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:
Attaches a locally running nbdkit instance to the kernel device /dev/nbd0. -b 512 can be omitted when using nbd-client(8) ≥ 3.19.
Alternative method using a Unix domain socket instead of a public TCP/IP socket. Use
nbdkit -U /tmp/socket to serve.
Queries whether /dev/nbd0 is attached or not.
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.
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:
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 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 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:
nbdkit(1), nbdkit-plugin(3), loop(4), losetup(8), mount(8), nbdfuse(1), nbd-client(8), modprobe(8), libguestfs(3), http://libguestfs.org.
Richard W.M. Jones
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