NAME
nbdkit-loop - use nbdkit with the Linux kernel client to create loop devices and loop mounts
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
nbdkit(1) itself, the main commands you will use are:
nbd-client localhost /dev/nbd0
Attaches a locally running nbdkit instance to the kernel device /dev/nbd0.
nbd-client -unix /tmp/socket /dev/nbd0
Alternative method using a Unix domain socket instead of a public TCP/IP socket. Use "nbdkit -U /tmp/socket" to serve.
nbd-client -d /dev/nbd0
Detaches /dev/nbd0.
nbd-client -c /dev/nbd0
Queries whether /dev/nbd0 is attached or not.
modprobe nbd
You may be need to run this command once to load the nbd client kernel module.
The nbd-client(8) and modprobe(8) commands must be run as root.
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
libguestfs(3) instead since it safely isolates
untrusted filesystems from the host.
Loop mount a
filesystem from a compressed file
If you have a filesystem or disk image in xz-compressed
format then you can use nbdkit-xz-filter(1) and
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
Loop mount a
filesystem from a web server
You can use 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 --filter=xz if the remote image is XZ-compressed.
Create a
giant btrfs filesystem
nbdkit is useful for testing the limits of Linux
filesystems. Using nbdkit-memory-plugin(1) you can
create virtual disks stored in RAM with a virtual size up to
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
Inject
errors into Linux devices
Using 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 /tmp/inject, and stop injecting errors by removing this file.
Write Linux
block devices in shell script
Using 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
SEE ALSO
nbdkit(1), nbdkit-client(1), nbdkit-plugin(3), loop(4), losetup(8), mount(8), nbdfuse(1), nbd-client(8), modprobe(8), libguestfs(3), http://libguestfs.org.
AUTHORS
Richard W.M. Jones
COPYRIGHT
Copyright Red Hat
LICENSE
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