Ok, let's say you have created your ext3 root filesystem,
and you want to use it with data=journal, instead of the
default data=ordered. You cannot umount it, so you try:
mount -o remount,data=journal
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Most likely, the command will fail with:
mount: / not mounted already, or bad option
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If you also run dmesg, you will see the error:
EXT3-fs: cannot change data mode on remount
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so you're stuck. Also, the root partitions gets
mounted before fstab is read, so... it's a bit
harder to get the options right.
If you cannot reboot the system, the only thing
you can try is to play with pivot_root or similar,
to try to temporarily move the root on tmpfs / ramdisk / ...
but would still need lot of restarting, mount --bind and
so on to move the services to the new root.
If you can afford to reboot, and you want data=journal
after every reboot, you can use:
tune2fs -o journal_data /dev/sda1
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where sda1 is your root partition. Alternatively, on LILO
or grub command line, you can pass the option: