Btrfs
Btrfs is a CoW (copy on write) filesystem with checksums, snapshots, compression and more.
Installation
The btrfs-progs package provides a number of user-space tools. To install the package:
# apk add btrfs-progs
When Installing Alpine linux, setup-disk can automatically set up a root filesystem with Btrfs using environment variables. Export ROOTFS before running setup-disk or setup-alpine like so:
# export ROOTFS=btrfs
The filesystem utilities and modules will automatically be set up.
Manual Installation
If btrfs is used for root filesystem, ensure that the initramfs is generated with the btrfs module, otherwise the system may fail to boot.
To do so edit the /etc/mkinitfs/mkinitfs.conf and ensure that "btrfs" is in the list of features as follows:
Contents of /etc/mkinitfs/mkinitfs.conf
After making the above change, issue the command to regenerate the initramfs:
# mkinitfs
Configuration
To load the btrfs kernel module immediately, use the following command:
# modprobe btrfs
To mount non-root partitions with btrfs filesystem automatically on startup:
# echo btrfs >> /etc/modules
Enabling btrfs-scan service from the btrfs-progs package loads the btrfs kernel module and scans for and registers Btrfs devices with the kernel. To enable it:
# rc-update add btrfs-scan boot
btrfs-scan service is recommended if btrfs file system is used.Mounting a subvolume
To mount a subvolume @alpine located in the btrfs partition /dev/nvme0n1p3, the command is:
# mount -o subvol=@alpine /dev/nvme0n1p3 /mnt
Mounting a subvolume on boot
To mount a volume on boot, add a new entry to your /etc/fstab file as follows:
Contents of /etc/fstab
If you use more specific mounting options like for example:
Contents of /etc/fstab
The option defaults is recommended for most use cases. More information about mounting can be found in the official Btrfs wiki
apk-snap
The apk-snap script from apk-snap package triggers filesystem snapshots before and after every apk commit using snapper utility.
The apk-snap package is currently available in testing repository. It can be safely installed by following the guidelines.
The {ic|apk-snap}} script provides necessary apk hooks and script that causes snapper utility to automatically take a pre and post snapshot before and after apk transactions, similar to how YaST does with OpenSuse. This provides a simple way to undo changes to a system after an apk transaction.
By default the / (root) snapshots taken by snapper are saved in the /.snapshot folder. To make it easier to manage the snapshots created by snapper, it is better to mainain it outside of /(root) folder. To achieve this, create a subvolume @snaps_root in the btrfs partition and mount the above subvolume on /.snapshot folder by having an entry in etc/fstab file as follows:
Contents of /etc/fstab
Bootloader
If the initrd and kernel are installed inside the btrfs root subvolume instead of EFI partition, configuring the bootloader properly is important. For sample configuration, refer to the following pages for rEFInd and GRUB bootloaders.
Btrbk
The backup tool btrbk can be used to take backups using btrfs specific capabilities to create atomic snapshots and transfer them incrementally to your backup locations.
To install the package btrbk, issue the command:
# apk add btrbk
To use default BusyBox, use the following global option in /etc/btrbk/btrbk.conf as follows:
Contents of /etc/btrbk/btrbk.conf
Troubleshooting
Mount failed
If you try mounting a Btrfs volume via your /etc/fstab and if it doesn't show up, this is related to #9539. Can't mount BTRFS volume using fstab. This could be because Btrfs does not know about the drives during boot. To avoid this issue enable the btrfs-scan service. The volume should mount correctly after a reboot.
Failed to fetch subvolume detail
In Alpine Linux, when running btrbk with BusyBox defaults, you may encounter the above error 'Skipping subvolume "/mnt/btrfs/@myvolume": Failed to fetch subvolume detail'. To troubleshoot, run the command
# btrbk -n -l debug -L -S run
If you see the error
WARNING: ... sh: readlink -v -e '/mnt/btrfs/@myhome' WARNING: ... readlink: unrecognized option: e
To resolve, either use the compat busybox option as mentioned in btrbk section or install the coreutils package.