Stop making swap partitions–use swap files instead

jjenders 2 days ago 37 commentsRead Article on gist.github.com

⚡ Community Insights

Discussion Sentiment

58% Positive

Analyzed from 1262 words in the discussion.

Discussion (37 Comments)Read Original on HackerNews

weinzierl•about 2 hours ago

There used to be a time when not only the OS required using partitions for optimum performace (swapfiles) but also applications.

In the late 90s databases were regularly set up in a way to store their data on raw partitions. There were other types of applications too that required partitions but for databases it was really common.

The practice really only died with OSes allowing apps to bypass the normal filesystem cache.

In Linux this is O_DIRECT and interestingly Linux was super late to the party because Linus hated it with a passion. Now, he was far from the only one, at least Alan Cox and Andrew Morton were skeptical, others as well. Linus was unusually extreme about it and that particular discussion is probably the single source of the majority of quotes people use against him.

mcculley•7 minutes ago

It would be nice if the VFS had a hook for providing swap pages, making this manual futzing less necessary.

crabbone•3 minutes ago

Oh, the irony of:

    echo "/swapfile none swap defaults 0 0" >> /etc/fstab

If we are going full modern, why not create a Systemd unit? ( https://www.freedesktop.org/software/systemd/man/latest/syst... ) if you need more info. If you didn't know this: /etc/fstab is not used directly after the pivot. Systemd parses it, creates units for every record in it, and that's what gets executed in reality. I can't imagine why would you want a swap file before pivot.

ducktective•about 1 hour ago

Tangential, but does anyone know why in 2026 and on Debian 13, my machine still hangs when some process exhausts RAM?

Is there really no higher-priority kernel process to prevent total freeze of the system and send a SIGKILL to the culprit process when such a scenario happens?

BoredomIsFun•6 minutes ago

It does not if you switch swapp off and use zram instead. I am typing right now on such a setup wityh 16 GiB ram and it occasionally, once a week or so, kills my firefox due to oom.

If you are you using disk swap - not sure why would if you have a SSD, but I once heard some justification for doing that - then install early OOM.

pndc•9 minutes ago

I had a 1GB Debian VM which started freezing (requiring a hard reboot) after a routine aptitude upgrade to apply security patches. It was indeed caused by low memory, but not out of memory as there was still enough swap space remaining.

The culprit turned out to be the kernel itself, and rolling back to a 6.1 series kernel made the problem go away. I see that Linus's love for vibe-coding is already paying dividends.

dev_hugepages•about 1 hour ago

This is called an oomkiller. The kernel has one but it kicks in very late and the kernel prefers to do page trashing instead of killing processes.

systemd-oomd should be integrated in systemd, you can configure it to your liking and see if it improves your problem.

ButlerianJihad•36 minutes ago

I believe that you mean: https://en.wikipedia.org/wiki/Thrashing_(computer_science)

Chris Siebenmann discusses when the OOM killer triggers: https://utcc.utoronto.ca/~cks/space/blog/linux/OOMKillerWhen

Chris disables systemd-oomd after it obliterates his X session with no explanation: https://utcc.utoronto.ca/~cks/space/blog/linux/SystemdOomdNo...

rovr138•29 minutes ago

> First off, this is exactly how systemd-oomd is supposed to behave under memory pressure. The documentation is specific on this; systemd-oomd itself says:

> > [...] If the configured limits are exceeded, systemd-oomd will select a cgroup to terminate, and send SIGKILL to all processes in it. [...]

> By having the user@.service template be enrolled in systemd-oomd, Fedora made the cgroup that systemd-oomd would select to be killed be all of your processes (across all of your sessions, if you have more than one). ...

Maybe *Fedora* has fixed or improved in the last 4 years. Or maybe they don't run Fedora.

afishhh•25 minutes ago

Set `/sys/kernel/mm/lru_gen/min_ttl_ms` at boot (see https://docs.kernel.org/admin-guide/mm/multigen_lru.html).

User-space OOM killers never really worked for me and imo are not a proper solution anyway. This option instead lets you make the kernel OOM killer actually work for desktop use.

Currently have it set to `1000` and it works very well for me (don't remember the last time I had a full system freeze due to OOM).

fc417fc802•11 minutes ago

> does anyone know why

In a nutshell, overcommit. It's more or less broken by design but it's also incredibly practical so pretty much everyone does it.

Couple that with the fact that it's difficult bordering on impossible to correctly determine the culprit. If you've got 16 GB RAM and the user launches 3 processes each of which attempts to use 8 GB who should you kill?

negrel•about 1 hour ago

OS are designed to fully exploit available resources, Linux tries its best before triggering an OOM kill.

I recommend using the earlyoom if you want more aggresive oom kill:

https://github.com/rfjakob/earlyoom

The README contains a lot of interesting information.

oomedfornight•17 minutes ago

A strange behavior I sometimes run into with earlyoom is that I try to start up some buggy software of mine and it seemingly never starts.

It took a long evening to figure out that it gets earlyoom'd immediately because it tries to allocate too much. Previously the very familiar hitching and freezing was a very easy sign of what kind of issue I was dealing with

trompetenaccoun•15 minutes ago

This has finally been fixed in the latest Ubuntu version (26), it now force closes the culprit.

CoastalCoder•30 minutes ago

Just mentioning this in case it's helpful:

If you know ahead of time which programs / processes are at risk of unacceptably high memory usage, check out "ulimit".

elektrontamer•about 1 hour ago

I always install earlyoom for that reason

pestatije•18 minutes ago

not tangential at all...try setting up a swap partition

jenders•12 minutes ago

Triggered

cfstras•about 1 hour ago

swap files make setting up hibernation a bit more complicated - in most cases the file has to be contiguous, e.g. you have to defragment it once after allocating. Then, you need to tell the bootloader the byte offset of the file on the partition.

jenders•11 minutes ago

If you want to share step-by-step, I’ll update the gist.

Kim_Bruning•about 1 hour ago

Every rule has an exception. Beware of this advice on zfs, for instance.

Meanwhile zswap (of no relation with zfs) is free performance. Is it a rule that everything starting with a 'z' must be cool?

jenders•4 minutes ago

That’s a good call out. I’ve never used ZFS on root. Been a ZFS user on Linux since the FUSE days but it has never seemed particular well suited (O_DIRECT support came late, ARC/page cache redundancy, etc)

optionalsquid•15 minutes ago

It also doesn't sound like a particularly good idea on BTRFS, judging by the linked documentation:

https://btrfs.readthedocs.io/en/latest/Swapfile.html

fc417fc802•3 minutes ago

And since you should probably be running a modern CoW filesystem then you probably shouldn't be using swapfiles ... is the conclusion I arrived at anyway. Either install multiple times more RAM than you need and enable zswap or else set aside a swap partition as has always been done.

dwedge•about 3 hours ago

Interestingly I always use swapfiles and this page made me realise there can be a performance impact if the filesystem is old - the lkml link says as long as the swap file isn't fragmented there's no impact

WalterGR•about 2 hours ago

In practice, there’s a potentially tremendous impact if you’re using a spinning metal disk. Sectors at the beginning of the disk (which is at the outer edge) are read/written way faster than sectors at the end. File systems tend to prefer writing files at the beginning. If the file system has been used for ‘a long time’ then it’s either fragmented - or it’s been defragmented and the free space is at the slow end of the drive.

One nice thing about swap partitions is that they can be fixed in place even before OS installation.

(Of course, if you’re using swap enough for this to be a huge factor, you probably have bigger problems. But I haven’t done the math.)

gblargg•about 2 hours ago

Indeed. I'd always put swap near the beginning, then a small root partition, then the rest. This reduced head movement for often-accessed data, and used the higher bandwidth of the outer tracks.

holowoodman•about 2 hours ago

Stop using partitions. Use LVM!

jenders•9 minutes ago

How often are you needing to change partition geometry after install, on a server?

holowoodman•4 minutes ago

Depends. Some customers do run a lot of "pets, not cattle" servers, and those can get a few extra hard disks if space runs out. Some security guidelines prescribe separate volumes/filesystems for e.g. /home or /var/log, so log spam doesn't kill the whole system as easily. But of course, this means that sometimes the initial sizes aren't right and you need to extend something. In those cases, which happen a few times per year, not using LVM would be a huge problem.

But even for the "frequently reinstalled cattle"-usecase, LVM has benefits, e.g. snapshots for backups, debugging and forensics.

mdspan•2 days ago

Swap files are also much easier to set up than partitions if you're using full disk encryption.

demomode•about 3 hours ago

Swap files are very fragile if you are using hibernation. For FDE I prefer LVM-over-LUKS and a dedicate partition

pasc1878•about 1 hour ago

MacOS seems to have no problem with hibernating and it and its predecessor NeXT have been using swap files for over 30 years.

killerstorm•37 minutes ago

MacOS has no problem with hibernation because hardware and software are designed together: it uses TPM to manage swap encryption. I assume "BIOS" is also optimized to enable fast boot on sleep.

Linux can work with TPM but it's never as smooth. (Unless I guess you make it custom for your specific setup.)

charcircuit•about 2 hours ago

Linux distros that do use swap files do so statically. It doesn't handle dynamically growing and shrinking it like other operating systems.

0xbadcafebee•about 2 hours ago

Unless you use the Linux package that does manage swap files dynamically.