Yesterday, I planned to try an upgrade from 6.x to 7.x on one of the servers at work, on the assumption that I could reasonably easily revert to 6.x if things went bad. My confidence in being able to do that was severely shaken when I saw "Upgrading metadata" messages for both geom_mirror disks when the 7.x kernel booted.
Whilst I did not need to revert, some examination of the geom_mirror source shows that (as I feared), a 6.x kernel will choke on a 7.x geom_mirror. I've looked through the various geom man pages, UPDATING, the relevant commit message and various mailing lists and am unable to find any mention of this. Having the kernel automatically alter persistent state on the host in a way that is incompatible with earlier kernels is bad enough. Doing so without any mechanism to revert the change and with no "heads-up" warning of the change is, IMO, unacceptable. I accept that it's sometimes necessary to make changes that are not backward compatible or that are difficult to revert, such changes are normally discussed in advance and come with heads-up warnings. I can't think of any previous case where simply test booting a kernel is enough to render your system unusable with an older kernel. -- Peter Jeremy Please excuse any delays as the result of my ISP's inability to implement an MTA that is either RFC2821-compliant or matches their claimed behaviour.
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