How To Join Several Partition Together To Form a Single Larger One On a Linux Using mhddfs

The mhddfs (fuse) file system allows to unite a several mount points (or directories) to the single one. So a one big filesystem is simulated and this makes it possible to combine a several hard drives or network file systems. This system is like unionfs but it can choose a drive with the most of free space, and move the data between drives transparently for the applications. While writing files they are written to a 1st hdd until the hdd has the free space (see mlimit option), then they are written on a 2nd hdd, then to 3rd etc. df will show a total statistics of all filesystems like there is a big one hdd. If an overflow arises while writing to the hdd1 then a file content already written will be transferred to a hdd containing enough of free space for a file. The transferring is processed on-the-fly, fully transparent for the application that is writing. So this behaviour simulates a big file system.

In this tutorial you will learn how to install and configure MHDDFS virtual storage pool on a Linux operating systems.

Our sample setup

For demo purpose I’ve three hard disks drive /dev/sdb1, /dev/sdc1, and /dev/sdd1 as follows:
# df
Sample outputs:

Fig.01 My 3 hard drives are mounted at /disk1,/disk2 and /disk3

/disk1:
total 28
drwxr-xr-x 2 root root  4096 Aug  8 14:25 app1
drwx------ 2 root root 16384 Aug  8 14:20 lost+found
-rw-r--r-- 1 root root  7545 Aug  8 14:26 resume.txt
 
/disk2:
total 28
drwxr-xr-x 2 root root  4096 Aug  8 14:25 app2
drwx------ 2 root root 16384 Aug  8 14:20 lost+found
-rw-r--r-- 1 root root  6303 Aug  8 14:26 party.jpg
 
/disk3:
total 40
drwxr-xr-x 2 root root  4096 Aug  8 14:25 app3
drwx------ 2 root root 16384 Aug  8 14:21 lost+found
-rw-r--r-- 1 root root 17080 Aug  8 14:26 output.log

Installation

Let us see how to install mhddfs package on different Linux distros.

Install mhddfs package on a Debian/Ubuntu/Mint Linux & Co

Type the following apt-get command to install mhddfs:
# apt-get install mhddfs
Sample outputs:

Fig.02: Install mhddfs package

Install mhddfs package on a Fedora/RHEL/CentOS Linux & Co

Turn on EPEL repo and type the following command:
# yum install mhddfs
Fedora Linux v22.x+ user type the following command:
# dnf install mhddfs
Sample outputs:

Fig.03: Install mhddfs rpm package

Configuration

First, create a new mount point directory called /virtual.data, enter:
# mkdir /virtual.data
To join all three drives (see fig.01) together, enter:
# mhddfs /disk1,/disk2,/disk3 /virtual.data -o allow_other
Sample outputs:

Fig.04: mhddfs in action

Filesystem           1K-blocks    Used Available Use% Mounted on
/dev/sda1             39428520 6109320  31293268  17% /
udev                     10240       0     10240   0% /dev
tmpfs                   811792    9084    802708   2% /run
tmpfs                  2029472     144   2029328   1% /dev/shm
tmpfs                     5120       4      5116   1% /run/lock
tmpfs                  2029472       0   2029472   0% /sys/fs/cgroup
tmpfs                   405896      16    405880   1% /run/user/1000
/dev/sdb1              4061888    8196   3827644   1% /disk1
/dev/sdc1              4061888    8196   3827644   1% /disk2
/dev/sdd1              4061888    8208   3827632   1% /disk3
/disk1;/disk2;/disk3  12185664   24600  11482920   1% /virtual.data

Also, note down ls -l command output:

# ls -l /virtual.data/
total 64
drwxr-xr-x 2 root root  4096 Aug  8 14:25 app1
drwxr-xr-x 2 root root  4096 Aug  8 14:25 app2
drwxr-xr-x 2 root root  4096 Aug  8 14:25 app3
drwx------ 2 root root 16384 Aug  8 14:20 lost+found
-rw-r--r-- 1 root root 17080 Aug  8 14:26 output.log
-rw-r--r-- 1 root root  6303 Aug  8 14:26 party.jpg
-rw-r--r-- 1 root root  7545 Aug  8 14:26 resume.txt

You can now copy files or create new directories as per your requirement:
# cd /virtual.data/
# mkdir Music
# rsync -avp /somewhere/* Music/
....
..

Update /etc/fstab

Update /etc/fstab file as follows:

mhddfs#/disk1,/disk2,disk3 /virtual.data/ fuse defaults,allow_other 0 0

How do I unmount mhddfs based fuse file systems

Use the umount command to detaches the /virtual.data/ file system:
# umount /virtual.data/

For more info type see mhddfs man page.