In most of the situations you would simply create a loopback device using “losetup” and mount it using the “-o loopback” options. But if you want to create a loopback file, want to partition it, and finally mount a subpartition, this option can not be used. Let see how you can create partitions inside a loopback image.
Creating loopback device
1. First create a file of size around 1GB using “dd” command.
# dd if=/dev/zero of=loopbackfile.img bs=100M count=10 10+0 records in 10+0 records out 1048576000 bytes (1.0 GB) copied, 1.26748 s, 827 MB/s
2. Create a loopback device on top of the file created in the above step.
# losetup -fP loopbackfile.img
3. To print the loop device generated using the above command use "losetup -a".
# losetup -a /dev/loop0: [64769]:4199216 (/root/loopbackfile.img)
Creating partitions inside loopback image using fdisk
1. Use the fdisk command to create partitions on the loopback device /dev/loop0. A primary parition of size 500MB is created as shown below.
# fdisk /dev/loop0
Welcome to fdisk (util-linux 2.23.2).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Device does not contain a recognized partition table
Building a new DOS disklabel with disk identifier 0x4d455ea1.
Command (m for help): n
Partition type:
p primary (0 primary, 0 extended, 4 free)
e extended
Select (default p): p
Partition number (1-4, default 1): 1
First sector (2048-2047999, default 2048):
Using default value 2048
Last sector, +sectors or +size{K,M,G} (2048-2047999, default 2047999): +500M
Partition 1 of type Linux and of size 500 MiB is set
2. Save the partition table and exit out of the fdisk utility.
Command (m for help): w The partition table has been altered! Calling ioctl() to re-read partition table. Syncing disks.
Creating filesystem and mounting it
1. Create the ext4 filesystem on the /dev/loop0p1 partition created in the above step.
# mkfs.ext4 /dev/loop0p1 mke2fs 1.42.9 (28-Dec-2013) Discarding device blocks: done Filesystem label= OS type: Linux Block size=1024 (log=0) Fragment size=1024 (log=0) Stride=0 blocks, Stripe width=0 blocks 128016 inodes, 512000 blocks 25600 blocks (5.00%) reserved for the super user First data block=1 Maximum filesystem blocks=34078720 63 block groups 8192 blocks per group, 8192 fragments per group 2032 inodes per group Superblock backups stored on blocks: 8193, 24577, 40961, 57345, 73729, 204801, 221185, 401409 Allocating group tables: done Writing inode tables: done Creating journal (8192 blocks): done Writing superblocks and filesystem accounting information: done
2. Mount the filesystem on the desired directory.
# mkdir /loopfs # mount -o loop /dev/loop0p1 /loopfs
3. Verify the size of mount point and the filesystem type.
# df -hP /loopfs Filesystem Size Used Avail Use% Mounted on /dev/loop1 477M 2.3M 445M 1% /loopfs
# mount | grep loopfs /dev/loop0p1 on /loopfs type ext4 (rw,relatime,seclabel,data=ordered)
