View source for Swap/de

<languages />
<br />
__TOC__

=Übersicht=

Der Auslagerungsspeicher wird zur Erweiterung des für die Ausführung von Programmen verfügbaren Speichers (RAM) verwendet.
* Ohne Auslagerungsspeicher werden bei Speichermangel die Anwendungen ''beendet'', was zu einem Absturz des gesamten Systems führen kann.
* Wenn dem System Auslagerungsspeicher zur Verfügung steht, kann der Kernel weniger häufig genutzte Speicherseiten von inaktiven Anwendungen / Diensten auf die Festplatte schreiben ("auslagern"), wodurch mehr echter Arbeitsspeicher für aktive Anwendungen zur Verfügung steht.


Es gibt drei verschiedene Möglichkeiten, wie der Swap in Manjaro verwaltet werden kann:
* Eine Auslagerungs-Partition
* Eine Auslagerungs-Datei
* Komprimierte Auslagerung (zswap)


Dieser Artikel versucht, so umfassend wie möglich zu sein, aber noch mehr Informationen finden Sie in den  [[Special:MyLanguage/#See_Also|unten verlinkten Artikel]]
=Brauche ich eine Auslagerung?=

Das ist eine Frage, die nicht beantwortet werden kann, ohne einen Blick auf Ihre Konfiguration zu werfen. Selbst bei reichlich verfügbarem Speicher wird dieser oft als Auffangnetz oder sogar manchmal aufgrund spezifischer Anwendungsanforderungen verwendet, also werfen Sie einen Blick auf die folgende nicht abschließende Liste:
* Wenn Sie den Ruhezustand verwenden: ''ja, Sie müssen auslagern!''
* Wenn Sie Dienste haben, die nicht immer aktiv sind, aber trotzdem ständig laufen: ''ja, Sie müssen auslagern!''
* Wenn Sie eine Anwendung haben, die anstelle von RAM direkt virtuellen Speicher für temporäre Speicherung zuweist: ''ja, Sie müssen auslagern!''
* Wenn Sie eine Anwendung haben, die ein Speicherleck hat: ''ja, Sie müssen auslagern!''
* Wenn Sie einen Server mit 1 TB RAM haben, den Sie als Desktop verwenden, ohne dass Anwendungen virtuellen Speicher zuweisen oder Speicherlecks haben: ''Nein, Sie müssen nicht auslagern!''
=Wie viel Auslagerungspeicher brauche ich?=

Die benötigte Swap-Größe ist sehr variabel und hängt von ''Ihren'' spezifischen Anwendungen und der Arbeitslast ab.  Es gibt keine allgemeingültige Formel für die Größe des Auslagerungsspeichers, ohne die Nutzung über einen bestimmten Zeitraum hinweg zu überwachen.  Ein vernünftiger Ansatzpunkt wäre:
* Bei weniger als 4 GB physischem Speicher (RAM) wird dringend empfohlen, dass der Swap-Speicher mindestens so groß sein sollte wie der RAM-Speicher. Außerdem wird empfohlen, dass der Swap-Speicher maximal doppelt so groß ist wie der RAM-Speicher, abhängig von der Größe des für das System verfügbaren Festplattenplatzes, wegen des abnehmenden Nutzens.
* Bei moderneren Systemen (>4 GB) sollte Ihr Auslagerungsplatz mindestens der Größe Ihres physischen Speichers (RAM) entsprechen, '''wenn Sie den Ruhezustand verwenden''', andernfalls benötigen Sie mindestens ROUNDUP(SQRT(RAM)), d.h. die Quadratwurzel Ihrer Arbeitsspeichergröße aufgerundet auf das nächste GB. Das Maximum ist auch hier das Doppelte der Arbeitsspeichergröße, ebenfalls wegen des abnehmenden Nutzens.
* Der einzige Nachteil, wenn Sie mehr Auslagerungsspeicher haben, als Sie tatsächlich benötigen, ist, dass der dafür reservierte Speicherplatz nicht für Anwendungs- oder Systemdaten verwendet werden kann.

The "diminishing returns" means that if you need more swap space than twice your RAM size, you would be better off adding more RAM as Hard Disk Drive (HDD) access is about 10³ slower then RAM access, so something that would take 1 second, suddenly takes more then 15 minutes! And on a Solid State Drive (SSD) the same operation that took 1 second in RAM will still take about 1 minute on that SSD!

Taking into account all of the above, this brings us to the following table:
(last 3 columns denote swap space)

        RAM   No hibernation    With Hibernation  Maximum
        1GB              1GB                 2GB      2GB
        2GB              2GB                 3GB      4GB
        3GB              3GB                 5GB      6GB
        4GB              4GB                 6GB      8GB

        RAM   No hibernation    With Hibernation  Maximum
        5GB              2GB                 7GB     10GB
        6GB              2GB                 8GB     12GB
        8GB              3GB                11GB     16GB
       12GB              3GB                15GB     24GB
       16GB              4GB                20GB     32GB
       24GB              5GB                29GB     48GB
       32GB              6GB                38GB     64GB
       64GB              8GB                72GB    128GB
      128GB             11GB               139GB    256GB
      256GB             16GB               272GB    512GB
      512GB             23GB               535GB      1TB
        1TB             32GB              1056GB      2TB
        2TB             46GB              2094GB      4TB
        4TB             64GB              4160GB      8TB
        8TB             91GB              8283GB     16TB
{{note|The largest server one of the authors of this wiki has ever installed had, indeed, 8TB of RAM}}
=Displaying Swap Information=

The command {{ic|swapon}} will display your current swap information.  For example:
 swapon
 NAME      TYPE      SIZE   USED PRIO
 /dev/sda7 partition  20G  44.3M   -2

The following script will:
* show whether zswap is active or not and if active, give zswap parameters if run with the {{$ic|sudo}} command
* display a list of all applications / services that take up swap and how much they take up in descending order

 #!/bin/bash
 #Check whether running as root
 if [ "$(whoami)" = 'root' ]; then
   dmesg | grep "zswap:" | grep --silent "load"
   if [[ $? -eq 0 ]]; then
     # zswap is active
     echo "zswap information:"
     grep --recursive --color=none . /sys/kernel/debug/zswap/
     read -n 1 -s -r -p "Press any key to continue"
   else
     echo "[warning] zwap not active. Continuing"
   fi
 else
   echo "[warning] Not running as root: skipping zswap info"
 fi
 for szFile in /proc/*/status ; do
   awk '/VmSwap|Name/{printf $2 "\t" $3}END{ print "" }' "$szFile"
 done | sort --key 2 --numeric --reverse | more
=Using a Swap Partition=

A swap partition is the traditional way of managing swap.  In this scenario, a dedicated partition (or partitions) are created for holding swap.
==Creating and Enabling a Swap Partition==

To create a swap partition, you need enough unallocated disk space to create an additional partition. If you do not have enough space, skip to the next section [[Special:MyLanguage/#Using_a_swap_file|#Using_a_swap_file]]. A swap partition can be created in any disk management / partition management tool and should be set as type {{ic|linuxswap}}.

Once you have a swap partition you will need to initialize the swap partition with {{ic|mkswap}}.  For example, if your swap partition is {{ic|/dev/sda3}}, you could use the command:
 sudo mkswap /dev/sda3

Next we need to enable the swap partition with the {{ic|swapon}} command. Following our example above this could be done with:
 sudo swapon /dev/sda3

In order to ensure that the swap is enabled at boot we can add an entry to {{ic|/etc/fstab}}.  It is best to use the UUID instead of the device name for this purpose.  You can add the line to fstab manually or using the command:
 sudo bash -c "echo UUID=$(lsblk -no UUID /dev/sda3) none swap defaults 0 0 >> /etc/fstab"

If you would like to have more than one swap partition, simply repeat the steps above for any additional partitions.

{{note|Be sure to replace /dev/sda3 in the above commands with your actual swap partition.}}
{{tip|''If you're using a HDD'' (spinning rust) put the swap partition *at the beginning of the disk* as the speed of the disk is higher on the inside tracks. On an SSD this doesn't matter.}}
=Using a Swapfile=

Using a swap partition has one major disadvantage:  Changing the size of swap or adding swap requires repartitioning the disk.  In current Linux kernels, it is possible to use a swap file instead of a dedicated partition and as from kernel 2.6 onwards there is no performance difference any more between the two.

==Creating and Enabling a Static Swapfile==
{{tip|There is no reason you can't have both a swap partition and a swapfile.  This is an easy way to add more swap without repartitioning.}}


First create and intialize the file to hold the swap.  For example, to create a 4GB swapfile, you could use the command:
 sudo fallocate -l 4G /swapfile
 sudo mkswap /swapfile


Set the appropriate permissions on the file.  It should be readable and writable only by {{ic|root}}.  This can be done with the command:
 sudo chmod u=rw,go= /swapfile


Next we need to enable the swapfile with the {{ic|swapon}} command.  Following our example above this could be done with:
 sudo swapon /swapfile


In order to ensure that the swap is enabled at boot we can add an entry to {{ic|/etc/fstab}}.  You can add the line to fstab manually or using the command:
 sudo bash -c "echo /swapfile none swap defaults 0 0 >> /etc/fstab"

==Swapfiles on BTRFS==

As of kernel 5.0 and higher, swapfiles are supported on btfrs.  They still require some special handling in addition to the above steps.

Prior to running the {{ic|fallocate}} step above, you should run these commands:
 sudo truncate -s 0 /swapfile
 sudo chattr +C /swapfile
 sudo btrfs property set /swapfile compression none


These commands create an empty swapfile, disable COW for that file and ensure that compression is disabled.

==Swapfiles on ZFS==

zfs doesn't support swapfiles, however you can achieve a similar benefit using a zvol as a swap volume.


Detailed instructions on how to accomplish this can be found in this [https://github.com/zfsonlinux/pkg-zfs/wiki/HOWTO-use-a-zvol-as-a-swap-device ZoL guide].
=Automated Swap Management with systemd-swap=
{{note|Using systemd-swap with on btrfs/zfs or with hibernation support requires special handling beyond the scope of this article}}

It is possible to automatically manage the size of your swapfile using {{ic|systemd-swap}}.  It can create swap files dynamically based on memory needs.  It is also an easy way to enable/manage [[#Using_zswap_with_systemd-swap|zswap]]

==Installing and Configuring systemd-swap==

First, install and enable {{ic|systemd-swap}}:
 pamac install systemd-swap
 sudo systemctl enable systemd-swap.service


Next enable dynamic swap management by creating a file {{ic|/etc/systemd/swap.conf.d/myswap.conf}} with the following parameters:
 zswap_enabled=0
 zram_enabled=0
 swapfc_enabled=1


If you would prefer a single command to create this file you could use:
 sudo bash -c 'echo -e "zswap_enabled=0\nzram_enabled=0\nswapfc_enabled=1" > /etc/systemd/swap.conf.d/myswap.conf'

==Remove Other Swap==

In order for systemd-swap to be effective you need to remove any traditional swap devices you have.


To display your current swap devices you can use the command {{ic|swapon}}.  For example, my test machine displays the following:
 swapon
 NAME      TYPE      SIZE USED PRIO
 /swapfile file        4G   0B   -2
 /dev/sda3 partition 8.4G   0B   -3


In this example there are two swap devices.  A swapfile and a swap partition.  To turn them off we can use the command {{ic|swapoff}}.
 sudo swapoff /swapfile
 sudo swapoff /dev/sda3


Next we need to remove them.  
The swap partition will need to be deleted using your favorite partitioning tool.  You can remove the swapfile by simply deleting it.
 sudo rm /swapfile


We need to also remove any lines related to swap from {{ic|/etc/fstab}}.  Edit the file with an [[Special:MyLanguage/Viewing_and_editing_configuration_files|appropriate text editor]] and remove or comment out the lines for the swap space.  In the example above, we would remove these lines:
 UUID=c2430cf1-8ea9-4422-a5c5-5a38779194c3 swap           swap    defaults,noatime 0 2
 /swapfile none swap defaults 0 0


Next reboot.  After the reboot, when running {{ic|swapon}} you should see something similiar to the following:
 NAME                           TYPE SIZE USED PRIO
 /var/lib/systemd-swap/swapfc/1 file 512M   0B   -2


systemd-swap will add and remove swapfiles as your memory usage dictates.

{{note|If you don't delete your existing swap partition it will be automatically activated by systemd even though you have removed it from fstab}}

==Using zswap with systemd-swap==

zswap is compressed swap kept in RAM. zswap keeps the most frequently used pages in RAM and writes less frequently used pages to the swap space on disk.  In many workloads, this will result in increased swap performance and less wear and tear on SSDs.

To enable zswap with systemd-swap, simply alter the instructions above and set Linux Kernel parameter {{ic|zswap_enabled=1}}
=Tuning & Performance Considerations=

Although swap seems like a great way to expand memory, excessive swap use will cause severe performance degradation, as mentioned before.

There are couple of parameters that can be used to tune swap utilization.  These are swappiness and vfs_cache_pressure.  To see your current settings for these you can use the following commands:
 cat /proc/sys/vm/swappiness
 cat /proc/sys/vm/vfs_cache_pressure

{{ic|swappiness}} controls how likely a page is to be transferred to swap.  This value represents the percentage of the free memory before activating swap. The lower the value, the less swapping is used and the more memory pages are kept in physical memory where:
* 0 disables swap
* 60 is the default value which is ideal for a server running a lot of services
* 100 is very aggressive swapping.

For ''most'' computers the recommended value is 10: Theoretically, this means to only start swapping when RAM usage reaches around 90 percent.

{{ic|vfs_cache_pressure}} is a percentage value that controls the tendency of the kernel to ''reclaim'' the memory which is used for caching of directory and inode objects.  The default value is 100.  Increasing this value will increase the rate in which these objects are removed from the RAM cache.  Decreasing it will allow these objects to be cached in memory longer, consuming additional RAM over time.  Depending on your specific workload, increasing or decreasing this value too far can have significant negative impacts on system performance.  Experimentation is needed to find the appropriate balance and the default value is reasonable.  In general, it is more common to optimize swappiness before experimenting with vfs_cache_pressure.

To set these values you can use the command {{ic|sysctl}}.  For example, to set the swappiness value to 10 you could use:
 sudo sysctl vm.swappiness=10


'''There is no preset defined answer on the optimal values for these parameters.'''  Experimentation is needed to find the optimum configuration for your specific hardware and workload.


=See Also=
* The Arch Wiki page on [https://wiki.archlinux.org/index.php/Swap swap]
* The Arch Wiki page on [https://wiki.archlinux.org/index.php/Zswap zswap]
* The Linux kernel documentation on [https://www.kernel.org/doc/Documentation/vm/zswap.txt zswap]
* The Linux kernel documentation on [https://www.kernel.org/doc/Documentation/sysctl/vm.txt swappiness and vfs_cache_pressure]
* The openSUSE guide to [https://doc.opensuse.org/documentation/leap/tuning/html/book.sle.tuning/cha.tuning.memory.html tuning memory]
* The [https://github.com/Nefelim4ag/systemd-swap systemd-swap page]
* The ZFS on Linux guide on [https://github.com/zfsonlinux/pkg-zfs/wiki/HOWTO-use-a-zvol-as-a-swap-device Swap Volumes]


[[Category:Contents Page]]