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.
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?
> 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.
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).
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
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?
Notably windows doesn't use overcommit, and degrades much more gracefully under memory pressure. The biggest tradeoff is the amount of disk space consumed by a page file that also has to reserve space for unused pages that have been allocated but never been swapped in. On linux you can turn overcommit off, but there's too much software written around the assumption that overcommit is on
Is that still the case today? Notably (IIUC) overcommit is required for certain security measures. I believe it was chromium that I noticed mmaping somewhere north of 1 TB of memory on startup so that it can do (again IIUC) something akin to ASLR internally.
On Windows you can achieve something like manual overcommit by calling VirtualAlloc with just MEM_RESERVE. That gives you a continuous space in your process's virtual address space, without actually backing it with any physical pages. Kind of like what a malloc does on linux
But where linux would automagically back those pages once you use them, Windows requires you to actually ask for those pages to be backed by something (physical memory or page file) by calling VirtualAlloc with MEM_COMMIT on the range you actually want to use
At a glance that seems like a much more sensible design. I guess it's dead in the water for posix on account of fork being CoW? This is quite the rabbit hole. I wonder if programming languages ought to be designed in such a way to accommodate a preemptive signal indicating allocation failure in place of a page fault? Rather than malloc returning null or etc.
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.
Ah nice. I was dealing with that in one of our environments where a security update ended up causing apt to use more memory than usual so the oom killer nuked our elasticsearch process to "free up some memory".
And since that happened on all nodes around the same time, it took out the entire cluster. If you are not familiar, random, uncontrolled node restarts in any kind of multi node database or search product are a great way to trigger outages. So, not great.
I've had quite a few encounters with the oomkiller killing processes that were important and didn't need killing. Or as I like to phrase it "killing the one reason this server exists".
These days the way to size a server is "have enough memory to run whatever you need running + at least half a GB for whatever apt might randomly demand at any point". And guess what, memory tends to be expensive in cloud environments so people tend to get vms with as little as half a GB of ram.
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.
Remember when distros used complicated partition setups, one for /root, one for /var one for /home, a swap partition etc. Was always a bad choice because one of them would be at 99% while others would linger below 10%
For swap, the best advice is to disable swap on your desktop, unless <8GB RAM. Really, I've never needed it and you probably won't either.
On the Desktop fine-grained partitioning is a bit questionable but on a server it's still quite practical. E.g. preventing the database or excessive logs from filling the disk space. The latter is also an attack vector actually.
Also generally swap is recommended on all systems. During normal use it should rarely be needed but if it's needed it prevents spurious crashes. E.g. make -j16 shouldn't result in a hard to comprehend crash because I miss 1 GB of RAM.
I'm not sure if that's a bad choice... BTW, I think STIG still requires it. So, if you want to be compliant with some (US) government requirements, you'd still have to put different bits of your system on different partitions / devices.
Why I'm not sure this is a bad idea: a pathological process that quickly captures the entire allocated disk space will be contained by this layout and will likely not affect the system as a whole. Consider, for example, a process that excessively logs into /var/log: once the filesystem mounted at /var fills up, it might fail / hang, but you will still be able to use stuff from /bin or /sbin. But, if your entire root filesystem fills up, you probably won't be able to run any applications at all.
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.
zswap indeed is the real magic almost silver bullet nowadays. Device backed swap is often not needed anymore and it solves a lot of the issues with freezing under memory pressure, though it may not be ideal for servers.
And if I have a separate otherwise unused ssd then using a swap partition simply is easier than creating a file system for it and mounting it somewhere. Block level backups also are easier.
What would be nice about swap files is if they could dynamically grow and shrink without a special setup.
Way too many variables on Linux for a one size fits all.
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). I can’t guarantee a consistent enough workload to optimize ZFS for and I don’t feel like it’s general purpose enough. Large object storage though? I’m sitting on a PB of ZFS.
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.
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
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.)
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.
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.)
I have FDE with keys in the TPM. It's pretty smooth bit required a one time additional step in Fedora. It's also probably not very secure, but my threat model is simple theft.
How so? It's a one time setup procedure of only a few steps to create a tiny "protective" ext4 (or whatever) partition at the front. Thereafter you configure crypttab to mount the partition via UUID with an offset and key. If you don't need to hibernate you can read the key from /dev/urandom.
(ZFS might be described as the love child of lvm and btrfs, 'cept ZFS got there first)
( Very briefly: you hand ZFS a stack of disks (in mirrors or raidz groups, for redundancy) and it makes a zpool. Then you carve the pool into datasets as needed. Each dataset looks like an ordinary directory and takes only the space its files actually use, so you never guess partition sizes again. And each one can have its own compression, snapshot schedule, quota, encryption, and send/receive backups, etc etc etc. )
ZFS is overrated and mostly a hobbyist/enthusiast thing.
Performance of zvol is worse than for LVM logical volumes. Last I've looked, zvol for swap is just asking for trouble, up to and including deadlocks. ZFS filesystems do have weird edge cases and incompatibilities, e.g. around free space reporting, quota and the like. And support with software vendors e.g. for enterprise distributions or backup software is spotty at best.
>> ZFS is overrated and mostly a hobbyist/enthusiast thing.
Not true at all.. but most large object storage redundancy has moved to the application layer. There are still workloads that are a great fit but I agree OpenZFS is currently not well suited as a general purpose FS.
Not a btrfs user but OpenZFS u til recently didn’t even support O_DIRECT and may still duplicate page cache in arc. It’s not well suited as a general purpose FS where you can’t anticipate the workload, like on a desktop.
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.
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.
It's a terrible idea to delegate swap, a capability that is expected to operate under severe memory pressure, to a subsystem that fails under such conditions.
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.
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?
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.
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...
> 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.
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).
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.
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
> 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?
Notably windows doesn't use overcommit, and degrades much more gracefully under memory pressure. The biggest tradeoff is the amount of disk space consumed by a page file that also has to reserve space for unused pages that have been allocated but never been swapped in. On linux you can turn overcommit off, but there's too much software written around the assumption that overcommit is on
Is that still the case today? Notably (IIUC) overcommit is required for certain security measures. I believe it was chromium that I noticed mmaping somewhere north of 1 TB of memory on startup so that it can do (again IIUC) something akin to ASLR internally.
On Windows you can achieve something like manual overcommit by calling VirtualAlloc with just MEM_RESERVE. That gives you a continuous space in your process's virtual address space, without actually backing it with any physical pages. Kind of like what a malloc does on linux
But where linux would automagically back those pages once you use them, Windows requires you to actually ask for those pages to be backed by something (physical memory or page file) by calling VirtualAlloc with MEM_COMMIT on the range you actually want to use
At a glance that seems like a much more sensible design. I guess it's dead in the water for posix on account of fork being CoW? This is quite the rabbit hole. I wonder if programming languages ought to be designed in such a way to accommodate a preemptive signal indicating allocation failure in place of a page fault? Rather than malloc returning null or etc.
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.
> switch swapp off and use zram instead
Or zswap instead of zram.
https://chrisdown.name/2026/03/24/zswap-vs-zram-when-to-use-...
https://linuxblog.io/zswap-better-than-zram/
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".
This has finally been fixed in the latest Ubuntu version (26), it now force closes the culprit.
Ah nice. I was dealing with that in one of our environments where a security update ended up causing apt to use more memory than usual so the oom killer nuked our elasticsearch process to "free up some memory".
And since that happened on all nodes around the same time, it took out the entire cluster. If you are not familiar, random, uncontrolled node restarts in any kind of multi node database or search product are a great way to trigger outages. So, not great.
I've had quite a few encounters with the oomkiller killing processes that were important and didn't need killing. Or as I like to phrase it "killing the one reason this server exists".
These days the way to size a server is "have enough memory to run whatever you need running + at least half a GB for whatever apt might randomly demand at any point". And guess what, memory tends to be expensive in cloud environments so people tend to get vms with as little as half a GB of ram.
I always install earlyoom for that reason
not tangential at all...try setting up a swap partition
Triggered
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.
Remember when distros used complicated partition setups, one for /root, one for /var one for /home, a swap partition etc. Was always a bad choice because one of them would be at 99% while others would linger below 10%
For swap, the best advice is to disable swap on your desktop, unless <8GB RAM. Really, I've never needed it and you probably won't either.
On the Desktop fine-grained partitioning is a bit questionable but on a server it's still quite practical. E.g. preventing the database or excessive logs from filling the disk space. The latter is also an attack vector actually.
Also generally swap is recommended on all systems. During normal use it should rarely be needed but if it's needed it prevents spurious crashes. E.g. make -j16 shouldn't result in a hard to comprehend crash because I miss 1 GB of RAM.
Id always suggest at least 2-4G of swap even if you have plenty of memory 32g+ because for that rare edge case where you'd really not want to OOM.
Agreed. Buy more RAM but these days that means taking a HELOC
I'm not sure if that's a bad choice... BTW, I think STIG still requires it. So, if you want to be compliant with some (US) government requirements, you'd still have to put different bits of your system on different partitions / devices.
Why I'm not sure this is a bad idea: a pathological process that quickly captures the entire allocated disk space will be contained by this layout and will likely not affect the system as a whole. Consider, for example, a process that excessively logs into /var/log: once the filesystem mounted at /var fills up, it might fail / hang, but you will still be able to use stuff from /bin or /sbin. But, if your entire root filesystem fills up, you probably won't be able to run any applications at all.
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.
If you want to share step-by-step, I’ll update the gist.
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?
zswap indeed is the real magic almost silver bullet nowadays. Device backed swap is often not needed anymore and it solves a lot of the issues with freezing under memory pressure, though it may not be ideal for servers.
And if I have a separate otherwise unused ssd then using a swap partition simply is easier than creating a file system for it and mounting it somewhere. Block level backups also are easier.
What would be nice about swap files is if they could dynamically grow and shrink without a special setup.
Way too many variables on Linux for a one size fits all.
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). I can’t guarantee a consistent enough workload to optimize ZFS for and I don’t feel like it’s general purpose enough. Large object storage though? I’m sitting on a PB of ZFS.
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
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.
Truth be told—- I don’t use swap and set `vm.swappiness=0` but in these difficult times, swap may be the hero we need.
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
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.)
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.
Swap files are also much easier to set up than partitions if you're using full disk encryption.
Swap files are very fragile if you are using hibernation. For FDE I prefer LVM-over-LUKS and a dedicate partition
MacOS seems to have no problem with hibernating and it and its predecessor NeXT have been using swap files for over 30 years.
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.)
I have FDE with keys in the TPM. It's pretty smooth bit required a one time additional step in Fedora. It's also probably not very secure, but my threat model is simple theft.
What's the benefit of using the TPM compared to a simple passphrase at boot, in that scenario?
How so? It's a one time setup procedure of only a few steps to create a tiny "protective" ext4 (or whatever) partition at the front. Thereafter you configure crypttab to mount the partition via UUID with an offset and key. If you don't need to hibernate you can read the key from /dev/urandom.
It would be nice if the VFS had a hook for providing swap pages, making this manual futzing less necessary.
Stop using partitions. Use LVM!
Stop using LVM. Use ZFS!
(ZFS might be described as the love child of lvm and btrfs, 'cept ZFS got there first)
( Very briefly: you hand ZFS a stack of disks (in mirrors or raidz groups, for redundancy) and it makes a zpool. Then you carve the pool into datasets as needed. Each dataset looks like an ordinary directory and takes only the space its files actually use, so you never guess partition sizes again. And each one can have its own compression, snapshot schedule, quota, encryption, and send/receive backups, etc etc etc. )
ZFS is overrated and mostly a hobbyist/enthusiast thing.
Performance of zvol is worse than for LVM logical volumes. Last I've looked, zvol for swap is just asking for trouble, up to and including deadlocks. ZFS filesystems do have weird edge cases and incompatibilities, e.g. around free space reporting, quota and the like. And support with software vendors e.g. for enterprise distributions or backup software is spotty at best.
>> ZFS is overrated and mostly a hobbyist/enthusiast thing.
Not true at all.. but most large object storage redundancy has moved to the application layer. There are still workloads that are a great fit but I agree OpenZFS is currently not well suited as a general purpose FS.
I'll consider it when zfs in in tree.
Not a btrfs user but OpenZFS u til recently didn’t even support O_DIRECT and may still duplicate page cache in arc. It’s not well suited as a general purpose FS where you can’t anticipate the workload, like on a desktop.
How often are you needing to change partition geometry after install, on a server?
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.
Oh, the irony of:
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.systemd is malware.
Linux distros that do use swap files do so statically. It doesn't handle dynamically growing and shrinking it like other operating systems.
Unless you use the Linux package that does manage swap files dynamically.
https://github.com/Tookmund/Swapspace
It's a terrible idea to delegate swap, a capability that is expected to operate under severe memory pressure, to a subsystem that fails under such conditions.