rdnbd (Read Only Network Block Device)

rdnbd allows you mount HTTP file directly!

$ go install github.com/agfn/rdnbd/cmd/rdnbd@latest
$ sudo rdnbd -device /dev/nbd0 -cache .cache https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/debian-12.5.0-amd64-netinst.iso
$ lsblk /dev/nbd0
$ sudo mkdir -p /mnt/debian && sudo mount -o ro /dev/nbd0p1 /mnt/debian

rdnbd relies on the Network Block Device. Ensure you have NBD installed:

$ ls -lh /dev/nbd*
brw-rw---- 1 root 6 43,   0 Jun 10 17:03 /dev/nbd0
...

If NBD is not installed, refer to the Install NBD section

How it Works

Prerequisites

• The HTTP server must support range requests, enabling partial file downloads.
• Running on Linux with the NBD driver.

The concept is straightforward: emulate a block device to mount it.

NBD (Network Block Device) is designed for this purpose. It requires a server to provide block data and a kernel driver to map block data to a block device /dev/nbdx.

With the help of go-nbd, I quickly built the NBD server. However, it was extremely slow due to network latency when mounting, so I implemented an on-disk cache to reduce download frequency.

The cache consists of two files:

• cache: stores cached block data.
• cache.idx: stores the cached block index.

When reading a block, it checks whether the block is cached by accessing idx = cache.idx[block_id]. If idx > 0, the block has been cached and can be read from cache[(block_id - 1)]. If the block is not cached, it downloads the block using HTTP range request.

Install NBD

Install NBD for WSL

1. Compile WSL Kernel with CONFIG_BLK_DEV_NBD=y

   # kernel project is quite large, so clone the files you just need with `--depth 1`
   $ git clone --depth 1 -b linux-msft-wsl-5.15.153.1 https://github.com/microsoft/WSL2-Linux-Kernel
   $ cp Microsoft/config-wsl .config
   $ sed -i '/CONFIG_BLK_DEV_NBD/d' .config
   $ echo 'CONFIG_BLK_DEV_NBD=y' >> .config
   $ make -j12

2. Restart WSL with Compiled Kernel.

   Copy kernel to Windows file system:

   # copy to E:\wsl\kernel
   $ cp arch/x86/boot/bzImage /mnt/e/wsl/kernel

   Config WSL kernel. Edit $env:UserProfile\.wslconfig file and add the following line:

   [wsl2]
   kernel=E:\\wsl\\kernel

   Restart WSL:

   $ wsl --shutdown
   $ wsl