How to fsck /usr with LVM?

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https://bbs.archlinux.org – Wasn't sure where to post this:I just applied a pile of updates to my Arch+MythTV PVR. Not necessarily related to my problem, but this update included the replacement for mtab. On reboot, I get dumped into single-user because /usr is dirty. Dirty dirty /usr. Of course, I cannot fsck it because I cannot unmount it. I'm using LVM: two 2TB drives mirrored, two VGs defined (rootvg and datavg). /usr is it's own filesystem within rootvg.Any suggestions on how to get this fixed? I assume I'll have to boot from a liveCD. Preferably something that will recognize the LVM stuff at boo (HowTos)