Btrfs guide to set up an LUKS-encrypted btrfs raid volume with included maintenance & recovery guide

btrfs-guide.md

Encrypted Btrfs storage setup and maintenance guide

Initial setup with LUKS/dm-crypt

This exemplary initial setup uses two devices /dev/sdb and /dev/sdc but can be applied to any amount of devices by following the steps with additional devices.

Create keyfile:

dd bs=64 count=1 if=/dev/urandom of=/etc/cryptkey iflag=fullblock
chmod 600 /etc/cryptkey

Encrypt devices:

cryptsetup -v -c aes-xts-plain64 -h sha512 -s 512 luksFormat /dev/sdb /etc/cryptkey
cryptsetup -v -c aes-xts-plain64 -h sha512 -s 512 luksFormat /dev/sdc /etc/cryptkey

Backup LUKS header:

cryptsetup luksHeaderBackup --header-backup-file ~/sdb.header.bak /dev/sdb
cryptsetup luksHeaderBackup --header-backup-file ~/sdc.header.bak /dev/sdc

Automatically unlock LUKS devices on boot by editing /etc/crypttab:

data1 UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx /etc/cryptkey luks,noearly #,discard (for SSDs)
data2 UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx /etc/cryptkey luks,noearly #,discard (for SSDs)
# Use 'blkid /dev/sdb' to get the UUID

Unlock encrypted devices now to create the filesystem in next step:

cryptsetup open --key-file=/etc/cryptkey --type luks /dev/sdb data1
cryptsetup open --key-file=/etc/cryptkey --type luks /dev/sdc data2

Create filesystem:

mkfs.btrfs -m raid1 -d raid1 /dev/mapper/data1 /dev/mapper/data2

Mount filesystem:

mount -t btrfs -o defaults,noatime,compress=zstd /dev/mapper/data1 /mnt/data

Automatically mount btrfs filesystem on boot by editing /etc/fstab:

UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx /mnt/data btrfs defaults,noatime,compress=zstd 0 2
# Add option 'autodefrag' to allow automatic defragmentation: useful for files with lot of random writes like databases or virtual machine images
# Use 'blkid /dev/mapper/data1' to get the UUID of the Btrfs partition, it is common for all raid devices

Maintenance

In a btrfs raid setup it is necessary to frequently run a btrfs scrub to check for corrupted blocks/flipped bits and repair them using a healthy copy from one of the mirror disks.

In the example below a systemd-timer is used to run an automatic btrfs scrub job each month.

/etc/systemd/system/btrfs-scrub.timer:

[Unit]
Description=Monthly scrub btrfs filesystem, verify block checksums
Documentation=man:btrfs-scrub

[Timer]
# first saturday each month
OnCalendar=Sat *-*-1..7 3:00:00
RandomizedDelaySec=10min

[Install]
WantedBy=timers.target

/etc/systemd/system/btrfs-scrub.service:

[Unit]
Description=Scrub btrfs filesystem, verify block checksums
Documentation=man:btrfs-scrub

[Service]
Type=simple
ExecStart=/bin/btrfs scrub start -Bd /mnt/data
KillSignal=SIGINT
IOSchedulingClass=idle
CPUSchedulingPolicy=idle

Recovery from device failure

Example with one failed device:

• /dev/mapper/data1 working device
• /dev/mapper/data2 failed device
• /dev/mapper/data3 new device
• /mnt/data mountpoint

In case of failing/failed device, mount in degraded mode with one of the working devices:

mount -t btrfs -o defaults,noatime,compress=zstd,degraded /dev/mapper/data1 /mnt/data

Find the the device ID of the missing disk by executing btrfs device usage /mnt/data:

# btrfs device usage /mnt/data

/dev/mapper/data1, ID: 1
   Device size:             7.28TiB
   Device slack:              0.00B
   Data,RAID1:              5.46TiB
   Metadata,RAID1:          7.00GiB
   System,RAID1:           32.00MiB
   Unallocated:             1.81TiB

missing, ID: 2
   Device size:             7.28TiB
   Device slack:              0.00B
   Data,RAID1:              5.46TiB
   Metadata,RAID1:          7.00GiB
   System,RAID1:           32.00MiB
   Unallocated:             1.81TiB

NOTE: Encrypt the new device before using it in the btrfs raid by following the steps above.

Start the replace operation in background by adding the new device to the btrfs raid using the device ID of the missing disk:

btrfs replace start 2 /dev/mapper/data3 /mnt/data

(Optional) Check the replace progress:

btrfs replace status /mnt/data

Once the replace operation has finished, the fstab entry is left unmodified:

UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx /mnt/data btrfs defaults,noatime,compress=zstd 0 2
# Use 'blkid /dev/mapper/data1' to get the UUID of the Btrfs partition, it is common for all raid devices