Author: Petros Koutoupis (petros@petroskoutoupis.com)

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 disk drives or even map them as caching nodes to slower local (or remote) disk drives. The same utilies 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.

RESTful API

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 - 127.0.0.1:9118/v1/listRapidDiskVolumes|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 127.0.0.1:9118/v1/createRapidDisk/128|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 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

Installing:

# make tools-install

Uninstalling:

# make tools-uninstall

Installing modules for DKMS support

# make dkms-install

Uninstalling modules for DKMS support

# make dkms-uninstall

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