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Why btrfs is great

Btrfs creates a single pool from multiple disks

Efficient use of the eMMC

If you find the eMMC is too small for your application you can just add an SDHC card to the pool and instantly more storage will be available. No need to mount or move files around.

Btrfs creates multiple subvolumes from a single pool

The factory partitioning based on ext4 (rootfs 1536MiB, boot 64MiB, update 704MiB, home the rest) made sense at the time.

The update partition was made large to be able to hold the OTA (over-the-air-update), while rootfs was 1GiB. However, at a certain point in time rootfs needed increasing as the expense of update, so OTA didn’t work any more. The result of course, 704MiB is not being used at all and the unused space in rootfs is still not available to home.

Partitioning scheme

With btrfs we now have a single rootfs partition with subvolumes @, @boot, @home.

The free space is available to all subvolumes.

Btrfs can take a snapshot of a subvolume

Btrfs can take a snapshot of a subvolume and keep that as a backup. Due to it being a COW file system the snapshot hardly takes up any space, only files that are modified are re-written to disk and take additional space.

This allows experimenting while quickly being able to fall back to the working subvolume and repair.

This also allows to take snapshot of the root (or other) image and use btrfs send to send it to the host as a backup.

Btrfs can have a 2nd root image

With btrfs we can have a 2nd root image that we can boot. For this we have reserved the subvolume @new. This allows care free experimenting with for instance a new yocto release or debian image.

It is easy to send the @new subvolume OTA:

cat edison-image-edison.snapshot | ssh root@edison.local "btrfs receive /mnt"

or use the utils/flash/btrfsFlashOta.sh to do that smoothly.

Alternatively, you can receive a subvolume from the host sent earlier as a backup.

Btrfs can have a second linux kernel

This is not exactly unique to btrfs. In general /boot (@boot) can hold multiple kernels. But considering that we can have also multiple root images, we have moved the modules directory to @modules and make that mount on /lib/modules.

This way, you can install a @new image with a new kernel, and boot that with the old tried and tested kernel. Or you can boot the @ old image with the new kernel.

Btrfs can resize the disk online

After updating using flashall btrfs filesystem resize max / can perform a resize on the mounted file system.

Btrfs has per file compression

As btrfs has per file compression we can increase available disk space, especially when there are many text or other easily compressible files on disk.

Compression can be done on the fly:

root@edison:~# mount | grep btrfs
/dev/mmcblk0p8 on / type btrfs (rw,relatime,compress=lzo,ssd,space_cache,subvolid=256,subvol=/@)
/dev/mmcblk0p8 on /boot type btrfs (rw,noatime,compress=lzo,ssd,space_cache,subvolid=257,subvol=/@boot)
/dev/mmcblk0p8 on /lib/modules type btrfs (rw,noatime,compress=lzo,ssd,space_cache,subvolid=258,subvol=/@modules)
/dev/mmcblk0p8 on /home type btrfs (rw,noatime,compress=lzo,ssd,space_cache,subvolid=261,subvol=/@home,x-systemd.automount)

Or on demand:

root@edison:~# btrfs filesystem defrag -r /
root@edison:~# btrfs fi us /
    Device size:                   3.55GiB
    Device allocated:              1.22GiB
    Device unallocated:            2.33GiB
    Device missing:                  0.00B
    Used:                        753.66MiB
    Free (estimated):              2.71GiB      (min: 1.54GiB)
    Data ratio:                       1.00
    Metadata ratio:                   2.00
    Global reserve:                2.54MiB      (used: 16.00KiB)

Data,single: Size:1.02GiB, Used:657.73MiB
   /dev/mmcblk0p8          1.02GiB

Metadata,DUP: Size:93.44MiB, Used:47.95MiB
   /dev/mmcblk0p8        186.88MiB

System,DUP: Size:8.00MiB, Used:12.00KiB
   /dev/mmcblk0p8         16.00MiB

   /dev/mmcblk0p8          2.33GiB

A more accurate analysis of compression results can be done using btrfs-compsize:

root@edison:~# /usr/bin/compsize /bin /boot /etc /home /lib /opt/ sbit /sketch/ /usr/ /var/
Processed 26316 files, 9056 regular extents (10118 refs), 17382 inline.
Type       Perc     Disk Usage   Uncompressed Referenced  
TOTAL       92%      680M         737M         764M       
none       100%      632M         632M         650M       
lzo         45%       48M         105M         113M       

Btrfs has online conversion from Ext

You can take your existing Edison with ext4 partitions and convert to btrfs without loosing your data. You can even undo this and go back to ext4. Of course some modification to the image are needed which you can do manually.

In principle you can do this without backing up your current installation.

Btrfs is supported from vanilla kernel and U-Boot

There may be other file systems that can do what btrfs can do, zfs comes to mind. However, btrfs support is in the vanilla kernel as well as in U-Boot. Turning on support is just a matter of flipping on the switch.

Less great things

Btrfs repair tools are dangerous to run on a mounted file system

As we have the kernel on @boot when the file system get’s corrupted we might not be able to boot the kernel and if we could might not be able to repair it because then it is already mounted.

To resolve this, we create a separate rescue image that has it own kernel and root image in a initrd. The initrd hold all the tools needed to convert an ext4 partition to btrfs or to repair damaged file systems.