make zram use lz4 compression algorithm

Question

Ubuntu 18, trying to make zram to use lz4 compression.

Tried modifying /usr/bin/init-zram-swapping file

#!/bin/sh
...........
...........
for i in $(seq ${NRDEVICES}); do
  DEVNUMBER=$((i - 1))
  echo $mem > /sys/block/zram${DEVNUMBER}/disksize
  echo lz4 > /sys/block/zram${DEVNUMBER}/comp_algorithm # <= added this line
  mkswap /dev/zram${DEVNUMBER}
  swapon -p 5 /dev/zram${DEVNUMBER}
done

But, after rebootzramctl still shows I'm using lzo algortihm

NAME       ALGORITHM DISKSIZE  DATA  COMPR TOTAL STREAMS MOUNTPOINT
/dev/zram3 lzo           466M  2.9M 614.2K    1M       4 [SWAP]
/dev/zram2 lzo           466M  2.9M 631.2K    1M       4 [SWAP]
/dev/zram1 lzo           466M  2.9M 536.1K 1020K       4 [SWAP]
/dev/zram0 lzo           466M    3M   610K    1M       4 [SWAP]

I also tried adding adding it inrc.local, but it throws device already in use warning, because as far as I know, the change needs to be done before mkswap and swapon runs.

Solution

This works:

# initialize the devices
for i in $(seq ${NRDEVICES}); do
  DEVNUMBER=$((i - 1))
  echo lz4 > /sys/block/zram${DEVNUMBER}/comp_algorithm
  echo $mem > /sys/block/zram${DEVNUMBER}/disksize
  mkswap /dev/zram${DEVNUMBER}
  swapon -p 100 /dev/zram${DEVNUMBER}
done

Note: it seems you need to define the compression algorithm before zram size. Set swapon -p to priority level 100 to use zram actively as it improves performance.

Comments

[Modestas]

Yes, it seems you need to define the compression algorithm before zram size. Set swapon -p to 100 swapiness to use zram actively as it improves performance.

[Sirajus Salekin]

thanks, i will check after going home. Too excited if this works :D

[Tomofumi]

IMHO I don't understand why you use swapon -p 100 here, rather than sysctl vm.swappiness=100?

[Modestas]

Tomofumi I don't know. Please tell if you find out.

[nrc]

swapon -p 100 indicates the priority amongst swap partitions. You may have for example, a priority 10 zram swap and a priority 20 hdd swap partition. swappiness is instead the threshold of free memory at which point the system starts to use the swap.