FOSDEM 2019 – Better loop mounting with NBD

My talk was accepted:

If you’re coming to FOSDEM, please come and say hello. In the meantime if you want to watch a rough early run-through of the talk, see:


1 Comment

Filed under Uncategorized



Looks great, and nbdkit compiles out of the box.

Leave a comment

Filed under Uncategorized

Gigatron – chips are down

Looks like I’ll need to spend some time “debugging” all my solder joints because although all the chips are in the blinkenlights aren’t lighting up 😔


Filed under Uncategorized

Gigatron – some assembly required

I don’t know if 74 chips have got smaller since I was a kid or if I’ve just got bigger but this is a test of dexterity.

There’s a reason why although I started out my career in hardware I soon decided to stick to software!

Amazingly nothing blew up when I connected the power.


Filed under Uncategorized

nbdkit inline scripts

I have proposed a patch for nbdkit, our flexible, pluggable Network Block Device server, to make writing Linux block devices into a (long) single command.

Here’s a simple block device with virtual size 1M that reads as zeroes:

nbdkit sh - <<'EOF'
    case "$1" in
        get_size) echo 1M ;;
        pread) dd if=/dev/zero count=$3 iflag=count_bytes ;;
        *) exit 2 ;;

Leave a comment

Filed under Uncategorized

nbdkit / FOSDEM test presentation about better loop mounts for Linux

I’ve submitted a talk about nbdkit, our flexible pluggable NBD server, to FOSDEM next February. This is going to be about using NBD as a better way to do loop mounts in Linux.

In preparation I gave a very early version of the talk to a small Red Hat audience.

Video link:

Sorry about the slow start. You may want to skip to 2 mins to get past the intro.

Summary of what’s in the talk:

  1. Demo of regular, plain loop mounting.
  2. Demo of loop mounting an XZ-compressed disk image using NBD + nbdkit.
  3. Slides about how loop device compares to NBD.
  4. Slides about nbdkit plugins and filters.
  5. Using VMware VDDK to access a VMDK file.
  6. Creating a giant disk costing EUR 300 million(!)
  7. Visualizing a single filesystem.
  8. Visualizing RAID 5.
  9. Writing a plugin in shell script (live demo).
  10. Summary.



Filed under Uncategorized

nbdkit + xz + curl

I’ve submitted a talk about nbdkit, our flexible, pluggable NBD server, to FOSDEM next year about how you can use nbdkit as a replacement for loopback mounts (or “loop mounts” as I was told off for not calling them last week). In preparation for that talk I ran through it in private to a small Red Hat audience on Monday. If I can I will release that video some time, but I may have to edit out Red Hat “super-secret” stuff first (or most likely not because there aren’t any secrets in it, but I’m still waiting for the internal video to be released).

Anyway this attracted a lot of interest and one question that was asked was why the xz plugin which lets you transparently open and uncompress XZ files on the fly was a plugin at all. Surely it would make more sense for it to be a filter? So it could be used not just to uncompress local files, but also xz-compressed cloud images over HTTPS.

The answer is yes it would! So I fixed it. XZ is now a filter (the plugin is left around but we’ll deprecate it eventually).

You can use it on top of the file plugin, curl plugin or other plugins:

$ nbdkit --filter=xz file file.xz
$ nbdkit --filter=xz curl

This is fun and you can use this to boot the cloud image entirely remotely:

$ qemu-system-x86_64 -machine accel=kvm:tcg \
    -cpu host -m 2048 \
    -drive file=nbd:localhost:10809,if=virtio

However it’s incredibly slow. One problem is that the Fedora mirror sites aren’t very happy about you issuing lots of small HTTP Range requests and I observed that they throttle the connection quite aggressively. The second problem is that the xz block size for these cloud images is too large.

The XZ format (or rather, LZMA format) is divided into streams and blocks. We don’t normally use streams, and many XZ files use a single block. But it’s possible to tell the xz program to use a smaller than default block size, and in that case the output is divided into indexed blocks. Note the block size applies to the uncompressed input, the compressed blocks will have varying sizes, but the index that is created lets us find the block boundaries easily. When a byte is requested we can use a binary search to take us quickly to the compressed block, uncompress it (and cache it), and answer the request. We will only uncompress at most one block instead of the whole file.

For disk images I normally advocate a 16M block size. The current cloud images use (I think) a 192M block size, so both a huge amount of data has to be read over HTTPS to read one uncompressed byte, plus we have to cache very large blocks in RAM.

As an experiment I recompressed the cloud image using xz --block-size=$((16 * 1024 * 1024)) and hosted it locally, and booting is much quicker (albeit still slow because the cloud image contains cloud-init).

But even better we already ship a variety of disk images compressed with a 16M block size for virt-builder here, and these can be booted directly too:

$ nbdkit -U - --filter=xz curl \ \
        --run \
    'qemu-system-x86_64 -machine accel=kvm:tcg -cpu host -m 2048 -drive file=$nbd,if=virtio'

… although you can’t log in because they all have locked root accounts (virt-builder normally customizes them after download).

1 Comment

Filed under Uncategorized