dnbd3 - distributed network block device (version 3)
The distributed network block device in version 3 (dnbd3) is a network protocol similar to nbd to implement a distributed block-based storage system. Such a distributed block-based storage system consists of dnbd3 components, namly one or more servers and several clients. Servers are meant to expose virtual disk images as block devices to clients using dnbd3. Clients request data blocks from servers and can implement a load balancing mechanism to connect to the fastest available server for data exchange.
This repository contains the source code for the following dnbd3 components:
- dnbd3: Linux kernel module client for dnbd3
- dnbd3-bench: Benchmark utility to test dnbd3
- dnbd3-fuse: Fuse client for dnbd3
- dnbd3-server: Server to serve virtual disk images for dnbd3
The dnbd3 components can be built for the following Linux kernel versions and Unix distributions:
- Archlinux with Linux kernel 5.9.x or 5.4.x
- Ubuntu 20.04 with Linux kernel 5.4.x
- CentOS 8 with Linux kernel 4.18.x
- FreeBSD 12.1 (only user space programs, eg. dnbd3-server)
Build
Preliminaries
A build of the dnbd3 components requires the installation of the following build tools and libraries under your supported Unix distribution.
Archlinux with Linux kernel 5.9.x or 5.4.x
pacman -S git \
make \
cmake \
gcc \
linux-headers \ # or linux-lts-headers
fuse2 \
jansson \
afl \
dpkg \
rpm-tools
Ubuntu 20.04 with Linux kernel 5.4.x
apt-get install git \
make \
cmake \
gcc \
linux-headers-generic \
libfuse-dev \
libjansson-dev \
rpm
Note that afl is not available on Ubuntu 20.04 and should be built from the original sources.
CentOS 8 with Linux kernel 4.18.x
yum install git \
make \
cmake \
gcc \
kernel-devel \
elfutils-libelf-devel \
fuse-devel \
jansson-devel \
rpm-build
Note that afl is not available on CentOS 8 and should be built from the original sources.
FreeBSD 12.1
pkg install git \
cmake \
pkgconf \
fusefs-libs \
jansson \
afl \
rpm4
Preparation
Before a build takes place, you should create a build directory inside the root folder of the repository. After that, change your working directory to that new directory as follows:
Configuration
A build of the dnbd3 components can be configured and customized by the following configuration variables (CMake cache entries):
| Variable |
Type |
Values |
Default value |
Description |
CMAKE_BUILD_TYPE |
STRING |
{Debug, Release} |
Debug |
Build configuration of the dnbd3 project. |
KERNEL_BUILD_DIR |
PATH |
{a .. z, A .. Z, /, _, -} |
/lib/modules/uname -r/build |
Path to Linux kernel modules to compile against. |
KERNEL_INSTALL_DIR |
PATH |
{a .. z, A .. Z, /, _, -} |
/lib/modules/uname -r/extra |
Path to install Linux kernel modules. |
DNBD3_KERNEL_MODULE |
OPTION |
{ON, OFF} |
ON |
Build the dnbd3 Linux kernel module. |
DNBD3_CLIENT_FUSE |
OPTION |
{ON, OFF} |
ON |
Enable build of dnbd3-fuse. |
DNBD3_SERVER |
OPTION |
{ON, OFF} |
ON |
Enable build of dnbd3-server. |
DNBD3_SERVER_FUSE |
OPTION |
{ON, OFF} |
OFF |
Enable FUSE-Integration for dnbd3-server. |
DNBD3_SERVER_AFL |
OPTION |
{ON, OFF} |
OFF |
Build dnbd3-server for usage with afl-fuzz. |
DNBD3_SERVER_DEBUG_LOCKS |
OPTION |
{ON, OFF} |
OFF |
Add lock debugging code to dnbd3-server. |
DNBD3_SERVER_DEBUG_THREADS |
OPTION |
{ON, OFF} |
OFF |
Add thread debugging code to dnbd3-server. |
DNBD3_RELEASE_HARDEN |
OPTION |
{ON, OFF} |
OFF |
Compile dnbd3 programs in Release build with code hardening options. |
DNBD3_PACKAGE_DOCKER |
OPTION |
{ON, OFF} |
OFF |
Enable packaging of Docker image. |
A value from the range of appropriate values can be assigend to each configuration variable by executing CMake once with the following command pattern:
cmake -D<VARIABLE>=<VALUE> [-D ...] ../.
Debug
In the Debug build configuration, all dnbd3 components can be built by calling make:
Optionally, the output files can be installed with superuser permissions on the local system using the Makefile target install:
sudo make install
sudo depmod -a # only required if DNBD3_KERNEL_MODULE is enabled
Packages
In the Release build configuration, installation packages can be built by calling the make target package:
This target creates a Debian installation package (*.deb), a RPM installation package (*.rpm) and a compressed archive (*.tar.gz) containing the built dnbd3 components.
Sources
In the Release build configuration, sources can be built by calling the make target source:
This target creates compressed archives (*_source.tar.gz and *_source.zip) containing the source code of this repository for code distribution purposes.
Docker image
A docker image of the built dnbd3 components can be created in the Release build configuration with the option DNBD3_PACKAGE_DOCKER=ON and DNBD3_KERNEL_MODULE=OFF. The image is based on Ubuntu 20.04 and a created docker container from it starts the embedded dnbd3-server automatically.
Before the image is built, make sure that your docker daemon runs and you are a member of the docker group to access the docker deamon without any super user privileges. Then, build the docker image by calling the following Make target:
The built docker image is saved as archive file (*_ubuntu-20-04_docker.tar) and can be deployed to other machines. On each machine, the created image can be loaded with the following docker client call:
docker image load -i *_ubuntu-20-04_docker.tar
After the image is loaded, a docker network needs to be available so that each docker container based on this image can establish a network connection. Therefore, a docker network called dnbd3 is created with the following docker client call:
docker network create --driver=bridge --subnet=192.168.100.0/24 dnbd3
If the network is present, docker containers with a name of form dnbd3-server<NUMBER> and an IPv4 address from the network's subnet can be created using docker client calls like the following ones:
docker container create --name dnbd3-server1 --ip 192.168.100.10 --network dnbd3 <IMAGE_TAG>
docker container create --name dnbd3-server2 --ip 192.168.100.50 --network dnbd3 <IMAGE_TAG>
docker container create --name dnbd3-server3 --ip 192.168.100.100 --network dnbd3 <IMAGE_TAG>
docker container create --name dnbd3-server4 --ip 192.168.100.123 --network dnbd3 <IMAGE_TAG>
Note that the image is already tagged with an IMAGE_TAG which is set to the current dnbd3 package version number and follows the format dnbd3:<DNBD3_VERSION>. The IMAGE_TAG can be reused to create a docker container. Finally, each container based on the image can be started with the following docker client call:
docker container start -a dnbd3-server<MUNBER>
Configuration of dnbd3-server
The dnbd3-server is started according to the following command line call.
dnbd3-server -c <CONFIG_DIR>
An operation of the dnbd3-server requires a configuration directory to provide proper functionality. The configuration directory should contain two configuration files, namely the alt-servers and the server.conf file.
Configuration file alt-servers
The alt-servers configuration file specifies the list of known alt-servers for the dnbd3-server. The configuration in the file is specified the INI file format as shown in the following.
[Address]
comment=Whatever
for=purpose # where purpose is either "client" or "replication"
namespace=some/path/
All fields in an INI section are optional. If the for key is missing, the dnbd3-server will be used for replication and will be propagated to clients that request a list of alt servers. The namespace key can be specified multiple times per INI section. If this key is missing, the server will be used for all image names. Otherwise, it will only be used for images which's name starts with one of the given strings.
If the dnbd3-server is not running in proxy mode, this file won't do much.
Configuration file server.conf
The server.conf file is the main configuration file of the dnbd3-server. The configuration in the file is specified the INI file format as shown in the following.
[dnbd3]
basePath=/srv/openslx/dnbd3 # virtual root of image files
serverPenalty=1234 # artificial acceptance delay for incoming server connections (µs)
clientPenalty=2345 # artificial acceptance delay for incoming client connection (µs)
isProxy=true # enable proxy mode - will try to replicate from alt-servers if a client requests unknown image
uplinkTimeout=1250 # r/w timeout for connections to uplink servers
Debugging
Debugging of the Linux kernel modules and the user space utility requires this project to be built in the Debug configuration.
Linux kernel module
The Linux kernel module dnbd3 supports the Linux kernel's dynamic debug feature if the Linux kernel is built with the enabled kernel configuration CONFIG_DYNAMIC_DEBUG. The dynamic debug feature allows the printing of customizable debug messages into the Linux kernel's message buffer.
Dynamic debug for the modules can be either enabled at module initialization or during operation. At module initialization, dynamic debug can be enabled by modprobe using the "fake" module parameter dyndbg:
modprobe dnbd3 dyndbg=+pflmt
The module parameter dyndbg customizes the debug messages written into the Linux kernel's message buffer. The specific value +pflmt enables all debug messages in the source code and includes function name (f), line number (l), module name (m) and thread ID (t) for each executed debug statement from the source code.
During operation, debug messages from debug statements in the code can be customized and enabled dynamically as well using the debugfs control file <DEBUG_FS>/dynamic_debug/control where DEBUG_FS is the mount point of a mounted DebugFS, eg. /sys/kernel/debug:
echo "module dnbd3 +pflmt" > <DEBUG_FS>/dynamic_debug/control
More information regarding the Linux kernel's dynamic debug feature can be found in the Linux kernel documentation.
Development notes
Resource locking in dnbd3
The order of aquiring multiple locks is very important, as you'll produce a possible deadlock if you do it in the wrong order. Take very good care of locking order if you have lots of functions that call each other. You might lose track of what's going on.
dnbd3-fuse
This is a list of used locks, in the order they have to be aquired if you must hold multiple locks.
mutexInit
newAltLock
altLock
connection.sendMutex
requests.lock
dnbd3-server
This is a list of used locks, in the order they have to be aquired if you must hold multiple locks. Take a look at the lock priority defines in src/server/locks.h for the effective order.
reloadLock
loadLock
remoteCloneLock
_clients_lock
_clients[].lock
integrityQueueLock
imageListLock
_images[].lock
uplink.queueLock
altServersLock
client.sendMutex
uplink.rttLock
uplink.sendMutex
aclLock
initLock
dirLock
If you need to lock multiple clients or images or etc at once, lock the client with the lowest array index first.
If the program logic would require to aquire the locks in a different order, you have to rework the code. For example, if you hold the lock for client 10 and you need to look up some other client. You must not simply fetch the _clients_lock now and then iterate over the clients until you find the one you need, as it violates the above order to first lock on the clients array and then the clients lock. Instead, you need to release client 10's lock, then lock on _clients_lock and iterate over the clients. Now you check if you either encounter the client you originally held the lock on, or the client you are looking for. You immediately lock on those two. You can then release the _clients_lock and work with both clients.
This described implementation advice is visualized in the following pseudo C code.
/* client10 is assumed to be a pointer to a client, which happens to be at index 10 */
lock (client10->lock);
/* ... */
/* we need another client */
unlock(client10->lock);
lock(_clients_lock);
client clientA = NULL, clientB = NULL;
for (i = 0; i < _num_clients; ++i) {
if (client[i] == client10) {
clientA = client[i];
lock(clientA.lock);
} else if (client[i].something == <whatever>) {
clientB = client[i];
lock(clientB.lock);
}
}
unlock(_clients_lock);
if (clientA && clientB) {
/* make sure we actually found both */
/* do something important with both clients */
}
if (clientA)
unlock(clientA.lock);
if (clientB)
unlock(clientB.lock);
