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: 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 -b 512 -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 -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

AUTHORS

Richard W.M. Jones

COPYRIGHT

LICENSE

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