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:

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

### 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

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 --priority 10
        done
        ;;
    stop)
        for n in $(seq $CPUS)
        do
            i=$((n - 1))
            swapoff /dev/zram$i && echo "zram: disabled disk $n of $CPUS" &
        done

        wait
        sleep .5
        modprobe --remove zram
        ;;
    *)
        echo "Usage: $(basename $0) (start | stop)"
        exit 1
        ;;
esac

# End of file

then make the script executable:

chmod +x /etc/init.d/zram

then instruct your system to start it at boot time, with the command (skip this on systemd)

apt-get install insserv
insserv zram