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Author: Petros Koutoupis (

About the RapidDisk Project

RapidDisk contains a set of advanced Linux RAM Drive and Caching kernel modules. The user space utilities allow you to dynamically allocate RAM as block devices to either use them as stand alone drives or even map them as caching nodes to slower local (or remote) disk drives. The same utilities provide users with the capability to export the same volumes across an NVMe Target network.


Caching Policies

Leverage a high speed RAM drive to add speed to a slower volume by utilizing the (Linux native) Device-Mapper framework. Enable a Write / Read-through or Write Around Least Recently Used or LRU (FIFO) cache.

Write-Through / Read-Through Caching

This is where an application treats cache as the main data store and reads data from it and writes data to it. The cache is responsible for reading and writing this data to the permanent storage volume, thereby relieving the application of this responsibility.

In this mode, all writes are cached to a RapidDisk RAM drive but are also written to disk immediately. All disk reads are cached. Cache is not persistent over device removal, reboots, or after you remove the Device-Mapper mapping. This module does not store any cache metadata on RapidDisk volumes but instead in memory outside of RapidDisk. You can map and unmap a cache drive to any volume at any time and it will not affect the integrity of the data on the persistent storage drive.

Write-Around Caching

Write Around caching shares some similarities with the Write-Through implementation. However, in this method, only read operations are cached and not write operations. This way, all read data considered hot can remain in cache a bit longer before being evicted.


The RapidDisk Daemon (rapiddiskd) enabled remote management of RapidDisk volumes. The management commands are simplified into a set of GET and POST commands. It operates over port 9118 by default. This can be changed when invoking the daemon with the use of a parameter. Either way, please ensure that the port is open for TCP within your firewall rules.

An example of a GET command:

# curl -s --output -|jq .
  "volumes": [
      "rapiddisk": [
          "device": "rd1",
          "size": 67108864
          "device": "rd0",
          "size": 67108864
      "rapiddisk_cache": [
          "device": "rc-wa_loop7",
          "cache": "rd0",
          "source": "loop7",
          "mode": "write-around"

An example of a POST command:

# curl -X POST -s|jq .
  "status": "Success"

Building and Installing the rapiddisk kernel modules and utilities

Change into the project's parent directory path.

To build the rapiddisk management utility, you will need to have the libjansson, libpcre2, libdevmapper and libmicrohttpd development library files installed on your host system.

You are required to having either the full kernel source or the kernel headers installed for your current kernel revision.

To build rapiddisk from source, you would type the following on the command line:

# make

To install rapiddisk (must execute with superuser rights: sudo):

# make install

To uninstall rapiddisk (must execute with superuser rights: sudo):

# make uninstall

The rapiddisk utility will install in /sbin/

For utility information please reference the rapiddisk manual page:

# man 1 rapiddisk

Inserting/Removing the rapiddisk / rapiddisk-cache kernel modules

Both modules are required to be loaded for the rapiddisk daemon to start.

To insert the rapiddisk module:

# modprobe rapiddisk

To remove the rapiddisk module:

# modprobe -r rapiddisk

To insert the rapiddisk-cache module:

# modprobe rapiddisk-cache

To remove the rapiddisk-cache module:

# modprobe -r rapiddisk-cache

Building and installing / uninstalling the tools ONLY


# make tools-install


# make tools-uninstall

Installing modules for DKMS support

# make dkms-install

Note - In dkms package versions 3.x and later, the REMAKE_INITRD option has been deprecated. As a result, the operating system's initrd is not remade on every dkms rebuild of the module. If this is desired, then it will need to be manually rebuilt per your operating system's procedures. Otherwise, you can install and enable the rapiddisk-on-boot functions which is supported on both Ubuntu and RHEL based Linux distributions.

Uninstalling modules for DKMS support

# make dkms-uninstall

Note - Please refer to the above note on initrd remakes during dkms rebuilds of the rapiddisk modules.

Managing the RapidDisk daemon service

After installation, to start the service via systemd:

# systemctl start rapiddiskd.service

To check the status of the service via systemd:

# systemctl status rapiddiskd.service

To stop the service via systemd:

# systemctl stop rapiddiskd.service

To start the service at boot via systemd:

# systemctl enable rapiddiskd.service

Managing RapidDisk as an NVMe Target

There are a few things that need to be known when using the NVMe Target features of the RapidDisk suite.

Loading the NVMe Target Kernel Modules

In order to map any RapidDisk device and export it in the NVMe Target framework, the nvmet and the nvmet-tcp or nvmet-rdma (or nvme-loop) kernel modules must be inserted.

# modprobe nvmet nvmet-tcp

Enabling NVMe Target Ports

At least one Ethernet interface will need to be configured as a target port to export the RapidDisk volume from.

# rapiddisk -i eth -P 1 -t tcp

Exporting Targets

When exporting a volume, a RapidDisk volume and a target port must be defined. If a host NQN is not defined, the administration utility will provide access to any host NQN. Note - a target can be exported across more than one target port.

# rapiddisk -e -b rd3 -P 1

If a host NQN is defined, access is restricted to only those host NQNs. Note - the following command example can be repeated multiple times to add additional host NQNs for the specified target export.

# rapiddisk -e -b rd3 -P 1 -H nqn.host1

Unexporting Targets

Unexporting RapidDisk volumes looks a bit different than exporting. If a host NQN is defined for a specified target, only that NQN will be removed from accessing the exported target.

# rapiddisk -x -b rd3 -H nqn.host1

Removing all allowed host NQNs will revert access to any and all host NQNs requesting access to the target.

If a target port is defined, the exported target will not be exported from the interface if one condition is met: the target has no defined allowed host NQNs.

# rapiddisk -x -b rd3 -P 1 -H nqn.host1


# rapiddisk -x -b rd3 -P 1

And if there are no defined allowed host NQNs and the target is not being exported across any target ports, the entire target is removed from the subsystem.