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I want to do a few experiments on the btrfs file system, but I don't want to make any changes to my existing partitions, and I want full control over things like device size.
Is it possible to create a file that looks like a block device that I can mount and unmount, and that will act like a block device such as running out of space?
The loop device is what you need for this. Run these commands as root:
truncate -s1G 1GB.img # Sparse allocation of a 1GB file
ld=$(losetup --show --find 1GB.img); echo "$ld"
You will now have a loop device (for example, /dev/loop0) that you can treat as a block device.
mkfs -t btrfs "$ld" # Device that was returned from losetup
mkdir -p /mnt/dsk
mount "$ld" /mnt/dsk
When you've finished, tidy up again
umount /mnt/dsk
losetup -d "$ld"
rm 1GB.img
If you want to create a partition table on the block device, make sure you always include the --partscan flag on the losetup command. This will create the associated devices, for example, /dev/loop0p1.
truncate -s1G 1G.img also works (and takes no space on disk initially).
May 30, 2017 at 21:25
mount file.img /mnt/point and mount will create the loop device by itself.
May 30, 2017 at 21:27
truncate is a much better option for a filesystem image, thank you. (For throwaway scripts I too often forget about truncate.) Re mount, the question asked for a block device, so I offered the result from losetup.