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=head1 NAME
nbdkit-loop - use nbdkit with the Linux kernel client to create loop
devices and loop mounts
=head1 DESCRIPTION
nbdkit (server) can be used with the Linux kernel nbd (client) in a
loop mode allowing any of the plugins supported by nbdkit to be turned
into Linux block devices.
In addition to L<nbdkit(1)> itself, the main commands you will use
are:
=over 4
=item nbd-client localhost /dev/nbd0
Attaches a locally running nbdkit instance to the kernel device
F</dev/nbd0>.
=item nbd-client -unix /tmp/socket /dev/nbd0
Alternative method using a Unix domain socket instead of a public
TCP/IP socket. Use C<nbdkit -U /tmp/socket> to serve.
=item nbd-client -d /dev/nbd0
Detaches F</dev/nbd0>.
=item nbd-client -c /dev/nbd0
Queries whether F</dev/nbd0> is attached or not.
=item modprobe nbd
You may be need to run this command once to load the nbd client kernel
module.
=back
The L<nbd-client(8)> and L<modprobe(8)> commands must be run as root.
=head2 Warning: Do not loop mount untrusted filesystems
Untrusted filesystems and untrusted disk images should not be loop
mounted because they could contain exploits that attack your host
kernel. Use the tools from L<libguestfs(3)> instead since it safely
isolates untrusted filesystems from the host.
=head2 Loop mount a filesystem from a compressed file
If you have a filesystem or disk image in xz-compressed format then
you can use L<nbdkit-xz-filter(1)> and L<nbdkit-file-plugin(1)> to
loop mount it as follows:
nbdkit --filter=xz file disk.xz
nbd-client localhost /dev/nbd0
mount /dev/nbd0p1 /mnt
=head2 Loop mount a filesystem from a web server
You can use L<nbdkit-curl-plugin(1)> to loop mount a filesystem from a
disk image on a web server:
nbdkit [--filter=xz] curl https://example.com/disk.img
nbd-client localhost /dev/nbd0
mount /dev/nbd0p1 /mnt
Use I<--filter=xz> if the remote image is XZ-compressed.
=head2 Create a giant btrfs filesystem
nbdkit is useful for testing the limits of Linux filesystems. Using
L<nbdkit-memory-plugin(1)> you can create virtual disks stored in RAM
with a virtual size up to S<2⁶³-1 bytes>, and then create filesystems
on these:
nbdkit memory $(( 2**63 - 1 ))
nbd-client localhost /dev/nbd0
Partition the device using GPT, creating a single partition with all
default settings:
gdisk /dev/nbd0
Make a btrfs filesystem on the disk and mount it:
mkfs.btrfs -K /dev/nbd0p1
mount /dev/nbd0p1 /mnt
=head2 Inject errors into Linux devices
Using L<nbdkit-error-filter(1)> you can see how Linux devices react to
errors:
nbdkit --filter=error \
memory 64M \
error-rate=100% error-file=/tmp/inject
nbd-client localhost /dev/nbd0
mkfs -t ext4 /dev/nbd0
mount /dev/nbd0 /mnt
Inject errors by touching F</tmp/inject>, and stop injecting errors by
removing this file.
=head2 Write Linux block devices in shell script
Using L<nbdkit-sh-plugin(3)> you can write custom Linux block devices
in shell script for testing. For example the following shell script
creates a disk which contains a bad sector:
#!/bin/bash -
case "$1" in
thread_model) echo parallel ;;
get_size) echo 64M ;;
pread)
if [ $4 -le 100000 ] && [ $(( $4+$3 )) -gt 100000 ]; then
echo EIO Bad block >&2
exit 1
else
dd if=/dev/zero count=$3 iflag=count_bytes
fi ;;
*) exit 2 ;;
esac
Create a loop from this shell script using:
nbdkit sh ./bad-sector.sh
nbd-client localhost /dev/nbd0
You can then try running tests such as:
badblocks /dev/nbd0
=head1 SEE ALSO
L<nbdkit(1)>,
L<nbdkit-client(1)>,
L<nbdkit-plugin(3)>,
L<loop(4)>,
L<losetup(8)>,
L<mount(8)>,
L<nbdfuse(1)>,
L<nbd-client(8)>,
L<modprobe(8)>,
L<libguestfs(3)>, L<http://libguestfs.org>.
=head1 AUTHORS
Richard W.M. Jones
=head1 COPYRIGHT
Copyright Red Hat
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