You can use the mkfs.btrfs command to create a Btrfs file system that is laid out across one or more block devices. The default configuration is to mirror the filesystem metadata across the devices. If you specify a single device, the metadata is duplicated on that device, unless you specify to use only one copy of the metadata. The devices can be whole block device(s), simple disk partitions, files, loopback devices (that is, disk images in memory), multipath devices, or LUNs that implement RAID in hardware.

See the mkfs.btrfs(8) manual page for more detailed information about the mkfs.btrfs command the various Btrfs configurations that you can create.

When you want to mount the file system, you can specify it by any of its component devices, for example:

sudo mkfs.btrfs -d raid10 -m raid10 /dev/sd[fghijk]

sudo mount /dev/sdf mountpoint

The mkfs.btrfs command automatically scans the new Btrfs devices into the kernel. To scan and assemble all of the relevant devices of the volume in the kernel you need to run the btrfs device scan command. You can undo this action by using the btrfs device scan --forget command. To assemble all of the relevant devices of the volume, you might need to run the btrfs device scan command.

You can obtain the RAID configuration for a mounted btrfs file system as follows:

sudo btrfs filesystem df mountpoint

Note that the btrfs filesystem df command displays more accurate information about the space that is used by a Btrfs file system than by using the df command.

You can also use the btrfs filesystem usage command to display information about all of the btrfs file systems that are on a system, for example:

sudo btrfs filesystem usage /btrfs

Overall:
    Device size: 1.95TiB
    Device allocated: 5.03GiB
    Device unallocated: 1.95TiB
    Device missing: 0.00B
    Used: 256.00KiB
    Free (estimated): 999.48GiB (min: 999.48GiB)
    Data ratio: 2.00
    Metadata ratio: 2.00
    Global reserve: 3.25MiB (used: 0.00B)

Data,RAID10: Size:2.00GiB, Used:0.00B (0.00%)
   /dev/nvme0n1p6 1.00GiB
   /dev/nvme0n1p7 1.00GiB
   /dev/nvme0n1p8 1.00GiB
   /dev/nvme0n1p9 1.00GiB

Metadata,RAID10: Size:512.00MiB, Used:112.00KiB (0.02%)
   /dev/nvme0n1p6 256.00MiB
   /dev/nvme0n1p7 256.00MiB
   /dev/nvme0n1p8 256.00MiB
   /dev/nvme0n1p9 256.00MiB

System,RAID10: Size:16.00MiB, Used:16.00KiB (0.10%)
   /dev/nvme0n1p6 8.00MiB
   /dev/nvme0n1p7 8.00MiB
   /dev/nvme0n1p8 8.00MiB
   /dev/nvme0n1p9 8.00MiB

Unallocated:
   /dev/nvme0n1p6 498.74GiB
   /dev/nvme0n1p7 498.74GiB
   /dev/nvme0n1p8 498.74GiB
   /dev/nvme0n1p9 498.74GiB

You can use the btrfs command to add or remove devices and rebalance the layout of the file system data and metadata across devices. The following table describes each of the commands that can use to perform these tasks.

You can compress a Btrfs file system to increase its effective capacity, as well as defragment it to increase I/O performance.

The following three compression types are supported:

• zlib

• lzo

• zstd

There are three ways in which you can enable compression:

• By using the subvolume property, for example:

  sudo btrfs subvolume create /btrfs/sv1 

  sudo btrfs property set /btrfs/sv1 compression zstd

• By using the mount option in one of the following ways:

  Use the lzo type to compress the file data on the whole file system:

  sudo mount -o compress=lzo /dev/sdb /btrfs

  Use the zstd type to compress a subvolume and then mount it at the /btrfs1 mount point:

  sudo mount -o compress=zstd,subvolume=sv1 /devsdb /btrfs1 

• By using defragment, for example:

  sudo btrfs filesystem defragment -czlib /btrfs1/akzo

You can enable compression at any point and only the new writes are compressed or defragmentation is run.

You can compress a Btrfs file system at the same time that you defragment it.

To defragment a Btrfs file system, use the following command:

sudo btrfs filesystem defragment filesystem_name

To defragment a Btrfs file system and compress it at the same time, use the following command:

sudo btrfs filesystem defragment -c filesystem_name

You can use the following command to defragment and optionally compress individual file system objects such as directories and files within a Btrfs file system:

sudo btrfs filesystem defragment [-c] file_name ...  

To set up automatic defragmentation, specify the autodefrag option when you mount the file system. However, note that automatic defragmentation is not recommended for large databases or for images of virtual machines.

You can use the btrfs command to increase the size of a mounted Btrfs file system as long as there is space on the underlying devices to accommodate the change. You also use the btrfs command to decrease its size, if the file system has sufficient available free space. Note that running the command does not have any effect on the layout or size of the underlying devices.

You would increase the size of /mybtrfs1 by 2 GB as follows:

sudo btrfs filesystem resize +2g /mybtrfs1

The following example shows how to set the size of /mybtrfs3 to 20 GB:

sudo btrfs filesystem resize 20g /mybtrfs3

The following command decreases the size of /mybtrfs2 by 4 GB:

sudo btrfs filesystem resize -4g /mybtrfs2