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I upgraded F10 to F11 successfully. Then I upgraded the filesystem in the following steps:

1. login as root user in multi user mode
2. read http://ext4.wiki.kernel.org/index.ph...system_to_ext4, and execute
   tune2fs -O extents,uninit_bg,dir_index /dev/mapper/VolGroup01-LogVol00
   
3. modify /etc/fstab, change the type of / to ext4
   
4. ========== reboot ==========
   (because fsck say running it on a mounted filesystem can cause filesystem damage, so i decide to reboot to single user mode first. maybe it's a mistake here)
   
5. try to boot F11 to single user mode, failed
   
6. ======== reboot from a SystemRescueCD-1.2.0 LiveCD ======
   
7. run e2fsck -fpDC0 /dev/mapper/VolGroup01-LogVol00 in SystemRescueCD-1.2.0 LiveCD, no error reported
   
8. ======== reboot ======
   
9. try to boot F11 normally(multi user mode), but it failed at: EXT3-fs: dm-0: couldn't mount because of unsupported optional features (40).

I tried rescue mode of Fedora-11-i386-netinst.iso after these steps above. The / partition can be found and mounted to /mnt/sysimage correctly. I can read/write files in / partition, and i can even yum new softwares in rescue mode, but it just can't be mounted when booting.