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Changing the eMMC filesystem

As with most computers, the speed of the N900's internal storage often poses a bottleneck, slowing down the device during heavy reads/writes. Changing the filesystem can mitigate the impact of this.

By choosing an appropriate filesystem for the content of your disk, you are able to speed things up. Vice versa, a bad filesystem choice can negatively impact performance. A different reason to switch filesystems besides performance reasons can be because of the feature set offered by different filesystems. For example, ext3 is considered mostly as a very stable filesystem while Reiser4 offers transparent compression.

Each filesystem got its own pros and cons. There is no 'one size fits all' type of filesystem; only filesystems each with their own use-cases. Therefore: do some extensive research before deciding to change your filesystem!

This wiki page is focused on changing the filesystem of the /home partition.

Prerequisites:

• The kernel you're running should support the filesystem you want to use. Either use kernel-power or compile a kernel yourself.
• Willingness to mess with Maemo's boot process (reflash risk!)

How to do it

Basically, you need to move /home to a temporary location, reformat its original partition with the filesystem of your choice, copy /home back and finally edit /etc/event.d/rcS-late. This can be a challenge as Maemo gets into a reboot loop when /home can't be mounted. Back up all your data!

Solution #1: Do everything on-device

This solution is based on Solution #7 in Repartitioning_the_flash.

1. Move /home as described in Solution #7 in Repartitioning_the_flash. Follow the described steps strictly until you reach the command which would actually repartition the flash (the sfdisk command). You don't need to follow any other steps from the mentioned solution from now onwards.
2. Assuming you've successfully moved /home (e.g. you got a fully booted phone with /home on a different partition), format /home its original partition to your filesystem of choice and mount it. For example:

   mkfs.ext4 /dev/mmcblk0p2
   mount -t ext4 -o rw,noatime,commit=1,data=writeback /dev/mmcblk0p2 /mnt
   

3. Copy your /home back to (now its reformatted) original partition:

   cp -a /home/* /mnt
   

   Don't forget to unmount /home/user/MyDocs prior to copying!

4. Now you should edit /etc/rcS-late to make Maemo use /home its original partition again and accept its new filesystem type.
   1. Making Maemo use /home its original partition again. In step 1, you've added the line:

      echo "/dev/mmcblk1p1 /home ext3 rw,noatime,errors=continue,commit=1,data=writeback 0 0" >> $tmp_fstab
      

      to /etc/event.d/rcS-late. Modify this line to represent the fstab entry of your new filesystem on /home its original partition. Examples include:

      #ext4 example
      echo "/dev/mmcblk0p2 /home ext4 rw,noatime,errors=continue,commit=1,data=writeback 0 0" >> $tmp_fstab
      

      #Reiser4 example
      echo "/dev/mmcblk0p2 /home reiser4 rw,noatime 0 0" >> $tmp_fstab
      

      Make sure you add no invalid mount options! Mount failure results in a reboot-loop. You can test the mount options by trying to mount the partition manually prior to making it permanent.

   2. Making Maemo accept the new filesystem type. Line no. 13, 18, 23, 24 and 26 of rcS-late got "ext3" hard-coded in it. We need to edit these lines to represent our new filesystem. Ext4 example:

      ##Begin of snippet
      script
          #mount /home; fix ext4 and retry mounting if 1st attempt fails
          mount_home ()
          {
          /bin/mount /home && grep -q "/home ext4 rw" /proc/mounts
          if [ $? -eq 0 ]
          then
            return 0
          else
            grep -q "/home ext4 ro" /proc/mounts
            if [ $? -eq 0 ]
            then
              umount /home
            fi
            HOME_DEV=`grep "/home ext4" /etc/fstab | cut -d' ' -f1`
            fsck.ext4 -y $HOME_DEV >> /var/lib/fsck_ext4_home.log 2>&1
            sync
            /bin/mount /home && grep -q "/home ext4 rw" /proc/mounts
            return $?
          fi
          }
      ##End of snippet
      

      Make sure line 24 contains a valid fsck.X command. Try running the command with all its parameters manually before making it permanent.

5. Reboot your phone. You're done now.

Some initial benchmarks

Note from author: The benchmarks below are pretty limited, feel free to substitute them with more extended/reliable ones.

How the benchmarks were performed: Reflashed & repartitioned phone in offline mode, SSH over usb. Used script (first round out of five dismissed in the averaged result):

#!/bin/sh
for run in `seq 1 5`
do
sync
echo 3 > /proc/sys/vm/drop_caches
sleep 1
time sh -c "cp [FILE/DIRECTORY] /mnt && sync"
done
 
df | grep /mnt

The files were copied from /dev/mmcblk0p1 (FS: reiser4, LZO compression) to /dev/mmcblk0p2 (FS: depends on test)

Used ext3 options:

mkfs.ext3 /dev/mmcblk0p2
mount -t ext3 -o rw,noatime,errors=continue,commit=1,data=writeback /dev/mmcblk0p2 /mnt

Used ext4 options:

mkfs.ext4 /dev/mmcblk0p2
mount -t ext4 -o rw,noatime,errors=continue,commit=1,data=writeback /dev/mmcblk0p2 /mnt

Used reiser4 options:

mkfs.reiser4 -o create=ccreg40,compress=lzo1,cluster=8K,formatting=smart /dev/mmcblk0p2
mount -t reiser4 -o noatime /dev/mmcblk0p2 /mnt

Averaged values of the "real time" as reported by time

Space reported by df after each benchmark (in total 747264 1K blocks available)

Do note that both the stock Maemo kernel and kernel-power lack Reiser4 support; you'll have to compile a kernel with Reiser4 support yourself.

Remarks about filesystems

Please extend this text as you encounter stuff worthy to take note of.

• Reiser4 has some problems with both cluster=8K and cluster=16K on the N900 ("Wrong level found in node" warnings in dmesg). The parameters cluster=32K and cluster=64K (the default value) work without error. Use these values to prevent potential data loss.

The following seems to have removed the warnings in dmesg, test consisted of creating a tarball of /home/opt (~400MiB), extracting it once mounted, removal of the original tar and then repeating for a tarball of the SDK (~500MiB).

Take note of the fsck.reiser4 stage which should be executed before the first mount.

mkfs.reiser4 -y -o create=ccreg40,compress=lzo1,compressMode=ultim,cluster=8K,fibration=ext_3_fibre,formatting=smart /dev/mmcblkXpY
fsck.reiser4 -y --build-fs /dev/mmcblkXpY
mount -t reiser4 -o rw,noatime,nodiratime /dev/mmcblkXpY /path/to/mount

Different filesystem in action