When Manual Partitioning for Qubes 4.2.4 Should I Declare a SWAP Partition? How Much?

General Discussion
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I’ve a Qubes-certified NovaCustom V56 laptop with 128GB RAM and TWO 4TB SSDs.

A popular rule of thumb for partitioning linux drives has been SWAP size = RAM size.

Does this hold for Qubes 4.2.4 or are there other Qubes-specific SWAP recommendations?

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It depends on your use case(s) and threat model.

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Obviously I don’t know your use case(s), but I would find it overkill to have swap if you already have 128gb of RAM. Further, for security purposes, swap is often set to zero anyway.

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I’ve never heard of a use case for a swap partition outside of extremely limited hardware (<16GB RAM); don’t create one.

A swap partition can only be used by dom0, which means it will go unused on your system, because very little ever happens in dom0. Swap space for your applications will be handled by each qube’s volatile volume, which is a separate block device.

Give dom0 6GB RAM and it will never ever get low on memory.

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To elaborate on this:

Whenever the system requests a new memory page, the memory manager must make an educated decision …

  1. Purge a page from the buffer cache.
  2. Purge a page from a mapping (which is effectively the same as 1 on most systems).
  3. Move a page that has not been accessed for a long time — preferably never — to swap. This could happen proactively, not necessarily at the very last moment.
  4. Kill your process or a random process (OOM).
  5. Kernel panic.

Options 4 and 5 are very undesirable and will only happen if the operating system has no other choice. Options 1 and 2 mean that you discard something you may need again soon. This negatively impacts performance.

Option 3 means you move something you probably won’t need anytime soon to slow storage. That’s fine, though, because now something you do need can use the fast RAM.

By removing option 3, you have effectively limited the operating system to either option 1 or 2. Reloading a page from disk is the same as reloading it from swap, except reloading from swap is less likely to happen (due to proper paging decisions being made).

In other words, disabling swap gains you nothing (performance wise) but limits the operating system’s options for dealing with a memory request. This might not be a disadvantage, but it very possibly may be (and will never be an advantage).

The one big caveat is security and secrecy: Could it be dangerous to store parts of your memory pages in a (more) persistent state?

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That seems to roughly track with the installer default, at least for the Btrfs installation layout: dom0 RAM is normally limited to 4096 MiB by a Xen command line parameter, and my swap partition is 4015 MiB. (Minus 16 MiB for a LUKS2 header, that would make it 3999 MiB of usable swap space. :thinking:)

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