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Copyright 2018 Sam Pullara

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

Network Block Device

The nbd-client on Linux allows you to mount a remote server implementing the NBD protocol as a local block device. This NBDServer exports volumes whose data is stored in FoundationDB. This gives you a highly scalable, high performance, reliable block device that you can then format and put a filesystem on it.

The NBDCLI command allows you to create new volume, list the volumes you have in your system, delete a volume or snapshot a current volume to another volume.


Bring up FoundationDB and then run the NBDServer. It will be listening on the default 10809 port.

java -jar nbdcli.jar server

Create a new 1G volume:

java -jar nbdcli.jar create -n [volume name] -s 1G 

On a Linux host, install ndb, create the block device, format it and mount it:

sudo apt-get update && apt-get install nbd
sudo modprobe nbd
sudo nbd-client -N [volume name] [host] /dev/nbd0
sudo mkfs.xfs /dev/nbd0
mkdir tmp
sudo mount /dev/nbd0 tmp

You may need to change the ownership on that directory to access it but you can now save files there and they will be backed by FoundationDB. Each volume can only be shared to a single nbd client at a time.

Under the covers

Each volume is a sparse array of bytes (FDBArray) stored in FoundationDB across many rows in the database. In addition each volume can have a parent whose sparse array shows through where the child volume hasn't written yet. Here is the interface that we implement:

public interface Storage {
  void connect();

  void disconnect();

  CompletableFuture<Void> read(byte[] buffer, long offset);

  CompletableFuture<Void> write(byte[] buffer, long offset);

  CompletableFuture<Void> flush();

  long size();

  long usage();


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