Tmpfs considered harmful

https://rwmj.wordpress.com/2012/09/12/tmpfs-considered-harmful/#content

by rwmj • 14 years ago

89 81 14 years ago

rwmj.wordpress.com

9 times out of 10, tmpfs is explicitly not for use mounted as /tmp. The only obvious exception I can think of are cluster nodes where the workload is specialized and the node has no local storage whatsoever.

I guess it only takes one silly person to connect "tmp" in the name to "/tmp", and the rest is history.

justincormack• 14 years ago

sysfs is mounted on /sys

debugfs is mounted on /sys/kernel/debug

securityfs is mounted on /sys/kernel/security

So tmpfs is mounted on /tmp? Can see that...

alexchamberlain4• 14 years ago

It makes sense in certain cases to use tmpfs for /tmp. However, I'm not sure this is on desktop machines, which (generally) have lower RAM than servers thrashing their /tmp directories.

noselasd• 14 years ago

Even in that case, using tmpfs on /tmp sounds like band aid. If there are applications that suffer performance because stuff it puts in /tmp , then change those applications - possibly by using an application specific tmpfs , or a new system wide tmpfs that doesn't overload something as ubiquitous /tmp

rwmj4• 14 years ago

The systemwide tmpfs already exists: /dev/shm.

praptak2• 14 years ago

Minor disagreement about the following: "Everyone must worry about whether their files need to survive a reboot"

I believe there was never such expectation of /tmp. On many systems it's being wiped periodically or even when the user's last session ends. The example app(s) that use /tmp for drafts need(s) to be fixed.

alexchamberlain4• 14 years ago

Totally agree, /tmp should be wiped at the end of each session.

sliverstorm• 14 years ago

Agreed; this prevents establishment of expectations otherwise.

dredmorbius• 14 years ago

The key is that it may be wiped. Files stored in /tmp are not guaranteed to survive boot. Whether or not they are there or not is a bit of a gamble (a regularly losing one should you choose tmpfs).

mccabe• 14 years ago

Surely thats what /var/tmp is intended for.

praptak2• 14 years ago

By "that" you probably mean "temporary files preserved between reboots". I agree :)

sehrope• 14 years ago

This is exactly what I use it for on my desktop. I have any app that I don't want to maintain longterm history for it's internal files (ex: Firefox browser cache) pointed to /tmp (mounted as tmpfs). As an added bonus if you're running your OS on an SSD (which you should be) you don't have it thrashed by constant writes/deletes.

The memory size issue is basically a non-issue at this point (pair of desktop 16 GB DIMMS are ~$80 and laptop isn't that much more).

RobAley4• 14 years ago

The memory size issue is very much still an issue. I happen to have globs of memory on my machine, but for the very reason that I have software to use it. If large tmp files start eating into the resources available to my programs, its an issue for me. Also, older hardware that has less memory available, and even newer hardware that can have more memory installed but with non-tech users who don't know how to upgrade it or even that its possible (or can't afford $80).

rwmj4• 14 years ago

Sure, but if the power goes down, there's at least a chance that I may be able to find that long email I was writing. With /tmp on tmpfs, it's gone.

colanderman• 14 years ago

That's why the GP considers mutt broken. Drafts should be stored in ~/.mutt or /var, not /tmp. Files in /tmp should not be expected to survive a process.

dubcroster• 14 years ago

I can't help but find that to be an issue of the MUA rather than that of the filesystem.

Although I agree with your points, I find that the design of one application alone cannot be a justifiable point.

chimeracoder3• 14 years ago

No, because /tmp is supposed to be assumed to be cleared on start up. If you want something to survive between reboots, /tmp is the last place you should put it, even if you're not using tmpfs.

I've never had a problem with this using mutt, but if I cared, I'd switch it to use /var/tmp.

Either way, if there's a problem, it's with the program, not the filesystem - no need to use a cannon to kill a fly.

ps20002• 14 years ago

I remember myself creating cron jobs to free wasted /tmp space

jaylevitt• 14 years ago

I can verify this - on my Fedora 12 box, I accidentally set up a test MySQL database into /tmp. The test database grew into my blog, and it worked great till I rebooted. Then I learned lots about recovering data. (I make regular backups, of coures, but who backs up /tmp?)

raverbashing2• 14 years ago

"Everyone must now be careful never to store a file in /tmp that might grow large"

If you're going to trash my /tmp with gigantic files please don't. Really

"it’s better to fix the filesystem to make it faster"

Yes, let's spend adding more complexity to the file system to work out every /tmp abuser out there. But his original point was avoiding adding complexity in the first place

So basically, he contradicts himself.

Mounting tmp as a memory backed fs is great (to which there may be better options than tmpfs), avoids (potentially) spinning up disks covered in rust for minor tasks.

lmm• 14 years ago

>Yes, let's spend adding more complexity to the file system to work out every /tmp abuser out there. But his original point was avoiding adding complexity in the first place

Better to add complexity once, in the filesystem, than in every program that writes to it.

w0utert• 14 years ago

> Mounting tmp as a memory backed fs is great (to which there may be better options than tmpfs), avoids (potentially) spinning up disks covered in rust for minor tasks.

I mount /tmp as a tmpfs for almost exactly this reason. I have an always-on home server/HTPC that runs the OS off of a small (40 GB) SSD, and I don't want to wear down the SSD with writes to /tmp, /var/tmp, /var/run, /var/log, etc. In theory, I don't want any writes at all, unless the thing is actually doing something useful. So I simply threw in 2GB extra RAM and mounted all these temporary/logging directories as tmpfs. Periodically (and on reboot) I zip the logs up and copy them to a backup directory just in case something fails.

Of course this runs the risk of losing temporary data and logs whenever the server goes down, but that's a trade-off I'm willing to make. It's been running just fine for over 2 years now. I'd do the same thing straight away if I ever need to set up some similar system.

pezz• 14 years ago

Yep - make your box do what you want.

Nothing stops you from changing fstab to make any mount tmpfs or disk.

RobAley4• 14 years ago

>If you're going to trash my /tmp with gigantic files please don't. Really

I write statistical programs that often generate temporary intermediate files of multiple GB size, which need to persist at the end of runs for trouble-shooting, partial re-runs etc. i.e. the software shouldn't delete them when it's finished, but they don't need to be kept long term. I kept forgetting to delete them when I finished a session and my disks kept filling up, so I decided to use /tmp and let the OS manage the space. Is there a better way to manage large tmp files than this that I'm missing?

raverbashing2• 14 years ago

Depends on your use case and how much of that complexity you wish to manage

In your case, what could work is having the new run use the same file names as the old run, or coming up with a rotation schedule (so you would keep the latest N runs)

But if having them on /tmp as it is working for you, great, no need to complicate things.

There are also speed considerations, if you're using these files as actual intermediate storage (as opposed to a log/debug dump) it may be interesting to have them written to a different disk (like an SSD)

mpyne• 14 years ago

Given that they should persist, the better way would be /var/tmp I would think, at least if persistence means "should survive reboot".

Even before shifting to tmpfs I'm pretty sure Fedora was erasing the contents of /tmp at startup (and I know Gentoo/OpenRC is).

There might be an even more appropriate FHS-compliant directory (/var/run perhaps?)

regularfry3• 14 years ago

Sounds like a job for /var/cache.

masklinn• 14 years ago

> If you're going to trash my /tmp with gigantic files please don't.

Where else are temporary files supposed to go, exactly? It's no abuse to put in /tmp temporary "scratch" data which have no reason to survive a process restart let alone a machine reboot, that's the mount point's explicit purpose.

notatoad2• 14 years ago

The /tmp on your system is not 'your' /tmp. The home directory is yours. /tmp exists pretty much solely as a place for programs to fill up with a mess of files, with the understanding that it will get automatically cleaned out and they aren't messing up the system proper. If a program needs to trash a place with large files, /tmp is where it should do it.

gizzlon• 14 years ago

Linkbait headline. Thought it was going to be about a gotcha with tmpfs in general, but it's only about one specific use of tmps in one specific -not even release- version of one linux distribution.

I get that the guy want's attention for this life-and-death issue, but linkbaiting is just mean :(

dmpk2k• 14 years ago

One of the hardest parts of writing a compiler is register allocation.

Not quite. There are many much more difficult problems in a competitive optimizing compiler before hitting that step.

You can get reasonable results from even a linear scan allocator, which is simple.

aidenn0• 14 years ago

It really depends on the architecture. There are some architectures with only 1 register, and others with many thousands of registers.

Current x86 implementations do a good job of being fast even when registers spill.

I've seen dual-issue Power cores (32ish registers) that only have 16k of 4-way dcache, which means register spills can easily kill performance.

asdfaoeu• 14 years ago

What's wrong with swapfiles?

keeperofdakeys2• 14 years ago

It should be mentioned that swapfiles are accessed directly, going around the filesystem. They can suffer from fragmentation if you don't create them when the filesystem is fresh, however.

buster2• 14 years ago

Oh, i hope they revert this change!

The big trouble for me would be "Everyone must now be careful never to store a file in /tmp that might grow large". I often use /tmp for all kinds of temporary storage. Be it a small text file or a GB big tar file.

ps20002• 14 years ago

Rare use case...

I remember CD burning softwares and torrent clients asking for special temp directories to be used for that.

Allowing every day applications to create GBs of tmp files would mean you would need to worry once your partition has less than 100 GB free.

However, nobody stops you from mounting /tmp anywhere else or even disable tmpfs.

buster2• 14 years ago

Very common for me, not so common for you. ;) And: when is use /tmp on my laptop, it gets cleared after reboot, so that's perfect for _temporary_ files..

keeperofdakeys2• 14 years ago

> "Everyone must now be careful never to store a file in /tmp that might grow large"

Be warned that when he says this, he doesn't mean it will eat all your memory, it will only grow upto a maximum size (by default half of your ram). When this is reached, it acts like any other filesystem that is full: you get write errors. There is actually another implementation called ramfs, which doesn't have such a size restriction. However, tmpfs is what is actually being implemented here. The ramfs itself is more of a 'toy' filesystem, also being quite limited, including not being able to use swap space (unlike tmpfs).

snorkel• 14 years ago

Bad title. tmpfs is good. Fedora maintainers assuming that all of /tmp should be in tmpfs is bad.

Apps are already in the habit of writing a lot of expendable garbage to /tmp that doesn't require the fast I/O of tmpfs. Clogging RAM with this junk is bad idea.

peterwwillis• 14 years ago

Example of exactly when to use hard disk and when to use tmpfs:

User browsing the web for a month without rebooting. Flash stores temporary files in /tmp/. For a month straight, Pandora is pumping out songs to /tmp/ as a cache. 43829 minutes in a month, imagine an average of 3 minutes per song that's 14609 songs, at an average 1.5 megabytes per song that's 7304 megabytes that's 7.13 gigabytes. Probably shouldn't be stored in RAM.

On the other hand, you have ganglia clusters of about 2,000 hosts. You have multiple gmetad's collecting data from different data sources and pooling it into your web interface box. It's updating ~10,000 tiny files every 30 seconds. Your i/o wait and system time is through the roof. You move the place gmetad writes to a tmpfs mount, and rsync to the disk every minute. Suddenly the i/o and system times are at 0.01%.

Tmpfs for /tmp/ is considered harmful, but extremely helpful in other cases.

armored_mammal• 14 years ago

My theory on this is that it's intended as a step to avoid SSDs writes.

chimeracoder3• 14 years ago

Exactly - this is a major benefit of using tmpfs on /tmp

dsr_• 14 years ago

The fact is, is that not every app that uses /tmp does so in a safe manner.

As long as that is true, it's not a good idea to use tmpfs for /tmp.

It's perfectly reasonable, however, to use tmpfs for, say, /var/local/tmp and let individual users point $TMP at that. Want to live fast and dangerously? Set $TMP.

alexchamberlain4• 14 years ago

This assumes processes use $TMP.

scotty79• 14 years ago

tmpfs is awesome to run rails from (i.e. for development).

IO bound becomes CPU bound so you're getting as good speed as you can get.

spartango• 14 years ago

I'd disagree with the more global point (in the title) that tmpfs is harmful: it's actually quite handy for all sorts of things.

As an amusing example, the SHM (shared memory) system in linux is actually implemented on top of tmpfs (/dev/shm is a tmpfs mount).

Other uses include CoW shadow mounting (when unioned to an immutable filesystem).

That said, the point that /tmp should not be a tmpfs mount is actually a decent one, just because we're used to the idea that /tmp is neither space-constrained (it's scratch space) nor extra performant.

krakensden• 14 years ago

I'm surprised no one is chanting "CADT" and linking to "What killed Linux on the desktop". This sort of subtle breakage over time is absolutely goddamn maddening.

notatoad2• 14 years ago

Why on earth would anybody ever use tmpfs for /tmp? If I'm writing a program that deals with a big file, and I want to temporarily write something to the filesystem, it is because I dont want it in memory. Why would you ever write to /tmp for any reason other than to get something out of ram for a while?

donavanm2• 14 years ago

Because a third party binary expects posix FS semantics for some data that does not require strong durability?

chimeracoder3• 14 years ago

I use /tmp all the time. I find myself needing to interact with things for which the filesystem/shell interface is appropriate, but for which I don't want to worry about having to clean out the directory before it gets too large.

Having it sit in RAM is great, because I get the speed without subjecting my disk to unnecessary writes - for certain operations, this can be a huge bonus.

lostnet• 14 years ago

Because tmpfs is RAM cached disk (swap) space just like any other filesystem?

Is there some better place than swap to accommodate /tmp?

B-Con• 14 years ago

This is very popular amongst people just working on desktop setups. They usually don't need /tmp to house anything too large. Arch Linux actually switched recently to mounting /tmp as tmpfs recently.

rogerbinns• 14 years ago

I have been using tmpfs for /tmp for many years (since 2005 IIRC). Note that I do have large amounts of swap space (typically 64GB).

One simple benefit is that files there do not get caught in backups - they are temporary detritus so I don't want backup space wasted by them.

The major benefit is that fsync is really slow on Linux. Typically it turns into a sync, and a lot of programs like to ensure you don't lose data by being fsync happy. For temporary stuff that is even more painful - you don't care about data loss - that is why it is in tmp in the first place.

TLDR: I prefer filesystem operations on transient/temporary files and data to run at the speed of RAM, and not end up in backups or waiting for syncs.

digikata• 14 years ago

tmpfs is useful for embedded systems where the primary filesystem is mounted read-only. Generally /tmp and /var may be tmpfs backed in that situation. Not sure why a desktop system would want this though.

donavanm2• 14 years ago

Here, let me man 7 hier that for you:

/tmp This directory contains temporary files which may be deleted with no notice, such as by a regular job or at system boot up.

binarycrusader• 14 years ago

Solaris has done this for many years; it's not the end of the world as the author believes.

tsahyt• 14 years ago

Running out of swap and memory is one thing, although unlikely considering how much of that stuff we've got nowadays. I for instance have 8gb of main memory and 10gb of swap space. 300mb are used after system boot (still too much). That leaves me with plenty of space to "waste" on temporary files in a tmpfs.

The Linux FHS explicitly states "Programs must not assume that any files or directories in /tmp are preserved between invocations of the program." That's not only reboots. It's not meant to be used to store drafts or anything that you might want to reopen at a later time. That's what we've got /var/tmp for. The FHS specifies as follows: "The /var/tmp directory is made available for programs that require temporary files or directories that are preserved between system reboots. Therefore, data stored in /var/tmp is more persistent than data in /tmp".

Tmpfs isn't harmful because of this, it's the default mutt configuration which is broken. If developers would have adhered to the standard the problem wouldn't exist. Since programs should adhere to the standard, because decisions like the one in question are made upon it, they should be reviewed and their default configurations should be changed in order to fix this issue.

Tmpfs considered harmful

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