NAME¶
losetup - set up and control loop devices
SYNOPSIS¶
Set up a loop device:
losetup [options] -f|loopdev file
Get info:
losetup [-l] [-a|loopdev]
losetup -j file [-o offset]
Recalibrate the size of a loop device:
losetup -c loopdev
Detach loop devices:
losetup -d loopdev ...
losetup -D
Prevent loop devices from getting listed:
losetup --remove loopdev ...
DESCRIPTION¶
losetup is used to associate loop devices with regular
files or block devices, to detach loop devices, and to query the status of a
loop device. If only the loopdev argument is given, the status of the
corresponding loop device is shown. If no option is given, all loop devices
are shown.
Note that the old output format (i.e., losetup -a) with
comma-delimited strings is deprecated in favour of the --list output
format.
It is possible to create multiple independent loop devices for the
same backing file. This setup may be dangerous, can cause data loss,
corruption, and overwrites. Use --nooverlap with --find
during setup to avoid this problem.
The loop device setup is not an atomic operation when used with
--find, and losetup does not protect this operation by any
lock. The number of attempts is internally restricted to a maximum of 16. It
is recommended to use for example flock(1) to avoid a collision in
heavily parallel use cases.
OPTIONS¶
The size and offset arguments may be followed by the
multiplicative suffixes KiB (=1024), MiB (=1024*1024), and so on for GiB,
TiB, PiB, EiB, ZiB and YiB (the "iB" is optional, e.g.,
"K" has the same meaning as "KiB") or the suffixes KB
(=1000), MB (=1000*1000), and so on for GB, TB, PB, EB, ZB and YB.
-a, --all
Show the status of all loop devices. Note that not all
information is accessible for non-root users. See also --list. The old
output format (as printed without --list) is deprecated.
-d, --detach loopdev...
Detach the file or device associated with the specified
loop device(s). Note that since Linux v3.7 kernel uses "lazy device
destruction". The detach operation does not return EBUSY error
anymore if device is actively used by system, but it is marked by autoclear
flag and destroyed later. Even if the device is not used, the loop device can
be destroyed later. If you need to wait for a complete removal of the loop
device, call udevadm settle after losetup.
-D, --detach-all
Detach all associated loop devices.
-f, --find [file]
Find the first unused loop device. If a file
argument is present, use the found device as loop device. Otherwise, just
print its name.
--show
Display the name of the assigned loop device if the
-f option and a file argument are present.
-L, --nooverlap
Check for conflicts between loop devices to avoid
situation when the same backing file is shared between more loop devices. If
the file is already used by another device then re-use the device rather than
a new one. The option makes sense only with --find.
-j, --associated file [-o
offset]
Show the status of all loop devices associated with the
given file.
-o, --offset offset
The data start is moved offset bytes into the
specified file or device. The offset may be followed by the
multiplicative suffixes; see above.
--loop-ref string
Set the reference string. The backwardly compatible
default is to use the backing filename as a reference in the loop setup ioctl
(aka lo_file_name). This option overrides this default behavior and sets the
reference to the string. The reference may be used by udevd in
/dev/loop/by-ref. The Linux kernel does not use the reference at all, but it
could be used by some old utils that cannot read the backing file from sysfs.
The reference is readable only for the root user (see --output +REF)
and it is restricted to 64 bytes.
--sizelimit size
The data end is set to no more than size bytes
after the data start. The size may be followed by the multiplicative
suffixes; see above.
-b, --sector-size size
Set the logical sector size of the loop device in bytes
(since Linux 4.14). The option may be used when creating a new loop device as
well as a stand-alone command to modify sector size of the already existing
loop device.
-c, --set-capacity loopdev
Force the loop driver to reread the size of the file
associated with the specified loop device.
-P, --partscan
Force the kernel to scan the partition table on a newly
created loop device. Note that the partition table parsing depends on sector
sizes. The default is sector size is 512 bytes, otherwise you need to use the
option --sector-size together with --partscan.
-r, --read-only
Set up a read-only loop device.
--direct-io[=on|off]
Enable or disable direct I/O for the backing file. The
default is off. Specifying either --direct-io or
--direct-io=on will enable it. But, --direct-io=off can be
provided to explicitly turn it off.
-l, --list
If a loop device or the -a option is specified,
print the default columns for either the specified loop device or all loop
devices; the default is to print info about all devices. See also
--output, --noheadings, --raw, and --json.
-O, --output column[,column]...
Specify the columns that are to be printed for the
--list output. Use --help to get a list of all supported
columns.
--output-all
Output all available columns.
-n, --noheadings
Don’t print headings for --list output
format.
--raw
Use the raw --list output format.
-J, --json
Use JSON format for --list output.
--remove loopdev...
Remove the specified loop device(s). The loop device must
be unused (not associated with any backing file). This option removes the
device node from the system. In most cases, --detach is sufficient for
normal use. See the LOOP DEVICE LIFECYCLE section below.
-h, --help
Display help text and exit.
-V, --version
Display version and exit.
LOOP DEVICE LIFECYCLE¶
Loop devices go through several states during their lifecycle:
Creation
When losetup associates a loop device with a
backing file, it automatically creates the device node (e.g.,
/dev/loop0) if it doesn’t already exist. This happens on-demand
when you use options like -f or specify a loop device explicitly. The
device is created using the LOOP_CTL_ADD ioctl.
Detachment
The -d (--detach) option disassociates the
backing file from the loop device using the LOOP_CLR_FD ioctl. After
detachment, the loop device node remains in /dev and can be reused for
another backing file. Use lsblk -a to see all loop device nodes,
including unused ones.
Removal
The --remove option completely removes the loop
device node from the system using the LOOP_CTL_REMOVE ioctl. The device
must be detached first (not associated with any backing file). After removal,
the device node disappears from /dev.
ENCRYPTION¶
Cryptoloop is no longer supported in favor of dm-crypt. For
more details see cryptsetup(8).
EXIT STATUS¶
losetup returns 0 on success, nonzero on failure. When
losetup displays the status of a loop device, it returns 1 if the
device is not configured and 2 if an error occurred which prevented
determining the status of the device.
NOTES¶
Since version 2.37 losetup uses LOOP_CONFIGURE ioctl
to set up a new loop device by one ioctl call. The old versions use
LOOP_SET_FD and LOOP_SET_STATUS64 ioctls to do the same.
ENVIRONMENT¶
LOOPDEV_DEBUG=all
enables debug output.
FILES¶
/dev/loop[0..N]
loop block devices
/dev/loop-control
loop control device
EXAMPLE¶
The following commands can be used as an example of using the loop
device.
# dd if=/dev/zero of=~/file.img bs=1024k count=10
# losetup --find --show ~/file.img
/dev/loop0
# mkfs -t ext2 /dev/loop0
# mount /dev/loop0 /mnt
...
# umount /dev/loop0
# losetup --detach /dev/loop0
AUTHORS¶
Karel Zak <kzak@redhat.com>, based on the original
version from Theodore Ts’o <tytso@athena.mit.edu>.
