:26
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