The MHDDFS package (known as the Multi-hdd FUSE filesystem) provides a method of joining a number of filesystems, of varying size, to appear as one large volume. The benefit of using this over something like lvm, is that if one of the hard drives dies, you only loose the data on that specific hard drive. The remaining drives will remain usable.
Mhddfs will automatically select which disk to write files to, based on the free space on the source drives. It will select the drive with the most free space.
Install mhddfs if its not already installed
$ sudo apt-get install -y mhddfs
Mount your source drives somewhere
(only needed if they are not mounted yet)
$ sudo mount /dev/sdb1 /mnt/sdb1 $ sudo mount /dev/sdc1 /mnt/sdc1 $ sudo mount /dev/sdd1 /mnt/sdd1
Show the mounts you want to combine
In my example, /dev/sdb1 is a 256mb drive, /dev/sdc1 is a 512mb drive, and /dev/sdd1 is a 1gb drive.
$ df -h Filesystem Size Used Avail Use% Mounted on <...irrelevant lines removed...> /dev/sdb1 243M 2.1M 225M 1% /mnt/sdb1 /dev/sdc1 487M 2.3M 456M 1% /mnt/sdc1 /dev/sdd1 991M 2.6M 922M 1% /mnt/sdd1
Note that sdd1 has the most free space out of the 3.
Create a folder to use as the mount point for mhddfs
$ sudo mkdir /mnt/storage
Run mhddfs and create the virtual mount point.
Concatenate your mount points, separated by commas.
$ sudo mhddfs /mnt/sdb1,/mnt/sdc1,/mnt/sdd1 /mnt/storage -o allow_other mhddfs: directory '/mnt/sdb1' added to list mhddfs: directory '/mnt/sdc1' added to list mhddfs: directory '/mnt/sdd1' added to list mhddfs: mount to: /mnt/storage mhddfs: move size limit 4294967296 bytes
You should now have the 3 mount points for the hdd’s mounted using mhddfs under /mnt/storage, and showing the total storage from the 3 mount points.
$ df -h Filesystem Size Used Avail Use% Mounted on <...irrelevant lines removed...> /dev/sdb1 243M 2.1M 225M 1% /mnt/sdb1 /dev/sdc1 487M 2.3M 456M 1% /mnt/sdc1 /dev/sdd1 991M 2.6M 922M 1% /mnt/sdd1 /mnt/sdb1;/mnt/sdc1;/mnt/sdd1 1.7G 6.8M 1.6G 1% /mnt/storage
Testing the new mhddfs virtual drive
To test the operation, create some files in the individual underlying mount points (sdb1, sdc1, and sdd1):
Create a file under sdb1
$ sudo touch /mnt/sdb1/file1
Show whats in the sdb1 mount point
$ ls -l /mnt/sdb1/ total 12 -rw-r--r-- 1 root root 0 Jun 4 23:16 file1 drwx------ 2 root root 12288 Jun 4 22:57 lost+found
Create a file under sdc1
$ sudo touch /mnt/sdc1/file2
Show whats in the sdc1 mount point
$ ls -l /mnt/sdc1/ total 12 -rw-r--r-- 1 root root 0 Jun 4 23:16 file2 drwx------ 2 root root 12288 Jun 4 23:04 lost+found
Create a file under sdd1
$ sudo touch /mnt/sdd1/file3
Show whats in the sdd1 mount point
$ ls -l /mnt/sdd1/ total 16 -rw-r--r-- 1 root root 0 Jun 4 23:16 file3 drwx------ 2 root root 16384 Jun 4 22:59 lost+found
If we look at /mnt/storage now, it should show the 3 files combined under the one mount point (and the lost+found folder).
$ ls -l /mnt/storage/ total 12 -rw-r--r-- 1 root root 0 Jun 4 23:16 file1 -rw-r--r-- 1 root root 0 Jun 4 23:16 file2 -rw-r--r-- 1 root root 0 Jun 4 23:16 file3 drwx------ 2 root root 12288 Jun 4 22:57 lost+found
If we create a file under /mnt/storage, mhddfs will work out which drive to store it on.
$ sudo touch /mnt/storage/file4
Show whats in /mnt/storage now
$ ls -l /mnt/storage/ total 12 -rw-r--r-- 1 root root 0 Jun 4 23:16 file1 -rw-r--r-- 1 root root 0 Jun 4 23:16 file2 -rw-r--r-- 1 root root 0 Jun 4 23:16 file3 -rw-r--r-- 1 root root 0 Jun 4 23:27 file4 drwx------ 2 root root 12288 Jun 4 22:57 lost+found
Now, if we look in all the individual drives, we can see it has saved ‘file4’ to the sdd1 partition.
Source mount point directory listing
$ ls -l /mnt/sd* /mnt/sdb1: total 12 -rw-r--r-- 1 root root 0 Jun 4 23:16 file1 drwx------ 2 root root 12288 Jun 4 22:57 lost+found /mnt/sdc1: total 12 -rw-r--r-- 1 root root 0 Jun 4 23:16 file2 drwx------ 2 root root 12288 Jun 4 23:04 lost+found /mnt/sdd1: total 16 -rw-r--r-- 1 root root 0 Jun 4 23:16 file3 -rw-r--r-- 1 root root 0 Jun 4 23:27 file4 drwx------ 2 root root 16384 Jun 4 22:59 lost+found
As you can see, the file has been stored under /mnt/sdd1 automatically. It determines this based on the drive that has the most free space. As noted earlier, that was /mnt/sdd1.
