Swap: Difference between revisions

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[[Category:Configuration]]

Swap ~~provides additional~~ virtual memory ~~by extending~~ physical RAM. This can be accomplished by using space on disk, such as [[#Swap file|swap file]] or [[#Swap partition|swap partition]], or through compression based methods like [[#Zram swap|zram]]. Additionally, [[#Zswap swap cache|zswap]] can act as a RAM-based compressed cache sitting in front of a traditional disk-based swap device.

Swap allows "cold" pages of virtual memory to be stored in places other than directly in the physical RAM, effectively allowing more pages to be stored. This can be accomplished by using space on disk, such as [[#Swap file|swap file]] or [[#Swap partition|swap partition]], or through compression based methods like [[#Zram swap|zram]]. Additionally, [[#Zswap swap cache|zswap]] can act as a RAM-based compressed cache sitting in front of a traditional disk-based swap device.

= Configuration =

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</syntaxhighlight>

A lower-impact option is <code>"discard=once"</code>, which runs discard exactly once when the swap is enabled, but does not continually issue discard commands as pages are ~~swapped out~~. This could make more sense depending on your hardware.

A lower-impact option is <code>"discard=once"</code>, which runs discard exactly once when the swap is enabled, but does not continually issue discard commands as pages are being overwritten. This could make more sense depending on your hardware.

<code>systemd-gpt-auto-generator(8)</code> does not automatically enable <code>discard</code>. Also, never enable <code>discard</code> on mdadm RAID setups, as ArchWiki reports that it causes lockup.

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swapDevices = [{

device = "/dev/~~sdXY~~";

device = "/dev/disk/by-partuuid/aaaaaaaaa-bbbb-cccc-dddd-0123456789ab";

randomEncryption.enable = true;

}];

</syntaxhighlight>

The selected device will have all its content made unusuable at every boot. Using a partuuid or partlabel is recommended because it is less subject to change when the overall partition scheme changes.

If you want to use TRIM, set <code>randomEncryption.allowDiscards</code> in addition to the <code>options</code>. This has the security implication of:

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You will need to weigh between the two.

Using a random key makes hibernation impossible. If you want to use hibernation, use a regular [[Full Disk Encryption]] (or partial disk encryption with LUKS) scheme with an unchanging key.

== Adjusting swap usage behaviour ==

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 [[Category:Configuration]]
-Swap provides additional virtual memory by extending physical RAM. This can be accomplished by using space on disk, such as [[#Swap file|swap file]] or [[#Swap partition|swap partition]], or through compression based methods like [[#Zram swap|zram]]. Additionally, [[#Zswap swap cache|zswap]] can act as a RAM-based compressed cache sitting in front of a traditional disk-based swap device.
+Swap allows "cold" pages of virtual memory to be stored in places other than directly in the physical RAM, effectively allowing more pages to be stored. This can be accomplished by using space on disk, such as [[#Swap file|swap file]] or [[#Swap partition|swap partition]], or through compression based methods like [[#Zram swap|zram]]. Additionally, [[#Zswap swap cache|zswap]] can act as a RAM-based compressed cache sitting in front of a traditional disk-based swap device.
 = Configuration =
@@ Line 142: / Line 142: @@
 </syntaxhighlight>
-A lower-impact option is <code>"discard=once"</code>, which runs discard exactly once when the swap is enabled, but does not continually issue discard commands as pages are swapped out. This could make more sense depending on your hardware.
+A lower-impact option is <code>"discard=once"</code>, which runs discard exactly once when the swap is enabled, but does not continually issue discard commands as pages are being overwritten. This could make more sense depending on your hardware.
 <code>systemd-gpt-auto-generator(8)</code> does not automatically enable <code>discard</code>. Also, never enable <code>discard</code> on mdadm RAID setups, as ArchWiki reports that it causes lockup.
@@ Line 154: / Line 154: @@
 <syntaxhighlight lang="nix">
 swapDevices = [{
-   device = "/dev/sdXY";
+   device = "/dev/disk/by-partuuid/aaaaaaaaa-bbbb-cccc-dddd-0123456789ab";
    randomEncryption.enable = true;
 }];
 </syntaxhighlight>
+The selected device will have all its content made unusuable at every boot. Using a partuuid or partlabel is recommended because it is less subject to change when the overall partition scheme changes.
 If you want to use TRIM, set <code>randomEncryption.allowDiscards</code> in addition to the <code>options</code>. This has the security implication of:
@@ Line 164: / Line 166: @@
 You will need to weigh between the two.
+Using a random key makes hibernation impossible. If you want to use hibernation, use a regular [[Full Disk Encryption]] (or partial disk encryption with LUKS) scheme with an unchanging key.
 == Adjusting swap usage behaviour ==