nbdkit-loop(1)

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 (C) 2013-2020 Red Hat Inc.

LICENSE
Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

•   Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

•   Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

•   Neither the name of Red Hat nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

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