zram and Swap

The zram kernel module creates compressed RAM-based block devices named /dev/zramX. These zram devices allow fast I/O and the compression provides RAM space savings.

Install zram

Install zram in Debian:

sudo apt install zram-tools

In order to enable zram, the computer needs to be rebooted.

Configure zram

The status of the zram module can be queried with systemctl status zramswap.service. The configuration and usage can be shown with zramctl:

$ sudo zramctl
NAME       ALGORITHM DISKSIZE DATA COMPR TOTAL STREAMS MOUNTPOINT
/dev/zram0 lz4           256M   4K   63B   20K       8 [SWAP]

Change the configuration parameters by editing the file /etc/default/zramswap:

ALGO=zstd
PERCENT=25

The will change the default compression algorithm from lz4 to zstd and the default size from 256 MiB to 25 % of the total RAM. The changes are only visible after rebooting the computer.

$ sudo zramctl
NAME       ALGORITHM DISKSIZE DATA COMPR TOTAL STREAMS MOUNTPOINT
/dev/zram0 zstd          7.8G   4K   59B   20K       8 [SWAP]

If the computer has a configured swap file or swap partition, the zram swap gets a higher priority (100) instead of the default of -2:

$ sudo swapon
NAME       TYPE      SIZE USED PRIO
/dev/dm-2  partition   4G   0B   -2
/dev/zram0 partition 7.8G   0B  100

When the computer uses suspend to disk, a swap file or swap partition is needed besides zram.