Create RAID array of image files

Question

OK, so I've got three image files in /home/, and they each reside on different physical drives:

image1.img

image2.img

image3.img

Each image is the same size, and I want to put the images themselves into RAID0.

How should I go about RAIDing them?

EDIT: Using mdadm I get this error:

EDIT: Output of cat /proc/mdstat

EDIT: Output of sudo gparted /dev/md0

Answer 1

To install the Linux software RAID you need to install the mdadm package.

sudo apt-get install mdadm

If you want to make a software RAID-0 from the three image files, you need to create loop devices for each image files:

sudo losetup /dev/loop1 image1.img
sudo losetup /dev/loop2 image2.img
sudo losetup /dev/loop3 image3.img

After you can create a RAID-0 array from them:

sudo mdadm --create /dev/md0 --level=0 --raid-devices=3 /dev/loop1 /dev/loop2 /dev/loop3

Answer 2

If your system matches the basic requirements you could use zfs (8 GB RAM, 64-bit system):

Add repo and update package list:

sudo add-apt-repository ppa:zfs-native/stable
sudo apt-get update

Install package:

sudo apt-get install ubuntu-zfs

Create a striped vdev (with no redundancy, but you asked for RAID0):

sudo zpool create vol0 ~/image[1-3].img

This creates the stripe and mounts it at /vol0.

sudo zfs create vol0/filesystem

This creates a zfs file system on the stripe and mounts it at /vol0/filesystem. Use

sudo zfs set mountpoint=/mnt/filesystem vol0/filesystem

if you want to change the mount point.

You can also add automatic compression:

sudo zfs create vol0/filesystem/compressed
sudo zfs set compression=on vol0/filesystem/compressed

Now everything you put into /mnt/filesystem/compressed will automatically be compressed.

Answer 3

I literally just did touch image#.img for each one. Is there something else I should do?

This is indeed the problem. The files are 0 bytes, and you cannot seek inside them. You have to make files that actually have a size. It can be a a sparse file if you want:

[/tmp]$ dd if=/dev/zero of=1.img bs=1M count=25 conv=sparse
25+0 records in
25+0 records out
26214400 bytes (26 MB, 25 MiB) copied, 0.00334608 s, 7.8 GB/s
[/tmp]$ dd if=/dev/zero of=2.img bs=1M count=25 conv=sparse
25+0 records in
25+0 records out
26214400 bytes (26 MB, 25 MiB) copied, 0.00347987 s, 7.5 GB/s
[/tmp]$ dd if=/dev/zero of=3.img bs=1M count=25 conv=sparse
25+0 records in
25+0 records out
26214400 bytes (26 MB, 25 MiB) copied, 0.00330069 s, 7.9 GB/s
[/tmp]$ du  1.img 
0       1.img

Now I can make loopdevices and assemble array:

[/tmp]$ sudo losetup loop0 1.img 
[/tmp]$ sudo losetup loop1 2.img 
[/tmp]$ sudo losetup loop2 3.img 
[/tmp]$ sudo mdadm --create /dev/md0 --level=0 --raid-devices=3 /dev/loop0 /dev/loop1 /dev/loop2
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.
[/tmp]$ sudo mkfs.ext4 /dev/md0 
mke2fs 1.42.11 (09-Jul-2014)
Discarding device blocks: done                            
Creating filesystem with 73728 1k blocks and 18432 inodes
Filesystem UUID: 8123197c-a9aa-434d-9233-103fe20727ed
Superblock backups stored on blocks: 
        8193, 24577, 40961, 57345

Allocating group tables: done                            
Writing inode tables: done                            
Creating journal (4096 blocks): done
Writing superblocks and filesystem accounting information: done
[/tmp]$ mkdir mnt/
[/tmp]$ sudo mount /dev/md0 mnt/
[/tmp]$ df -h mnt/
Filesystem      Size  Used Avail Use% Mounted on
/dev/md0         66M  1.3M   60M   3% /tmp/mnt
[/tmp]$ cat /proc/mdstat 
Personalities : [raid0] 
md0 : active raid0 loop2[2] loop1[1] loop0[0]
      73728 blocks super 1.2 512k chunks

unused devices: <none>
[/tmp]$ du -h 1.img 2.img 3.img 
1.3M    1.img
1.6M    2.img
1.6M    3.img

Sparse files will grow up to the maximum size, as data is written to them. They will not shrink if files inside the filesystem is deleted.