I previously used to create image files using dd, set up a filesystem on them using mkfsand mount them to access them as mounted partitions. Later on, I have seen on the internet that many examples use losetup beforehand to make a loop device entry under /dev, and then mount it. I could not tell why one would practically need an image file to behave as a loop device and have its own /dev entry while the same behaviour can be obtained without all the hassle.
Summary: In a real-life scenario, why do we need a /dev/loopXentry to be present at all, when we can just mount the fs image without it? What’s the use of a loop device?
Answers:
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Method 1
Mounts, typically, must be done on block devices. The loop driver puts a block device front-end onto your data file.
If you do a loop mount without losetup then the OS does one in the background.
eg
$ dd if=/dev/zero of=/tmp/foo bs=1M count=100 100+0 records in 100+0 records out 104857600 bytes (105 MB) copied, 0.0798775 s, 1.3 GB/s $ mke2fs /tmp/foo mke2fs 1.42.9 (28-Dec-2013) .... $ losetup $ mount -o loop /tmp/foo /mnt1 $ losetup NAME SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE /dev/loop0 0 0 1 0 /tmp/foo $ umount /mnt1 $ losetup $
You may need to call losetup directly if your file image has embedded partitions in it.
eg if I have this image:
$ fdisk -l /tmp/foo2
Disk /tmp/foo2: 104 MB, 104857600 bytes, 204800 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x1f25ff39
Device Boot Start End Blocks Id System
/tmp/foo2p1 2048 204799 101376 83 Linux
I can’t mount that directly
$ mount -o loop /tmp/foo2 /mnt1
mount: /dev/loop0 is write-protected, mounting read-only
mount: wrong fs type, bad option, bad superblock on /dev/loop0,
missing codepage or helper program, or other error
But if I use losetup and kpartx then I can access the partitions:
$ losetup -f /tmp/foo2 $ losetup NAME SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE /dev/loop0 0 0 0 0 /tmp/foo2 $ kpartx -a /dev/loop0 $ mount /dev/mapper/loop0p1 /mnt1 $
Method 2
File systems expect to read from and write to block devices, but image files aren’t block devices. Loop devices provide a block device on top of a file (or another block device, optionally with remapping).
There’s no need to consider loop devices when mounting images in many cases because mount takes care of everything for you; but loop devices are still involved. losetup -l -a will show them.
See also What is the difference between mount and mount -o loop.
Method 3
You seem to be on Linux and Linux uses a wrong name for that feature.
I invented that feature in 1988 on SunOS-4.0 and I call that feature fbk – File emulates BlocK device.
The background is that the device driver emulates a block device on top of a plain file. You need this as a filesystem cannot use a plain file as a background storage for a filesystem. It rather needs a block device and this is what fbk emulates.
Since a while some people made the program mount a bit more clever and there are mount implementations that automatically create a fbk instance for a file in case that the mount program detects that the argument that is expected to be a block device appears to be a plan file instead.
Method 4
Even if it was not needed in the background for mounting filesystems from files, you would still need it for any setup using a driver or program that absolutely expects a block device. Think nbd (network block device) servers, compound block device drivers like mdraid, lvm etc….
All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0