ext3 -> ext4 conversion of "/" and old Grub

[merilius]

Hi,

I am running F17 with / and /boot mounted as ext3. I want to convert / to ext4 to gain some performance, but I still have GRUB in my MBR installed from the F15 times (and GRUB2 package installed but unused). I had bad experience with GRUB2 and don't want to switch to GRUB2 yet.

Will GRUB be able to boot my system after the conversion? (I am going to convert the filesystem offline of course from some boot media)
At
https://ext4.wiki.kernel.org/index.p...system_to_ext4
they say

If you convert your root filesystem ("/") to ext4, and you use the GRUB boot loader, you will need to install a version of GRUB which understands ext4. Your system may boot OK the first time, but when your kernel is upgraded, it will become unbootable (press Alt+F+F to check the filesystem).

But I don't have GRUB installed. Will it work or am I asking for trouble?

Met

As far as I remember F15 grub is EXT4 aware.

Don't forget to change the fstab / mount type to ext4 and redo initramfs afterwards

su
mv /boot/initramfs-$(uname -r).img /boot/initramfs-$(uname -r)-backup.img
dracut /boot/initramfs-$(uname -r).img $(uname -r)

[jpollard]

You won't really gain any performance.

The performance improvement comes from files containing extents. So until such files are created, there won't be any performance improvements...

Also note, once extents ARE used, you can't go back to ext3.

[merilius]

@Yellowman: Thanks. Are ext4/3/2 drivers compiled into the kernel statically in Fedora? lsmod does not show any such modules in use.
I have already mounted my ext3 "/" with ext4 driver (it is compatible this way).
Does grub look at kernel image + initramdisk only, which resides on separate /boot partition in my case, or actually looks at "/" too? I have no need to convert /boot :-)

@jpollard: Yes, I am aware of this. But updates will come and eventually files will be overwritten. I hope to gain improvement over time.

[george_toolan]

Do you have a SSD? The only improvement is the support for the trim command ;-)

[merilius]

So, I performed the conversion. Edited fstab, recreated initramfs, booted from some rescue CD based on Gentoo and executed tune2fs and e2fsck. All seems to work fine. Have an impression that yum works faster now (maybe it is not a placebo effect).

[Gareth Jones]

It might do, as yum is modifying files and installing new ones, so extents may come into play. However, it's worth noting that in recent Fedora kernels, the ext4 driver is used for ext2/3 mounts too. On the whole, it's still worth enabling full ext4 features though.