DOKK / manpages / debian 10 / nbdkit / nbdkit-loop.1.en
nbdkit-loop(1) NBDKIT nbdkit-loop(1)

nbdkit-loop - use nbdkit with the Linux kernel client to create loop devices and loop mounts

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:

which attaches a locally running nbdkit instance to the kernel device /dev/nbd0. -b 512 can be omitted when using nbd-client(8) ≥ 3.19.
which detaches /dev/nbd0, and:
which queries whether /dev/nbd0 is attached or not.
which may be needed to load the nbd client kernel module.

The nbd-client(8) and modprobe(8) commands must be run as root.

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.

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 -b 512 localhost /dev/nbd0
 mount /dev/nbd0p1 /mnt

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 -b 512 localhost /dev/nbd0
 mount /dev/nbd0p1 /mnt

Use --filter=xz if the remote image is XZ-compressed.

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 -b 512 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

Using nbdkit-error-filter(1) you can see how Linux devices react to errors:

 nbdkit --filter=error \
        memory size=64M \
        error-rate=100% error-file=/tmp/inject
 nbd-client -b 512 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.

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
     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 -b 512 localhost /dev/nbd0

You can then try running tests such as:

 badblocks /dev/nbd0

nbdkit(1), nbdkit-plugin(3), loop(4), losetup(8), mount(8), nbd-client(8), modprobe(8), libguestfs(3), http://libguestfs.org.

Richard W.M. Jones

Copyright (C) 2013-2018 Red Hat Inc.

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2019-01-26 nbdkit-1.10.3