How to set up properly zram and swap

Question

I'm configuring & compiling new 3.0 kernel. One of the goodies I planned to use for some time (by patching) that was merged into 3.0 is zram.

Is it possible to set both hdd swap and zram swap so the zram is used first and only spilled pages are put into actual swap?

Answer 1

swapon have -p switch which sets the priority. I can set up:

swapon -p 32767 /dev/zram0
swapon -p 0 /dev/my-lvm-volume/swap

Or in /etc/fstab:

/dev/zram0              none swap sw,pri=32767 0 0
/dev/my-lvm-volume/swap none swap sw,pri=0     0 0

EDIT: Just for a full solution - such line may be helpful as udev rule:

KERNEL=="zram0", ACTION=="add", ATTR{disksize}="1073741824", RUN="/sbin/mkswap /$root/$name"

Answer 2

Sidenote: because of per-cpu locking, it is important to have as many zram-swaps as CPUs (modprobe zram_num_devices=n zram) instead of a single big one. RTFM!