Moment of truth

Tomorrow morning, I go out to the colo box and replace the existing one with my spiffy “new” Yellow Menace. I’ve tested the hell out of this one and it can handle all three domUs and the dom0 all doing dd’s from /dev/zero to the hard disk over and over again, which will be a nice change from the existing one freezing up throwing ext3 errors whenever I’m doing something disk intensive.

My plan is to pull out the old one, move the disks to the new one, boot it up, make sure I can log into the dom0 from home, and then go home to tinker with it. If things go as I expect, the services that live on here, like this blog, my mailing lists, my photo gallery, the Rochester Flying Club web site and others should only be off the air for an hour or so.

Then I’ll test the hell out of the old box when I get it home to see if I can reproduce the problem, and then see if I can fix the problem somehow, maybe with a new IDE cable. Then I’ll know how to advertise it on eBay – either as a working box, or a box with a suspect IDE controller but a probably fine SCSI controller.

My new colo box

It arrived last night. So of course I had to spend the whole damn night setting it up, in spite of the fact that the colo guys won’t be there to let me in for a few days.

Isn’t it pretty? As you can see, it’s a former Google Search Appliance, but without the Google software. Google evidently didn’t want anybody messing with it, and they even included a modem that you were supposed to connect to it in the case of problems so they could remotely diagnose and/or fix it. Which leads to problem number 1.

See that faceplate offset from the box in this picture? When it first came, that faceplate obscured access to the disk caddies, the floppy and the CD-ROM. Which would cause some difficulties installing new hard drives and booting from a CD to install the OS. The screws holding the faceplate on had had their (whatever you call the part the screwdriver goes in) rounded out so you couldn’t use a screwdriver to remove them. That necessitated solution number 1.

We ran out to the hardware store and bought a Dremel tool, and used that to cut grooves in the top of the screws so I could get them off. Once that was done, it was remarkably easy to get everything up and running. The drives are in caddies that slide out the front, and each drive is on a separate IDE controller – it appears that /dev/hda is the first disk, /dev/hdc is the second, /dev/hdd is the CD-ROM, /dev/hdc is the third disk. The only catches in the installation are:

  • There is a BIOS password that I don’t know – there is a jumper that says it will clear the CMOS, but I haven’t had the nerve to use it in case it clears something else that I don’t want cleared.
  • There are two ethernet jacks, and the one recognized by the Linux controller as eth0 is not the one you’d expect. Also, if you set the jumper to disable the ethernet controller, both seem to stop working. Either that, or the other one is a different type of ethernet controller that the Xen kernel can’t deal with.

I’d been using my Windows box as a sacrificial test machine with a couple of scratch disks to emulate what I have at the colo facility to play around. For the last couple of weeks, I’d been convinced that this was going to mean that I was going to have to take the box home to transfer the disks and install the latest Xen kernel, because I couldn’t get it to work consistently. But the new box has two things going for it – it has three disk slots, and the current colo boxes are using LVM. Experimenting with the new box showed that I could install a basic Debian Sarge on it, upgrade it to Debian Etch, use the Xen packages in Debian Etch to get up and running, and then slap the disks from my test machine in, and even though they’re on different devices than they were on the test machine, LVM automagically recognized them and I was able to mount the domU partitions exactly the same as I had in the test machine. From there, it was a simple matter to copy the new /lib/modules/2.6.18-3-xen-686 to the partitions, make a few small changes to the domU config files, and start them up.

This is great – it means that the way things are now, I can take the new box over to the colo, swap the hard drives over, and boot it up and it will be in a state that I can do the rest of the setup from home, with no data loss and only a few hours of downtime. That is currently scheduled for next Wednesday.

By all means, Paul, …

Yesterday I went to CompUSA to buy a new PCI IDE controller – one of my external USB drives that I bought to perform networked backups of my colo keeps losing its mind, and I was thinking that it might be that the USB controller (either the interface card or the one in the external box) isn’t up to heavy data transfer, so I thought it might be good to move it “indoors” as it were.

I installed the controller and a 250Gb hard drive. The system found it at /dev/hdg – I guess I put it in the second of the two IDE controllers on the card. I made /dev/hdg a physical volume (pv) under LVM2, made it a volume group (vg) and put a logical volume (lv) on it for my mp3 collection. After I moved my mp3s from where they’d lived before, on /dev/hdb, I wiped /dev/hdb and made it part of the vg, and made another vg for the colo backup. Yesterday I also discovered the “--link_dest” argument to rsync, so I can keep several days worth of backups in much less space.

Tonight I’m going to rip that hard disk out of the external USB drive and put it on my currently eviscerated Windows machine to see if IBM/Hitachi’s “DFT” drive function tester can find any problems with it. If not, I’ll add it to the vg and increase the size of the mp3 lv.

Tomorrow my new colo box should arrive, unless UPS does their customary screw-ups. I’m scheduled to go out to the colo facility on Thursday. I’m going to move the old drives to the new box, and upgrade to the newest version of Xen. I’ve practiced upgrading to the new Xen on my currently eviscerated Windows box (that’s why it’s eviscerated, I had to put scratch disks in it) and it didn’t go well, but I think I know what I did wrong. I also tried a full install of Debian on the dom0, and was able to save the domUs when I tried that.

If that goes well, I’ll be up again in a few hours. If it doesn’t go well, I’ll bring it home and work on it overnight, and I’m tentatively scheduled to go back to the colo on Friday.

The ultimate Heisenbug?

We’ve got a problem that happens apparently at random times at a few customer sites, but which we’ve been unable to reproduce in the lab. I’m not sure if that means it’s a Heisenbug or just a really nasty Bohr-bug.

The part of the system that is affected are three programs:

  • One that generates events, called “tixd
  • One that is responsible for collecting events from all the programs in the system (not just these three) and delivering them to subscribers, called the “EventBroker” or “eb
  • One that subscribes to the events that the “tixd” generates, which we call the “scheduled

What has been happening on these customer sites is that after days or weeks of proper operation, for no apparent reason, the “tixd” would say that it’s generating an event, but the “scheduled” wasn’t getting them any more. The customer would notice the problem, sometimes a day or two later, complain that things weren’t happening that were supposed to happen, our service people would restart the whole system, and everything would start working again.

This bug has been happening for ages now, and every time I get called in to look at their logs because I wrote the “scheduled” and all the fingers point to me. But I couldn’t find any reason why “scheduled” would stop responding to events, or would unsubscribe from events. A few builds ago, Tom put some debug into his “eb” that would log every event that came in and which subscribers it was being delivered to. He also logged subscribes and unsubscribes. And so we waited.

Today, it finally happened again. And this time, I’ve got the logs that show:

  • At 6am, an event is generated by the tixd, and the eb delivers it to the subscriber scheduled
  • Between 10am and 11am, there is a flurry of event subscribes and unsubscribes, all unrelated to scheduled. But some of these unsubscribes are caused when events are being delivered to subscribers that have exited without unsubscribing.
  • At about 1am, there is another event generated by the tixd, and the eb receives it but says there are no subscribers found.

At this point, because the eb log shows no unsubscribe coming from scheduled, I’d say it’s not my bug and pass it off to Tom, the author of the eb. But unfortunately, my employer declined to renew Tom’s contract at the end of last year, so he no longer works here. He dodged this bullet by only 5 days. And so I’ve got to figure out why this is happening. Lucky me.