---
title: BTRFS
---

[toc]
## Utils
`# pacman -S btrfs-progs`  

## Fstab example
`UUID=2dc70a6e-b4cf-4d94-b326-0ba9f886cf49 /mnt/tmp btrfs defaults,noatime,compress-force=zstd,space_cache=v2,subvol=@ 0 0`

Options:  
```
defaults       -> Use whatever defaults
noatime        -> Reading access to a file is not recorded
compress-force -> Send all files through the compression algorithm
space_cache    -> Increases performance by mapping unallocated blocks
subvol         -> Subvolume to mount
```

## Filesystem usage
Show storage allocated, used and free  
`# btrfs fi usage (mountpoint)`  

```
allocated: space used
unallocated: actual free space
Used: amount of data stored
Free: free storage based on "Used"
```

Start rebalance of datachunks filled less than 70%  
`# btrfs balance start --b -dusage=70 -musage=70 (mountpoint)`  

Check status of rebalance  
`# btrfs balance status -v (mountpoint)`  

## Disable CoW
Disable copy on write for folders (Only works on new files)  
`$ chattr +C (path)`  

## Device errors
Error counts for a given mountpoint  
`# btrfs dev stat (mountpoint)`  

## Compression

### Algorithms
```
zlib: Slow, but strong compression, level 1-9
lzo : Fastest, weak compression
zstd: [Recommended] Medium, newer compression standard than the others, only works on newer kernels, level 1-15
```

Enable compression for existing files  
`# btrfs filesystem defragment -r -v -c(alg) (path)`  
*It is impossible to specify the level of compression wanted.*  

Add `compress=(alg)` to `/etc/fstab`  

To specify a level of compression (zlib and zstd) use `compress=(alg):(level)` in fstab.  
For zstd compression it is recommended to use `compress-force=zstd:(level)`  

## Subvolumes
List  
`# btrfs subv list (path)`  

Create  
`# btrfs subv create (path)`  

Mount a subvolume  
`# mount -o subvol=@(subvolname) /dev/sdXX /(mountpoint)`  

## Snapshots
TODO

## RAID
An array can be mounted by specifying one of its members.  
`# mount /dev/sdXX /mnt`  

All members of an array have the same UUID, which can be mounted through fstab.  

### RAID 1
On filesystem creation  
`# mkfs.btrfs -m raid1 -d raid1 /dev/sdXX /dev/sdYY`  

### RAID 5
On filesystem creation  
*It is recommended not to use raid5/6 for metadata yet*  
`# mkfs.btrfs -m raid1 -d raid5 /dev/sdXX /dev/sdYY /dev/sdZZ`  

### RAID 10
On filesystem creation  
`# mkfs.btrfs -m raid10 -d raid10 /dev/sdXX /dev/sdYY /dev/sdZZ /dev/sdQQ`  

### Convert to single device
First, the files have to be collected on one device.  
*DUP on system and metadata should only be used on HDDs. Use single on SSDs*  
`# btrfs balance start -f -sconvert=dup,devid=(id) -mconvert=dup,devid=(id) -dconvert=single,devid=(id) /(mountpoint)`  

Now unused devices can be removed  
`# btrfs device delete /dev/sdYY /(mountpoint)`  

### Replace dying/dead device in RAID array
Show arrays that are available  
`btrfs fi show`  

From my testing the log has to be dropped before btrfs will mount the incomplete array  
`btrfs rescue zero-log /dev/sdXX`  

Mount with these options to be able to fix it  
`mount -o rw,degraded /(mountpoint)`  

The ID has to be replaced with the ID of the **missing** device!  
`btrfs replace start -B (id) /dev/sdYY /(mountpoint)`  

Query the status of the repace  
`btrfs replace status /(mountpoint)`  

Balance the filesystem at the end  
`btrfs balance /(mountpoint)`