'nbd' is a Haskell library to implement NBD (Network Block Device) servers. This can be used to implement block-devices in userspace using the Haskell programming language, using the Conduit library.

A very simple demonstration server is included in this repository, see bin/server.hs. The server can export multiple files (all as a different device), and currently only supports read-only access.

To test, build the library and binary using cabal, then run something like:

$ dd if=/dev/urandom of=blocks bs=1024 count=65536
$ md5sum blocks
7604d00d72b9b8f8cb10e709a1ac3075  blocks
$ ./dist/build/nbd-demo-server/nbd-demo-server blocks

Now in another shell, as root:

# modprobe nbd
# # Connect to the server, creating /dev/nbd0
# nbd-client localhost /dev/nbd0 -N blocks
Negotiation: ..size = 64MB
bs=1024, sz=67108864 bytes
# # Read the data and validate the hash
# dd if=/dev/nbd0 bs=4096 | md5sum
16384+0 records in
16384+0 records out
67108864 bytes (67 MB) copied, 0.265507 s, 253 MB/s
7604d00d72b9b8f8cb10e709a1ac3075  -
# # Disconnect /dev/nbd0
# nbd-client -d /dev/nbd0
Disconnecting: que, disconnect, sock, done