How to create loop partition from already existing partition - linux

Question

I believe /images/backups is using the space in /images ?
/dev/sdb1 820G 645G 135G 83% /images
/dev/loop0 296G 296G 0 100% /images/backups
I've a similar kind of partition in another machine /images which is 500G free, and I would want to take out 350G for /images/backups, how to do it ?
Is it right that, it is a simple loop mount which can give specified amount of space or we should create a NULL file of required size and mount ? If so, what are the mount options should be used to specify the size ?

Answer 1

You'll need to create the destination with a fixed size, but can use a "sparse file" which doesn't actually have any blocks written to it yet (and which thus doesn't actually consume space until you write to it).
For instance:
dd if=/dev/zero of=file.img bs=1 count=0 seek=20G
will create a sparse file preallocated to 20GB. That said, actually writing 20GB of zeros to disk up-front (making the file non-sparse) will be faster on writes and lead to less fragmentation.
This can be attached to a loopback device with the losetup command, have a filesystem created, and be mounted:
losetup /dev/loop1 file.img
mke2fs -j /dev/loop1
mount /dev/loop1 /mnt/somewhere
If you want to know if an existing file is sparse, the following will do the trick (on a system with GNU tools; some of the below is not supported in a pure POSIX environment):
{
read block_count block_size file_size
if (( block_count * block_size < file_size )) ; then
echo "Sparse"
else
echo "Non-Sparse"
fi
} < <(stat --format='%b %B %s'$'\n' /images/backups.img)