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Stop making swap partitions—use swap files instead

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Re: Stop making swap partitions—use swap files instead

#121

Post by jeroenhd »

optionalsquid wrote: post #27
Kim_Bruning wrote: post #11

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?

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

Swapfiles on BTRFS seem to work great if you allocate them early (with the command linked in your article) and put them in a different subvolume than the rest of your system.

BTRFS in general seems more I/O constrained than ext4 or xfs so it's probably still worse to put a swapfile on a BTRFS partition, but so far my systems are doing fine.

Re: Stop making swap partitions—use swap files instead

#122

Post by klodolph »

topspin wrote: post #76
RiverCrochet wrote: post #71

Earlier quoted context omitted.

was this before mmap(2)?

mmap(2) in Unix predates Linux itself by several years (1988 Sun, 1990 BSD). mmap(2) was present in Linux well before any of the high performance databases, file systems and other applications that use(d) O_DIRECT appeared on Linux. O_DIRECT was resisted by Torvalds and others, but it's there today, and used by supposedly important platforms. Nvidia even has an API to DMA data to and from GPUs that uses O_DIRECT. The…

Well-put

Re: Stop making swap partitions—use swap files instead

#123

Post by [deleted] »

Aurornis wrote: post #101
koala_man wrote: post #98

> Swap files have had the same performance characteristics as swap partitions for more than 20 years I thought so too, then I benchmarked it on a (spinning rust) HDD and found it was significantly slower: https://www.vidarholen.net/contents/blog/?p=1110 tl;dr: "In this test, using a swap file was surprisingly 50%+ slower than simply allocating a swap partition at the start of the drive, in spite of the low fragmentat…

> hen I benchmarked it on a (spinning rust) HDD Yes, but as your article notes HDDs have different performance characteristics depending on where the sectors are physically located on the platter. If you could force the swapfile to the same location on the platter it shouldn't perform differently. Most people are going to be using solid state where this doesn't matter.

[deleted]

Re: Stop making swap partitions—use swap files instead

#124

Post by joshAg »

throw0101d wrote: post #91
joshAg wrote: post #80

Earlier quoted context omitted.

Don't worry, it's not just you: https://lkml.org/lkml/2019/8/4/15 It's because linux is a toy OS. Specifically, it overcommits memory in the hope/assumption that it won't all be used at once, but doesn't have a way to gracefully degrade when applications collectively want to use more memory(+swap) than it actually has. You can turn off overcommit, but applications are designed with the overcommitting feature in mind,…

> It's because linux is a toy OS. Specifically, it overcommits memory …by default. It can be disabled via a sysctl : * https://www.kernel.org/doc/Documentation/vm/overcommit-accou...

You're right. I'm glad the very next sentence in my comment landed.

The problem with turning it off is that the system and applications have been architected assuming that it will be on, so things like fork/execing a memory heavy processes or allocating memory inside a cgroup (which still pretends overcommit is enabled and there's still no way to disable that assumption) that used to work fine might break with no good way to get them to work again. This comment (and siblings) have more specifics: https://news.ycombinator.com/item?id=27794237#27795199

Re: Stop making swap partitions—use swap files instead

#125

Post by jeroenhd »

killerstorm wrote: post #19
pasc1878 wrote: post #12

Earlier quoted context omitted.

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.)

These days with cryptenroll and friends, using the TPM is hardly an issue anymore. LVM and swapfiles both work well if you just create one large "everything but EFI" encrypted volume.

You'd need to keep secure boot on, preferably with your own keys loaded, to get all of the benefits, and that's a bit harder to automate.

Windows does all of this and more. It's harder on Linux because of a lack of tooling/interest/business incentives, and because on average Linux users want more control over their hardware than Windows users.

Re: Stop making swap partitions—use swap files instead

#126

Post by fc417fc802 »

jeroenhd wrote: post #119
wongarsu wrote: post #40

Earlier quoted context omitted.

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

Windows also does a neat trick Linux lacks: automatically adding more swap, up to a limit. Systems with loads of RAM barely lose any storage to swap, but once they do get hit, they can get many gigabytes of swap space without user interaction. I believe macOS does it too, of course. I'm sure there are many reasons why Linux can't do that by default, but it's a real shame.

I assume that's primarily because the stance of most distros would require something like that to be strictly opt-in. I don't know if it's possible to trigger a service based on overall swap usage? But given that the oom killer exists I don't see why it couldn't be trivially repurposed to add swap files on the fly.

Re: Stop making swap partitions—use swap files instead

#127

Post by jwilk »

cesarb wrote: post #106

> fallocate / chmod / mkswap Why not "mkswap --size ... --file ...", which does these three things and more? For instance, according to the mkswap man page, "[...] sets the nocow attribute for newly created files [...]" which is a detail that seems to be missing from this gist.

Also, fallocate+chmod is racy: between the two calls, an attacker could open the file, and then keep it open until next reboot. OTOH, mkswap creates the swap file with the correct permissions straight away.

The --size and --file options are relatively new, though: they were added in util-linux v2.40, released in 2024.

Re: Stop making swap partitions—use swap files instead

#128

Post by simoncion »

jenders wrote: post #32
holowoodman wrote: post #9

Stop using partitions. Use LVM!

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

My short answer is "Not frequently, but when I do, I'd be very angry about having to screw around repartitioning a disk instead of fiddling with an LV, VG, or some PVs.".

I use LVM on all my non-appliance Linux systems, [0] both servers and desktops. Given that few (no?) filesystems support online shrinking but every one that you'd use as '/' supports online growing, my strategy is to take a fairly conservative guess at how big my various subdirectories need to be and leave most of my available space unallocated. Later on, if I -say- find that I need more space in '/' or '/home', or prevent a blowup in '/var/log' from eating up all the space in '/' by putting that on a separate LV, etc, etc, it's just a few commands [1] and the job's done.

[0] ...someone else has configured the storage scheme for the appliances, and I'm not going to override their choices...

[1] ...and some data copying in the case of backing a subdirectory with a new volume...

Re: Stop making swap partitions—use swap files instead

#129

Post by fpoling »

rini17 wrote: post #86

Use zram instead. This from crappy 8G RAM multimedia/browsing minipc I'm on right now. The SSD is quite slow and browser is memory hungry so it was an improvement from on disk swap. $ zramctl NAME ALGORITHM DISKSIZE DATA COMPR TOTAL STREAMS MOUNTPOINT /dev/zram0 zstd 3G 482.9M 86.7M 91.8M 4 [SWAP]

On x64 CPU for laptops the memory bandwidth is rather limited and a fast compressor like lz4 can saturate it. As the result latest SSD are faster then z-ram when using hardware encryption on SSD.

Re: Stop making swap partitions—use swap files instead

#130

Post by linsomniac »

SoftTalker wrote: post #93

For a while since RAM sizes have gotten so big I would set up linux servers without swap at all. I figured what is a 2GB swap file going to do if the system has already exhausted 128GB of real RAM? I never saw any issues doing this, but I learned that linux ideally wants some swap space, which it will use for some kind of housekeeping stuff.

I have tried to forego swap because I had hoped that it would avoid swap thrashing, where performance goes into the toilet; I'd rather a process was killed but I could still access the system, than be unable to login to it.

But I've found that even with no/little swap, the system can get into high memory pressure situations (even easier because it can't swap out unused pages of libraries and the like), and still go into a sort of thrashing state even though it isn't swapping.

My main workstation is a Dell XPS15 with 32GB of RAM and I had 8GB of swap on it. About every month the system load would skyrocket even though it had plenty of mem+cache+swap available. If I killed off my browser and slack, load would return to normal, and I could use the system for a few more days or a week before it happened again.

I struggled with this for a year or more, before (for reason's I don't remember), I added another 8GB of swap. Within the next day 80% of the extra swap was used right up. I decided to swapoff that new file and create a 64GB swapfile and just see what happened. It got to around 20-30GB IIRC before it leveled off. And, more importantly, over the next 100+ days I never had that thrashing happen again.

That was with Ubuntu 22.04. I just recently (May-ish) switched over to a NixOS setup with 64GB of swapfile, and it's been very stable.

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