Union filesystem (FUSE) like unionfs, aufs and mhddfs. Merge multiple paths and mount them, similar to concatenating.

Get it here: https://github.com/trapexit/mergerfs or from OS package repository.

Compared to (older) alternatives mergerfs seems very stable over the past months I’ve been using it. It offers multiple options on how to spread the data over the used drives.

Optionally SnapRAID can be used to add parity disk(s) to protect against disk failures (https://www.snapraid.it).

Create/mount pool

Example using 5 devices /dev/sd[b-f]

Disks are already partitioned have a fs

for i in {b..f}; do
  mkdir /mnt/sd${i}1
  mount /dev/sd${i}1 /mnt/sd${i}1 && \
  mkdir /mnt/sd${i}1/mfs
done && \
mkdir /mnt/mergerfs && \
mergerfs -o defaults,allow_other,use_ino /mnt/sd*/mfs /mnt/mergerfs

And here’s the result from ‘df’:

/dev/mapper/sdb1             3.6T  100M  3.5T  1% /mnt/sdb1
/dev/mapper/sdc1             3.6T  100M  3.5T  1% /mnt/sdc1
/dev/mapper/sdd1             3.6T  100M  3.5T  1% /mnt/sdd1
/dev/mapper/sde1             3.6T  100M  3.5T  1% /mnt/sde1
/dev/mapper/sdf1             3.6T  100M  3.5T  1% /mnt/sdf1
mergerfs                      18T  500M  8.5T  1% /mnt/mergerfs

Changing pool

remove old drive from mergerfs pool

xattr -w user.mergerfs.srcmounts -/mnt/data1 /mnt/pool/.mergerfs

add new drive

xattr -w user.mergerfs.srcmounts +/mnt/data4 /mnt/pool/.mergerfs

some other mount options (-o)

• use_ino make mergerfs supply inodes
• fsname=example-name name in df
• no_splice_write fixes page errors in syslog

https://github.com/trapexit/mergerfs#mount-options

Pool info

xattr -l /mnt/mergerfs/.mergerfs
# or:
mergerfs.ctl -m /mnt/mergerfs list values

mergerfs.ctl -m /mnt/mergerfs info