Btrfs: Difference between revisions

= Installation of NixOS on btrfs =

== Partition the disk ==

== Format partitions and create subvolumes ==

== Mount the partitions and subvolumes ==

== Install NixOS ==

= Configuration =

== Compression ==

<code>nixos-generate-config</code> doesn't detect mount options automatically. To enable compression, you must specify them manually and other mount options in your <code>configuration.nix</code>:

<nowiki>

</nowiki>

 

 

 

 

 

 

 

Creating a separate subvolume for the swap file is optional. It is not required for functionality but can help with organization or snapshot management. Be sure to regenerate your <code>hardware-configuration.nix</code> if you choose to do this.

Finally, define a swap file in your configuration and run <code>nixos-rebuild switch</code>:

 

  size = 8*1024; # Creates an 8GB swap file

 

{{note| On systems where you do need to manually prepare a swap file on Btrfs, you can use <code>btrfs filesystem mkswapfile</code> utility, e.g.: <br><code># btrfs filesystem mkswapfile --uuid clear /swap/swapfile</code>}}

For more NixOS swap configuration options, see [[Swap]]. Additonal Btrfs swapfile usage can be found at [https://btrfs.readthedocs.io/en/latest/Swapfile.html the Btrfs docs].

== Scrubbing ==

Btrfs filesystems by default keep checksums for all files, to monitor if the file has changed due to hardware malfunctions or other external effects.

Scrubbing is the process of checking file consistency, which may use checksums and/or duplicated copies of data, from raid for example. Scrubbing may be done "online", meaning you don't need to unmount a subvolume to scrub it.

You can enable automatic scrubbing withː

Automatic scrubbing by default is performed once a month, but you can change that withː

By default, autoscrub will scrub all detected btrfs mount points. However, in case of mounted nested subvolumes (e.g. the example above where <code>/nix</code> and <code>/home</code> are nested subvolumes under <code>/</code>), you only need to scrub the topmost one. So an example configuration may look like this:

The result of the periodic auto scrub will be saved to the system journal, and you can check the status of the last scrubː

You can also start a scrub in the background manuallyː

You can check the status of the ongoing scrubbing process with the same <code>status</code> command as above.

== Deduplication ==

 

== Subvolumes ==

 

<syntaxhighlight lang="bash">

To create a new subvolume at a specified location:

To remove an existing subvolume:

=== Top-level vs nested subvolumes ===

In btrfs, subvolumes can be created either at the top level of the filesystem or within other subvolumes.

* Top-level subvolumes are created directly under the filesystem's root. By default, the root volume ID is 5. Top-level subvolumes are easier to snapshot, roll back or destroy independently. This is good for things such as <code>/home</code> or <code>/nix</code>.

* Nested subvolumes are created inside an existing subvolume or directory within the filesystem. All nested subvolumes inherit the mount status of their parent unless mounted separately. This layout is useful for organizing related subvolumes under a common namespace. For example, a top-level subvolume such as <code>/srv/nfs</code> can contain multiple nested subvolumes like <code>/srv/nfs/export1</code> and <code>/srv/nfs/export2</code>.

 

 

 

 

 

To take a read-only (<code>-r</code>) snapshot called <code>home_snapshot_202302</code> of the subvolume mounted at <code>/home</code>

You can also snapshot the root subvolume. But keep in mind that nested subvolumes are '''not''' part of a snapshot. So if you have subvolumes <code>/nix /home</code>, taking a snapshot of <code>/</code> will not include them.

Make snapshot read-write againː

However, changing read-only property of a snapshot in-place may [//lore.kernel.org/linux-btrfs/06e92a0b-e71b-eb21-edb5-9d2a5513b718@gmail.com/ causes issues] with any future incremental send/receive. Instead, a read-only snapshot itself (being a simple subvolume) can be snapshotted again as a read-write snapshot like this:<syntaxhighlight lang="bash">

{{warning|1=this will delete the current <code>/home</code> and restore the snapshot! <code>/home</code> must be unmounted for this operation}}

After this you can mount <code>/home</code> again.

== Transfer snapshot ==

Sending the snapshot <code>/snapshots/nixos_snapshot_202302</code> compressed to a remote host via ssh at <code>root@192.168.178.110</code> and saving it to a subvolume mounted on a directory at <code>/mnt/nixos</code>

 

= Tips and tricks =

 

 

 

== Installation with encryption ==

You can use any device partition for your bootloader. Note that this bootloader is unencrypted by default:

<code>mkfs.vfat -n BOOT "$DISK"p1</code>

=== Creating subvolumes ===

<code>umount /mnt</code>

Once the subvolumes have been created, mount them with the desired options.

 

Example with [https://facebook.github.io/zstd/ Zstandard compression] and noatime:

== Convert Ext3/Ext4 system partition to Btrfs ==

{{Warning|Note that migrating your existing root filesystem can cause data loss or make your system unbootable. Make sure to backup the partition or your files. Proceed only if you know what you're doing!}}

To convert the existing filesystem (Ext3/4) to Btrfs, boot into a NixOS live system and run the following commandː<syntaxhighlight lang="sh">

Next, mount the converted filesystem and chroot into itː<syntaxhighlight lang="sh">

</syntaxhighlight>If the Grub bootloader is used and it doesn't get reinstalled correctly, you can run the following command inside chrootː<syntaxhighlight lang="sh">

</syntaxhighlight>If the conversion was successful and no rollback is required, the backup image which was stored by btrfs-convert can be removed withː<syntaxhighlight lang="sh">