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?


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!

  • 1
    [citation needed]? I cannot find the recomendation in Linux documentation or Google. – Maciej Piechotka Nov 28 '11 at 23:28
  • By default max_comp_streams seems to be 1 according to documentation. I haven't checked if that parameter existed in 2011 but it seems to be a better option then multiple swap files. – Maciej Piechotka May 21 '15 at 19:27
  • Testing with the package zram-config under Ubuntu (and likely debian) I noted that the configuration defaults to 1 per core or thread (2 on an old E2140 dual core and 4 on an i3-3220) – Elder Geek Feb 29 '16 at 4:03
  • interesting, and this also seems quite clear to understand by reading /etc/init/zram-config.conf – Aquarius Power Nov 18 '16 at 19:44

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"

Select how many zram devices do you want by passing


to kernel parameters or directly to module (without zram.) . Default value is 1. zram will be able utilize as many cpus as number of devices you create.

Prepare zram0 device, set size to 1GB.

echo 1000000000 > /sys/block/zram0/disksize

Alternatively you can do it by udev rule. Create swap on zram0

mkswap /dev/zram0

and enable it with higher priority than other swap devices

swapon -p 32767 /dev/zram0
  • What kernel and distro versions are you writing for here ? – Cbhihe Aug 29 '16 at 15:23
  • For the record, proceeding as you advise on a 14.04.5 desktop Ubuntu, fails. In particular, when I try to set the priorities of zram0 and zram1 to value 10 on a 2 nuclei Intel T9300 box, it throws me with: swapon: /dev/zram0 or 1: swapon failed: Device or resource busy. – Cbhihe Aug 29 '16 at 15:26

When you enable zram, it will automatically have priority.

For ubuntu 16.04: /usr/bin/init-zram-swapping

You can edit that file (make a backup 1st), to lower the used real ram, I changed the mem line to this:

mem=$(((totalmem / 4 / ${NRDEVICES}) * 1024))

For some reason there seems to be a lot of misinterpretation of https://www.kernel.org/doc/Documentation/blockdev/zram.txt

It clearly states "2) Set max number of compression streams Regardless the value passed to this attribute, ZRAM will always allocate multiple compression streams - one per online CPUs - thus allowing several concurrent compression operations. The number of allocated compression streams goes down when some of the CPUs become offline. There is no single-compression-stream mode anymore, unless you are running a UP system or has only 1 CPU online.

To find out how many streams are currently available: cat /sys/block/zram0/max_comp_streams"

But repetitively there is a common urban myth max streams is 1.

Its plainly not true.

The two OS where zram has proven effective Chrome OS & Android you a single device. Also they tweak page-cluster

page-cluster controls the number of pages up to which consecutive pages are read in from swap in a single attempt. This is the swap counterpart to page cache readahead. The mentioned consecutivity is not in terms of virtual/physical addresses, but consecutive on swap space - that means they were swapped out together.

It is a logarithmic value - setting it to zero means "1 page", setting it to 1 means "2 pages", setting it to 2 means "4 pages", etc. Zero disables swap readahead completely.

The default value is three (eight pages at a time). There may be some small benefits in tuning this to a different value if your workload is swap-intensive.

Lower values mean lower latencies for initial faults, but at the same time extra faults and I/O delays for following faults if they would have been part of that consecutive pages readahead would have brought in.

echo "0" > /proc/sys/vm/page-cluster to force single page

Much seems to originate from zram_config the debian/ubuntu package that for some reason seems to have very little correlation with the kernel documents for zram and has bred a series of Chinese whispers that in essence could be completely wrong.

With file swap do you create a swap drive for each core? Maybe that might answer your questions. Also to back this up Googles Chrome OS & Android which successfully employ with the above page-cluster as its not matching a disk so latency can be improved, single devices.

Also for a sys-admin what is important actual mem usage or vm mem usage? Most examples show creation via disk_size and totally ignore mem_limit. disk_size=uncompressed vm size. mem_limit=actual mem footprint limit.

It sort of makes disk_size choice confusing as its a virtual max size dependent on comp_alg ratio and the overhead of 0.1% of the size of the disk when not in use and really is a guesstimate of mem_limit * (approx 2 - 4) of frugal vs optimism.

zram_config doesn't even check for previous service usage and overwrites whilst a simple check of the zram sys class like the below will.

createZramSwaps () {
        totalmem=$(free|awk '/^Mem:/{print $2}')
        mem=$((( totalmem * MEM_FACTOR / 100 / BIG_CORES ) * 1024))

        # Check Zram Class created
        if [ ! -d "${ZRAM_SYS_DIR}" ]; then
                modprobe zram
                echo ${COMP_ALG_SWAP} > /sys/block/zram${RAM_DEV}/comp_algorithm
                echo ${mem} > /sys/block/zram${RAM_DEV}/disksize
                mkswap /dev/zram${RAM_DEV}
                swapon -p ${SWAP_PRI} /dev/zram${RAM_DEV}
                RAM_DEV=$(cat /sys/class/zram-control/hot_add)
                echo ${COMP_ALG_SWAP} > /sys/block/zram${RAM_DEV}/comp_algorithm
                echo ${mem} > /sys/block/zram${RAM_DEV}/disksize
                mkswap /dev/zram${RAM_DEV}
                swapon -p ${SWAP_PRI} /dev/zram${RAM_DEV}

        if [ "$BIG_CORES" -gt 1 ];then
                for i in $(seq $((BIG_CORES - 1))); do
                        RAM_DEV=$(cat /sys/class/zram-control/hot_add)
                        echo ${COMP_ALG_SWAP} > /sys/block/zram${RAM_DEV}/comp_algorithm
                        echo ${mem} > /sys/block/zram${RAM_DEV}/disksize
                        mkswap /dev/zram${RAM_DEV}
                        swapon -p ${SWAP_PRI} /dev/zram${RAM_DEV}

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