zram (previously called compcache) can create RAM based block devices. It is a module of the Linux kernel since 3.2.

See https://code.google.com/p/compcache/ and Zram for more info.

Installation and use

Starting at buster (current testing) it's possible to simply install the zram-tools package and customize /etc/default/zramswap for your needs.

For systems using a standard SysV-based init system, it's possible to copy the following script to /etc/init.d/zram:

#!/bin/sh
### BEGIN INIT INFO
# Provides:          zram
# Required-Start:    $local_fs
# Required-Stop:     $local_fs
# Default-Start:     S
# Default-Stop:      0 1 6
# Short-Description: Use compressed RAM as in-memory swap
# Description:       Use compressed RAM as in-memory swap
### END INIT INFO

# Author: Antonio Galea <antonio.galea@gmail.com>
# Thanks to Przemysław Tomczyk for suggesting swapoff parallelization
# Distributed under the GPL version 3 or above, see terms at
#      https://gnu.org/licenses/gpl-3.0.txt

FRACTION=75

MEMORY=`perl -ne'/^MemTotal:\s+(\d+)/ && print $1*1024;' < /proc/meminfo`
CPUS=`nproc`
SIZE=$(( MEMORY * FRACTION / 100 / CPUS ))

case "$1" in
  "start")
    param=`modinfo zram|grep num_devices|cut -f2 -d:|tr -d ' '`
    modprobe zram $param=$CPUS
    for n in `seq $CPUS`; do
      i=$((n - 1))
      echo $SIZE > /sys/block/zram$i/disksize
      mkswap /dev/zram$i
      swapon /dev/zram$i -p 10
    done
    ;;
  "stop")
    for n in `seq $CPUS`; do
      i=$((n - 1))
      swapoff /dev/zram$i && echo "disabled disk $n of $CPUS" &
    done
    wait
    sleep .5
    modprobe -r zram
    ;;
  *)
    echo "Usage: `basename $0` (start | stop)"
    exit 1
    ;;
esac

then make the script executable:

chmod +x /etc/init.d/zram

then instruct your system to start it at boot time, with the command

  insserv zram