Zram

zram (previously called compcache) Article description::a Linux kernel feature and userspace tools for creating compressible RAM-based block devices. It has been included as a module of the mainline Linux since kernel version 3.14. Starting with kernel version 3.15, zram supports multiple compression streams and the ability to change the compression algorithms in use.

Introduction

The zram kernel module enables support for creating compressed block devices in RAM. These block devices can then be used for swap or general purpose RAM disks. The two most popular uses for it are swap to extend the available amount of RAM to processes and /tmp.

The RAM used for the block device is dynamically obtained and released up to its predefined uncompressed maximum size. The way it extends the amount of available RAM to a system is by using a portion of the RAM as compressed swap. It can therefore hold more pages of memory in the compressed swap than the amount of actual memory used.

Typically it compresses to a 3:1 ratio. So, 1 GiB of swap uses only 333 MiB of RAM on average. The compression ratio including memory used for disk overhead varies depending on the percent of maximum space used. This may typically vary from 1.5:1 for a 1.5 GiB disk with only 5% space used, to over 3:1 when nearly full. It also is much faster at swapping pages than typical hard disk swap.

My experience with using it, my system is still fully functional, with only slight slow downs at times. This is for a Xfce4 desktop with several apps and emerge running with PORTAGE_NICENESS=10. The memory and swap spaces were nearly maxed out. Intel Core2 Quad core 2.6Ghz, 4 GiB RAM. I had 4 - 1.5 GiB zram disks for swap, plus 1 GiB partition of hard drive swap as backup.

At one point during linking chromium, I saw the system using just over 5 GiB of zram swap, while using about 1.2 GiB of RAM, about 100 MiB of hard disk swap. The desktop was still responsive.

Caveats/Cons

Prior to kernel 3.15, each zram device contains its own compression buffer, memory pools, and other metadata as well as per-device locks. This can become a serious bottleneck for multi-core machines. To work around this problem, zram is capable of initializing multiple devices. For this reason, the recommended amount of devices for swap is 1 per CPU core for kernels prior to 3.15.

Another caveat for systems with limited memory, non swap use can reduce the amount of available memory to run applications.

When using this with a really fast SSD (e.g. Samsung 840 Pro), avoid setting rc_parallel=YES in /etc/rc.conf. Depending on the size of the zram partitions and the speed of the RAM, some swap partitions and filesystems might not be ready when the swap and localmount services are started.

In such case, if you absolutely have to use parallel, consider removing these services from runlevel boot and adding them to default instead.

Enabling zram support in kernel

Enable the following options are needed in the kernel's configuration file:

<pre>
Memory Management options  --->
    <M> Memory allocator for compressed pages
    [*]   Export zsmalloc statistics
Device Drivers  --->
    [*] Block devices --->
        <M> Compressed RAM block device support
        [*]     Write back incompressible or idle page to backing device
        [*]     Track zRam block status
</pre>

It is recommended that zram be built as a loadable module. This allows you to change number of zram devices without reboot, by deactivating zram devices and re-loading module with new parameters. If you have zram built-in, then you can only change the number of devices at boot time by using the kernel boot parameter:

zram.num_devices=#

In order to use the LZ4 compression algorithm, you must also enable it in kernel config:

<pre>
Cryptographic API --->
    *** Compression ***
    <*> LZ4 compression algorithm
</pre>

Initialization

<syntaxhighlight lang="bash">load_on_start="yes"

unload_on_stop="yes"
 
num_devices="2"

type0="swap"
flag0=
size0="512"
maxs0=2
algo0=lz4

type1="/tmp"
flag1="ext4"
size1="512"</syntaxhighlight>

Created symlink /etc/systemd/system/local-fs-pre.target.wants/zram_var_tmp.service → /etc/systemd/system/zram_var_tmp.service.

<syntaxhighlight lang="bash">#!/bin/bash

modprobe zram

SIZE=6144
echo $(($SIZE*1024*1024)) > /sys/block/zram0/disksize

mkswap /dev/zram0

swapon /dev/zram0 -p 10</syntaxhighlight>

<syntaxhighlight lang="bash">#!/bin/bash

swapoff /dev/zram0

echo 1 > /sys/block/zram0/reset

modprobe -r zram</syntaxhighlight>

<syntaxhighlight lang="bash">KERNEL=="zram0", SUBSYSTEM=="block", DRIVER=="", ACTION=="add", ATTR{disksize}=="0", ATTR{disksize}="512M", RUN+="/sbin/mkswap $env{DEVNAME}"
KERNEL=="zram1", SUBSYSTEM=="block", DRIVER=="", ACTION=="add", ATTR{disksize}=="0", ATTR{disksize}="512M", RUN+="/sbin/mkswap $env{DEVNAME}"
KERNEL=="zram2", SUBSYSTEM=="block", DRIVER=="", ACTION=="add", ATTR{disksize}=="0", ATTR{disksize}="512M", RUN+="/sbin/mkfs.ext4 $env{DEVNAME}"
# if you want lz4 support (since kernel 3.15) and without ext4 journaling 
# KERNEL=="zram2", SUBSYSTEM=="block", DRIVER=="", ACTION=="add", ATTR{initstate}=="0", ATTR{comp_algorithm}="lz4", ATTR{disksize}="512M", RUN+="/sbin/mkfs.ext4 -O ^has_journal -L $name $env{DEVNAME}"</syntaxhighlight>

<syntaxhighlight lang="text">/dev/zram0              swap                    swap            pri=16383                                                       0 0
/dev/zram1              swap                    swap            pri=16383                                                       0 0
/dev/zram2              /tmp                    ext4            defaults                                                        0 0</syntaxhighlight>

<syntaxhighlight lang="modula3">options zram num_devices=3</syntaxhighlight>

<syntaxhighlight lang="modula3">zram</syntaxhighlight>

Checking that zram is used

Check if zram is mounted as swap:

/dev/zram0                              partition       2097148 2816    16383

Check if zram is mounted as directories:

/dev/zram1 /var/tmp/portage ext4 rw,nosuid,nodev,block_validity,discard,delalloc,barrier,user_xattr,acl 0 0

See also

• Portage TMPDIR on tmpfs — Building packages in tmpfs both speeds up emerge times and reduces HDD/SSD wear.

External resources

• zram in official kernel documentation.