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 -b 512 localhost /dev/nbd0
           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.

       nbd-client -d /dev/nbd0
           which detaches /dev/nbd0, and:

       nbd-client -c /dev/nbd0
           which queries whether /dev/nbd0 is attached or not.

       modprobe nbd
           which may be needed 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 -b 512 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 -b 512 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 -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

   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 -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.

   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 -b 512 localhost /dev/nbd0

       You can then try running tests such as:

        badblocks /dev/nbd0

SEE ALSO

       nbdkit(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) 2013-2018 Red Hat Inc.

LICENSE

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