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thanks scan_ffs(8)

Posted by Felix Kronlage on Sunday, April 06, 2008

While updating machines at our colocation in Frankfurt last friday, Bernd and I hit a pretty harsh bug. Because of changes in the disklabel code, the disklabel of our main customer mailserver got updated by the OpenBSD kernel upon booting with the new OpenBSD kernel. After updating all packages and configurations on the machine we gave the server another reboot. Next the machine got stuck in the first-stage bootloader, complaining that there is no OS to boot.

Quite baffled, we booted bsd.rd kernel just to see that there is no disklabel present. Now, usually you have backups of /var/backups, where OpenBSD keeps informations like this around. Of course, these files were not in the backup (time to blush on my part). Thanks to a hint from our fellow OpenBSD developer Henning, we gave scan_ffs a try. Since scan_ffs is not present in the ramdisk kernels, we compiled it statically and placed in on a USB stick. scan_ffs scans a disk and outputs the partitions it finds. With a bit of thinking, we could then create a disklabel with the correct data. Once we had /var mounted, we could use the saved disklabel there to make sure the label is correct.

The manpage for scan_ffs contains the following advise:

The basic operation of this program is as follows:
1. Panic. You usually do so anyways, so you might as well get it over
with. Just don’t do anything stupid. Panic away from your machine.
Then relax, and see if the steps below won’t help you out. 

Very true. While we did not panic, the best thing to do in such situations is to think and evaluate your options and possible recovery methods and don’t be quick on trying to fix it, likely you will make it worse ;)

Thanks scan_ffs!