zram-generator is a generator that creates systemd units to format and use compressed RAM devices, either
as swap or a file system.
The generator will be invoked by systemd early at boot. The generator will then:
1. read configuration files from {/etc,/lib}/systemd/zram-generator.conf[.d] (see zram-generator.conf(5)
for details);
2. generate systemd.swap(5) and/or systemd.mount(5) units into TARGET_DIR and connect them to
swap.target or local-fs.target as appropriate;
3. ensure the zram module is loaded and create the requested devices.
The generator does nothing if run inside a container (as determined by systemd-detect-virt(8)--container).
The generator also understands the kernel command-line option systemd.zram. See zram-generator.conf(5)
for details.
Setting the ZRAM_GENERATOR_ROOT environment variable makes the generator run in test mode, in which case
containerisation is ignored and step 3 is skipped.
For the ramifications of ZRAM_GENERATOR_ROOT on config handling, see zram-generator.conf(5).
Generated dev-zramN.swap units depend on systemd-zram-setup@zramN.service, which will:
1. read configuration files from {/etc,/lib}/systemd/zram-generator.conf[.d] (see zram-generator.conf(5)
for details);
2. set the desired compression algorithm, if any; if the current kernel doesn't understand the specified
algorithm, a warning is issued, but execution continues;
3. set the desired blockdev size and format it as swap with systemd-makefs(8).
Generated path-to-mount-point.mount units depend on systemd-zram-setup@zramN.service. The effect is
similar to what happens for swap units, but of course they are formatted with a file system.
When the unit is stopped, the zram device is reset, freeing memory and allowing the device to be reused.
zram-generator implements systemd.generator(7).
Applyingconfigchanges
This generator is invoked in early boot, and the devices it configures will be created very early too, so
the easiest way to apply config changes is to simply reboot the machine.
Nevertheless, sometimes it may be useful to add new devices or apply config changes at runtime. Applying
new configuration means restarting the units, and that in turn means recreating the zram devices. This
means that filesystemsaretemporarilyunmountedandtheircontentslost, and pagesaremovedoutofthecompressedswapdevice into other memory. If this is acceptable, systemctlrestartsystemd-zram-setup@zramN or systemctlrestartsystemd-zram-setup@* may be used to recreate a specific
device or all configured devices. (If the device didn't exist, restart will create it.) If the way the
device is used (e.g. the mount point or file system type) is changed, systemctldaemon-reload needs to be
called first to recreate systemd units. If a device or mount point is removed from configuration, the
unit should be stopped before calling daemon-reload. Otherwise, systemd will not know how to stop the
unit properly.
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