Converting Ext4 to JFS

Because I have an older laptop and disk I/O can really bottleneck on the motherboard, I decided to move from the ext4 filesystem to JFS. Recently, I’ve used ext4 because it was fairly fast and definitely reliable; however, with the kernel moving to 2.6.30 new data integrity features have been added that slow it fairly noticeable on an eight year old computer. Moving to JFS has made a fair difference in improving the speed of the system, it’s caveat being that it that journals only metadata (not metadata and data like ext3/4)).

Backup, Convert, Restore

The JFS filesystem utilities will be needed (for Debian/Ubuntu):

sudo apt-get install jfsutils

Reboot to a rescue CD, and backup partition(s)/disks onto another drive. For this example two partitions are used: one for root, one for home. Mount root, home, and then the backup drive:

mkdir /mnt/{,,}
mount /dev/ /mnt/
mount /dev/ /mnt/
mount /dev/ /mnt/

Create the backup directories:

mkdir -p /mnt//backup-rsync/{root,home}

Backup both partitions:

rsync -axS /mnt// /mnt//backup-rsync/root
rsync -axS /mnt// /mnt//backup-rsync/home

Check integrity of backup, then create a JFS filesystem on both partitions:

mkfs.jfs /dev/
mkfs.jfs /dev/

Restore the backup contents back to the root and home partitions; first method:

rsync -axS /mnt//backup-rsync/root/ /mnt/
rsync -axS /mnt//backup-rsync/home/ /mnt/

Or this method to be sure files are defragmented (JFS is somewhat prone to fragmentation, heavy use may require occasional defragmenting):

(cd /mnt//backup-rsync/root/ && tar -cS -b8 --one -f - .) | (cd /mnt/ && tar -xS -b8 -p -f -)
(cd /mnt//backup-rsync/home/ && tar -cS -b8 --one -f - .) | (cd /mnt/ && tar -xS -b8 -p -f -)

Updating the System

The system needs to know of the filesystem changes. Changing apparent root from the rescue CD to the current Linux install is done by:

cd /mnt/
mount -t proc proc proc/
mount -t sysfs sys sys/
mount -o bind /dev dev/
chroot . /bin/bash

Update the chrooted system current mounts file:

grep -v rootfs /proc/mounts > /etc/mtab

The fstab file (the static filesystem configuration) needs to be updated. The information that will need adjusting is: the UUID (possibly), the filesystem type, and options. The UUID’s (unique disk identifiers) may have changed, they can be appended onto the fstab file (so that they can be easily moved) like this:

blkid /dev/ | cut -d "\"" -f 4 >> /etc/fstab
blkid /dev/ | cut -d "\"" -f 4 >> /etc/fstab

Edited /etc/fstab with set UUID, type, and options:

#                            
...
# /dev/sda2
UUID=5d9753dd-f45f-425a-85e2-25746897fdfa / jfs   noatime,errors=remount-ro 0 1
# /dev/sda4
UUID=d3f9eafd-1117-4c75-a309-b21dece655d1 /home jfs noatime                 0 2
...

noatime lessens disk writes by not creating a timestamp every time a file is accessed (it isn’t seen as very useful anymore since it was developed primarily for servers with statistics in mind).

JFS supposedly works very well with the Deadline Scheduler; the Grub configuration need to specify to use it. This example is for Grub2 though it is similar with original Grub; edit /etc/default/grub and append:

...
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash elevator=deadline"
...

The other Grub configurations need to be updated with the new information:

update-grub

Then the Grub bootloader will have to be re-installed to the MBR (I think this is because the version of Grub put on the MBR has directions on how to be able to find its configurations for a specific filesystem).

grub-install /dev/  # Disk here is more likely and not partition

Exit the chroot and unmount temporary filesystems:

exit
umount {proc,sys,dev}

Reboot.

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