Zram: Difference between revisions

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== Introduction ==
== Introduction ==


'''zram''' is a Linux kernel module to create RAM based block devices <ref>https://www.kernel.org/doc/Documentation/blockdev/zram.txt</ref>.
<code>zram</code> is a Linux kernel module to create RAM based block devices <ref>https://www.kernel.org/doc/Documentation/blockdev/zram.txt</ref>.
It's main feature distinguishing it from the classical ramdisks such as '''tmpfs''' and '''ramfs''' is the on-the-fly compression via LZ4 or LZO,
Its main feature distinguishing it from the classical ramdisks such as <code>tmpfs</code> and <code>ramfs</code> is the on-the-fly compression via LZ4, LZO (and LZO-RLE) or ZSTD,
LZ4 being the default <ref>https://en.wikipedia.org/wiki/Zram</ref>.
LZO-RLE (since 5.1) being the default <ref>https://en.wikipedia.org/wiki/Zram</ref>.
LZO compression is more efficient, but LZ4 is faster at compression/decompression.
LZO compression is more efficient, but LZ4 is faster at compression/decompression. ZSTD should be between LZO and LZ4 in speed.


Use cases for '''zram''' are as compressed swap disk or as general purpose RAM disk, providing high throughput and low latency.
Use cases for <code>zram</code> are compressed swap disks or general purpose RAM disks, providing high throughput and low latency.
The compression is of special interest for low memory devices. The huge drawbacks are of course the possible data loss
The compression is of special interest for low memory devices. The huge drawbacks are of course the possible data loss
on power failure and the limited disk size.
on power failure and the limited disk size.


With '''zswap''' there exists a RAM-based compressed cache for swap devices as alternative.
With <code>zswap</code> there exists a RAM-based compressed cache for swap devices as alternative.
'''zswap''' provides the ability to compress swapped pages and stores less frequent pages to a physical disk, which is therefore required <ref>https://en.wikipedia.org/wiki/Zswap</ref>.
<code>zswap</code> provides the ability to compress swapped pages and stores less frequent pages to a physical disk, which is therefore required <ref>https://en.wikipedia.org/wiki/Zswap</ref>.


== Setup and usage ==
== Setup and usage ==


The easiest way to manage '''zram''' block devices is via the '''zramctl''' utility provided by '''util-linux''' which is already installed on any Proxmox VE.
The easiest way to manage <code>zram</code> block devices is via the <code>zramctl</code> utility provided by <code>util-linux</code> package which is already installed on any Proxmox VE.


For this to work, the '''zram''' kernel module, part of the Linux mainline kernel since version 3.14, needs to be loaded as Proxmox VE does not loads this module by default.
For this to work, the <code>zram</code> kernel module, which has been part of the Linux mainline kernel since version 3.14, must be loaded, as Proxmox VE does not load this module by default.


=== Enable a zram-disk ===
=== Enable a zram-disk ===


To load the '''zram''' module, run the following command in a root shell:
To load the <code>zram</code> module, run the following command in a root shell:
  modprobe zram
  modprobe zram


This will load the kernel module and create a new block device accessible under '''/dev/zram0'''.
This will load the kernel module and create a new block device accessible under <code>/dev/zram0</code>.
With a initial disk size of 0B the zram-disk is rather useless, so one might change the size to something usable, e.g. 4GiB.
With an initial disk size of 0B the zram-disk is rather useless, so one might change the size to something usable, e.g. 4GiB.
This is done with the command:
This is done with the command:
  zramctl --size 4G /dev/zram0
  zramctl --size 4G /dev/zram0


Statistics for the newly created block device can be obtained via '''zramctl'''.
Statistics for the newly created block device can be obtained via <code>zramctl</code>.
For example, we can display the properties for the newly created zram-disk with:
For example, we can display the properties for the newly created zram-disk with:
  # zramctl
  # zramctl
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  swapon /dev/zram0 -p 10
  swapon /dev/zram0 -p 10


The '''-p''' option of '''swapon''' gives this swap device a higher priority over others, using the zram-disk prefered over other swap devices.
The <code>-p</code> option of <code>swapon</code> gives this swap device a higher priority over others. Using the zram-disk is then preferred over other swap devices.


=== Disable the zram-disk ===
=== Disable the zram-disk ===
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  zramctl --reset /dev/zram0
  zramctl --reset /dev/zram0


For further detail please refere to the manpage '''man zramctl''' or <ref>https://www.kernel.org/doc/Documentation/blockdev/zram.txt</ref>.
For further details please refer to the manpage <code>man zramctl</code> or <ref>https://www.kernel.org/doc/Documentation/blockdev/zram.txt</ref>.


== Permanent setup ==
== Permanent setup ==
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This is based on a setup described in <ref>https://wiki.archlinux.org/index.php/Improving_performance#Zram_or_zswap</ref>.
This is based on a setup described in <ref>https://wiki.archlinux.org/index.php/Improving_performance#Zram_or_zswap</ref>.
To permanently set up a zram-disk as swap device, perform the following steps.
To permanently set up a zram-disk as swap device, perform the following steps.
Add a file '''/etc/modules-load.d/zram.conf''' with the content '''zram''' by running:
Add a file <code>/etc/modules-load.d/zram.conf</code> with the content <code>zram</code> by running:


  echo "zram" > /etc/modules-load.d/zram.conf
  echo "zram" > /etc/modules-load.d/zram.conf


This will ensure the kernel module is loaded at boot. Next one needs a udev rule to automatically setup the disk size and make a swap device.
This will ensure the kernel module is loaded at boot. Next, you need a udev rule to automatically set the hard drive size and create a swap device.
For this, run:
For this, run:


  echo 'KERNEL=="zram0", ATTR{disksize}="4G" RUN="/sbin/mkswap /dev/zram0", TAG+="systemd"' > /etc/udev/rules.d/99-zram.rules
  echo 'KERNEL=="zram0", ACTION=="add", ATTR{comp_algorithm}="zstd", ATTR{disksize}="4G", RUN+="/sbin/mkswap /dev/zram0", TAG+="systemd"' > /etc/udev/rules.d/99-zram.rules


Replace the '4G' disksize with the value you want for your system. As last step, define the device as swap disk in your '''/etc/fstab''' by running:
Replace the '4G' disk size and the <code>comp_algorithm</code> with the values you want for your system. In the last step, define the device as a swap disk in your <code>/etc/fstab</code> by executing:


  echo "/dev/zram0 none swap defaults,pri=10 0 0" >> /etc/fstab
  echo "/dev/zram0 none swap defaults,pri=10 0 0" >> /etc/fstab


Make sure to use '''>>''' and not '''>''' in this command, otherwise you will overwrite your
Make sure to use <code>>></code> and not <code>></code> in this command, otherwise you will overwrite the existing configuration.
current fstab entries.
An additional option 'nofail' can be set in case `/dev/zram0` is not always available on boot to treat it as optional.


=== Setting up multiple disks at boot ===
=== Setting up multiple disks at boot ===
If you wish to create more than one zram-disk at boot, you can to pass the number of disks you want to have to the kernel module, which is achieved by:
If you wish to create more than one zram-disk at boot, you can pass the number of disks you want to have to the kernel module, which is achieved by:


  echo options zram num_devices=N > /etc/modprobe.d/zram.conf
  echo options zram num_devices=N > /etc/modprobe.d/zram.conf


where N should be replaced by the number of zram-disks you want to create. Further you have to setup udev rules to format and fstab entries to automout also these disks.
where N should be replaced by the number of zram-disks you want to create. Further you have to setup udev rules to format and fstab entries to automount also these disks.
 
== Alternative Setup using zram-tools ==
 
=== Setup and usage ===
 
An easy alternative way to manage <code>zram</code> block devices is via <code>zram-tools</code> which can be installed on your Proxmox VE.
 
apt install zram-tools
 
=== Enable zram ===
After installation, the systemd service “zramswap.service” automatically activates zram as swap device with the default configuration from <code>/etc/default/zramswap</code>
 
<pre>
# Compression algorithm selection
# speed: lz4 > zstd > lzo
# compression: zstd > lzo > lz4
# This is not inclusive of all that is available in latest kernels
# See /sys/block/zram0/comp_algorithm (when zram module is loaded) to see
# what is currently set and available for your kernel[1]
# [1]  https://github.com/torvalds/linux/blob/master/Documentation/blockdev/zram.txt#L86
#ALGO=lz4
 
# Specifies the amount of RAM that should be used for zram
# based on a percentage the total amount of available memory
# This takes precedence and overrides SIZE below
#PERCENT=50
 
# Specifies a static amount of RAM that should be used for
# the ZRAM devices, this is in MiB
#SIZE=256
 
# Specifies the priority for the swap devices, see swapon(2)
# for more details. Higher number = higher priority
# This should probably be higher than hdd/ssd swaps.
#PRIORITY=100
</pre>
 
The status of the zram can be checked with the following command:
 
<pre>
# zramswap status
NAME      ALGORITHM DISKSIZE DATA COMPR TOTAL STREAMS MOUNTPOINT
/dev/zram0 lz4          256M  4K  63B  20K      40 [SWAP]
</pre>
 
The additional zram swap space should now be shown in the following command:
 
<pre>
free -h
              total        used        free      shared  buff/cache  available
Mem:          7.8Gi      1.3Gi      6.4Gi        28Mi      267Mi      6.4Gi
Swap:          255Mi          0B      255Mi
</pre>
 
=== Disable zram ===
 
To disable your zram-disk you have to execute the following command:
 
zramswap stop
 
=== Configuration ===
There are three things you can configure:
 
* compression
* amount of memory (default is 256MB)
* priority
 
The size of the zram device can be configured either by absolute value (<code>SIZE</code>) or percent (<code>PERCENT</code>) in <code>/etc/default/zramswap</code>. e.g.
 
PERCENT=20
 
To apply the new configuration, restart <code>zramswap</code>.
 
zramswap restart
 
=== Limitation ===
 
When configuring <code>zram</code>, the size of a zram device controls the maximum uncompressed amount of data it can store, not the maximum compressed size. You can configure the zram's size to be equal to or even greater than your system's physical RAM capacity, as long as the compressed size on physical RAM will not exceed your system's physical RAM capacity.
 
Using zramswap with a physical swap is not recommended. Moving pages between zramswap and physical swap can cause high latencies and destabilize your system.
 


== References ==
== References ==

Latest revision as of 09:52, 18 December 2024


Warning: There are some reports that zRam may cause soft lockups on certain hardware, if you run into such issues try to disable zRam

Introduction

zram is a Linux kernel module to create RAM based block devices [1]. Its main feature distinguishing it from the classical ramdisks such as tmpfs and ramfs is the on-the-fly compression via LZ4, LZO (and LZO-RLE) or ZSTD, LZO-RLE (since 5.1) being the default [2]. LZO compression is more efficient, but LZ4 is faster at compression/decompression. ZSTD should be between LZO and LZ4 in speed.

Use cases for zram are compressed swap disks or general purpose RAM disks, providing high throughput and low latency. The compression is of special interest for low memory devices. The huge drawbacks are of course the possible data loss on power failure and the limited disk size.

With zswap there exists a RAM-based compressed cache for swap devices as alternative. zswap provides the ability to compress swapped pages and stores less frequent pages to a physical disk, which is therefore required [3].

Setup and usage

The easiest way to manage zram block devices is via the zramctl utility provided by util-linux package which is already installed on any Proxmox VE.

For this to work, the zram kernel module, which has been part of the Linux mainline kernel since version 3.14, must be loaded, as Proxmox VE does not load this module by default.

Enable a zram-disk

To load the zram module, run the following command in a root shell: 

modprobe zram

This will load the kernel module and create a new block device accessible under /dev/zram0. With an initial disk size of 0B the zram-disk is rather useless, so one might change the size to something usable, e.g. 4GiB. This is done with the command: 

zramctl --size 4G /dev/zram0

Statistics for the newly created block device can be obtained via zramctl. For example, we can display the properties for the newly created zram-disk with: 

# zramctl
NAME       ALGORITHM DISKSIZE DATA COMPR TOTAL STREAMS MOUNTPOINT
/dev/zram0 lzo             4G   0B    0B    0B       4

To make this device a swap device run the two following commands: 

mkswap /dev/zram0
swapon /dev/zram0 -p 10

The -p option of swapon gives this swap device a higher priority over others. Using the zram-disk is then preferred over other swap devices.

Disable the zram-disk

To disable the swap and also zram-disk we first run: 

swapoff /dev/zram0

to disable it as swap device and finally reset the size of the zram-disk to zero by: 

zramctl --reset /dev/zram0

For further details please refer to the manpage man zramctl or [4].

Permanent setup

This is based on a setup described in [5]. To permanently set up a zram-disk as swap device, perform the following steps. Add a file /etc/modules-load.d/zram.conf with the content zram by running: 

echo "zram" > /etc/modules-load.d/zram.conf

This will ensure the kernel module is loaded at boot. Next, you need a udev rule to automatically set the hard drive size and create a swap device. For this, run: 

echo 'KERNEL=="zram0", ACTION=="add", ATTR{comp_algorithm}="zstd", ATTR{disksize}="4G", RUN+="/sbin/mkswap /dev/zram0", TAG+="systemd"' > /etc/udev/rules.d/99-zram.rules

Replace the '4G' disk size and the comp_algorithm with the values you want for your system. In the last step, define the device as a swap disk in your /etc/fstab by executing: 

echo "/dev/zram0 none swap defaults,pri=10 0 0" >> /etc/fstab

Make sure to use >> and not > in this command, otherwise you will overwrite the existing configuration. An additional option 'nofail' can be set in case `/dev/zram0` is not always available on boot to treat it as optional.

Setting up multiple disks at boot

If you wish to create more than one zram-disk at boot, you can pass the number of disks you want to have to the kernel module, which is achieved by: 

echo options zram num_devices=N > /etc/modprobe.d/zram.conf

where N should be replaced by the number of zram-disks you want to create. Further you have to setup udev rules to format and fstab entries to automount also these disks.

Alternative Setup using zram-tools

Setup and usage

An easy alternative way to manage zram block devices is via zram-tools which can be installed on your Proxmox VE. 

apt install zram-tools

Enable zram

After installation, the systemd service “zramswap.service” automatically activates zram as swap device with the default configuration from /etc/default/zramswap 

# Compression algorithm selection
# speed: lz4 > zstd > lzo
# compression: zstd > lzo > lz4
# This is not inclusive of all that is available in latest kernels
# See /sys/block/zram0/comp_algorithm (when zram module is loaded) to see
# what is currently set and available for your kernel[1]
# [1]  https://github.com/torvalds/linux/blob/master/Documentation/blockdev/zram.txt#L86
#ALGO=lz4

# Specifies the amount of RAM that should be used for zram
# based on a percentage the total amount of available memory
# This takes precedence and overrides SIZE below
#PERCENT=50

# Specifies a static amount of RAM that should be used for
# the ZRAM devices, this is in MiB
#SIZE=256

# Specifies the priority for the swap devices, see swapon(2)
# for more details. Higher number = higher priority
# This should probably be higher than hdd/ssd swaps.
#PRIORITY=100

The status of the zram can be checked with the following command: 

 # zramswap status
NAME       ALGORITHM DISKSIZE DATA COMPR TOTAL STREAMS MOUNTPOINT
/dev/zram0 lz4           256M   4K   63B   20K      40 [SWAP]

The additional zram swap space should now be shown in the following command: 

free -h
               total        used        free      shared  buff/cache   available
Mem:           7.8Gi       1.3Gi       6.4Gi        28Mi       267Mi       6.4Gi
Swap:          255Mi          0B       255Mi

Disable zram

To disable your zram-disk you have to execute the following command: 

zramswap stop

Configuration

There are three things you can configure:

  • compression
  • amount of memory (default is 256MB)
  • priority

The size of the zram device can be configured either by absolute value (SIZE) or percent (PERCENT) in /etc/default/zramswap. e.g. 

PERCENT=20

To apply the new configuration, restart zramswap. 

zramswap restart

Limitation

When configuring zram, the size of a zram device controls the maximum uncompressed amount of data it can store, not the maximum compressed size. You can configure the zram's size to be equal to or even greater than your system's physical RAM capacity, as long as the compressed size on physical RAM will not exceed your system's physical RAM capacity.

Using zramswap with a physical swap is not recommended. Moving pages between zramswap and physical swap can cause high latencies and destabilize your system.


References

  1. ↑ https://www.kernel.org/doc/Documentation/blockdev/zram.txt
  2. ↑ https://en.wikipedia.org/wiki/Zram
  3. ↑ https://en.wikipedia.org/wiki/Zswap
  4. ↑ https://www.kernel.org/doc/Documentation/blockdev/zram.txt
  5. ↑ https://wiki.archlinux.org/index.php/Improving_performance#Zram_or_zswap
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