Running out of disk space in production
alt-romes.github.io184 points by romes 5 days ago
184 points by romes 5 days ago
A neat trick I was told is to always have ballast files on your systems. Just a few GiB of zeros that you can delete in cases like this. This won't fix the problem, but will buy you time and free space for stuff like lock files so you can get a working system.
Better fill those files with random bytes, to ensure the filesystem doesn’t apply some “I don’t actually have to store all-zero blocks” sparse-file optimization. To my knowledge no non-compressing file system currently does this, but who knows about the future.
XFS, Ext4, btrfs etc… all support sparse files, so any app can cause problems you can try it with:
dd if=/dev/zero of=sparse_file.img bs=1M count=0 seek=1024
Much harder problem than the file system layers to deal with because the stat size will look smaller usually.
Creating sparse files requires the application to purposefully use special calls like fallocate() or seek beyond EOF, like dd with conv=sparse does. You won't accidentally create a sparse file just by filling a file with zeros.
It is an observability issue, even zabbix tracked reserve space and inodes 20 years ago.
Will dedupe,compression,sparse files you simply don’t track utilization by clients view, which is what du does.
The concrete implementation is what matters and what is, as this case demonstrates, is what you should alert on.
Inodes, blocks, extents etc.. are what matters, not the user view of data size.
Even with rrdtool you could set reasonable alerts, but the heuristics of someone exploding a sparse file with a non-sparse copy makes that harder.
Rsync ssh etc… will do that by default.
Yep, btrfs will happily do this to you. I verified it the hard way
Well btrfs supports compression so that’s understandable. However I personally prefer to control compression manually so it only compresses files marked by me for compression using chattr(1).
I've switched to that also. It surely wastes some space but being able to reason about file space is worth it to me for now
If I recall correctly:
dd if=/dev/urandom of=/home/myrandomfile bs=1 count=NI just use fallocate to create a 1GB or 2GB file, depending on the total storage size. It has saved me twice now. I had a nasty issue with a docker container log quickly filling up the 1GB space before I could even identify the problem, causing the shell to break down and commands to fail. After that, I started creating a 2GB file.
My choice has always been `shred`:
$ sudo truncate --size 1G /emergency-space
$ sudo shred /emergency-space
If you want to do it really quickly
openssl enc -aes-256-ctr -pbkdf2 -pass pass:"$(date '+%s')" < /dev/zero | dd of=/home/myrandomfile bs=1M count=1024
That's very cool. Sadly running that exact command gets an incomplete file and error "error writing output file". It suggests adding iflag=fullblock (to dd). Running that makes a file of the correct size. But still gives "error writing output file". I suppose that occurs because dd breaks the pipe.
Weird, I could have sworn that used to work, maybe I wrote the notes down wrong.
Easiest alternative I guess is to pipe through head. It still grumbles, but it does work
openssl enc -aes-256-ctr -pbkdf2 -pass pass:"$(date '+%s')" < /dev/zero | head -c 10M > fooFwiw you can also do this with
head -c 1G /dev/urandom > /home/myrandomfile
bs=1 is a recipe for waiting far longer than you have to because of the overhead of the system calls. Better bs=N count=1
That’s also not great if you’re trying to make a 10 gigabyte file. In that case, use bs=1M and count=SizeInMB.
Modern computers are crazily overengineered...
Most current desktops (smaller than your usual server) won't have any problem with the GP's command. Yours is still better, of course.
Similarly, I always leave some space unallocated on LMV volume groups. It means that I can temporarily expand a volume easily if needed.
It also serves to leave some space unused to help out the wear-levelling on the SSDs on which the RAID array that is the PV¹ for LVM. I'm, not 100% sure this is needed any more² but I've not looked into that sufficiently so until I do I'll keep the habit.
--------
[1] if there are multiple PVs, from different drives/arrays, in the VG, then you might need to manually skip a bit on each one because LVM will naturally fill one before using the next. Just allocate a small LV specially on each and don't use it. You can remove one/all of them and add the extents to the fill LV if/when needed. Giving it a useful name also reminds you why that bit of space is carved out.
[2] drives under-allocate by default IIRC
Not needed. All your unused/unfilled space is that space for wear-leveling. It wasn't needed even back then besides some corner cases. And most importantly 10% of the drive in ~2010 were 6-12GB, nowadays it's 50-100GB at least.
Empty space is good for wear-leveling but enforcing a few percent extra helps.
> And most importantly 10% of the drive in ~2010 were 6-12GB, nowadays it's 50-100GB at least.
Back then you were paying about $2 per gigabyte. Right now SSDs are 1/15th as expensive. If we use the prices from last year they're 1/30th, and if we also factor in inflation it's around 1/50th.
So while I would say to use a lower percentage as space increases, 50-100GB is no problem at all.
> A neat trick I was told is to always have ballast files on your systems.
ZFS has a "reservation" mechanism that's handy:
> The minimum amount of space guaranteed to a dataset, not including its descendants. When the amount of space used is below this value, the dataset is treated as if it were taking up the amount of space specified by refreservation. The refreservation reservation is accounted for in the parent datasets' space used, and counts against the parent datasets' quotas and reservations.
* https://openzfs.github.io/openzfs-docs/man/master/7/zfsprops...
Quotas prevent users/groups/directories (ZFS datasets) from using too much space, but reservations ensure that particular areas always have a minimum amount set aside for them.
Typo; link should be:
* https://openzfs.github.io/openzfs-docs/man/master/7/zfsprops...
Addendum: there's also the built-in compression functionality:
> When set to on (the default), indicates that the current default compression algorithm should be used. The default balances compression and decompression speed, with compression ratio and is expected to work well on a wide variety of workloads. Unlike all other settings for this property, on does not select a fixed compression type. As new compression algorithms are added to ZFS and enabled on a pool, the default compression algorithm may change. The current default compression algorithm is either lzjb or, if the lz4_compress feature is enabled, lz4.
* https://openzfs.github.io/openzfs-docs/man/master/7/zfsprops...
Also if you VMs on a disk backed by ZFS it's trivial to extend those disks provided you actually do have space on the real disk. (Even automatic with LXC).
This trick is actually used by some banking apps.
They fill app their mobile apps with junk data just to make the APK/IPA bigger. So if they need to push an urgent update, they won't have users that can't update because their phones are full to the brim.
I know two Italian banks that do it, Unicredit and Intesa. The latter was on the news when a user found out that one of the filler files was a burp recording [1].
[1] https://www.ilfattoquotidiano.it/2024/12/20/intesa-san-paolo... (in Italian)
Doesnt this create an arms race situation where every 'critical' app claims a larger diskspace than necessary, just in case, and accelerates the issue?
Kinda sorta, but there's a limit where users will typically install X apps and apps of Y size need Z extra space to update. User content would fill up the rest. I would imagine a typical 256 gb phone is probably over this limit and people who take lots of videos/photos just need to clean up their phone a little more often.
But you still need a bunch of extra space to download and unpack the new version, and there are so many apps that need to share space, and a banking app should only need about 0.1% of a phone's storage...
Whoever gave them that idea was doing a bad deed.
This saved us a couple times. At least until I had time to add monitoring to their old system to track disk usage. It was also helpful to use a tool called ncdu. It helps you visualize where most disk space is getting used up to track down the problem.
I always called it a “bit-mass”. Like a thermal mass used in freezers in places where the power is not very stable.
I knew I didn’t invent the concept, as there’s so many systems that cannot recover if the disk is totally full. (a write may be required in many systems in order to execute an instruction to remove things gracefully).
The latest thing I found with this issue is Unreal Engines Horde build system, its so tightly coupled with caches, object files and database references: that a manual clean up is extremely difficult and likely to create an unstable system. But you can configure it to have fewer build artefacts kept around and then it will clear itself out gracefully. - but it needs to be able to write to the disk to do it.
Now that I think about it, I don’t do this for inodes, but you can run out of those too and end up in a weird “out of disk” situation despite having lots of usable capacity left.
I did this too, but i also zipped the file, turns out it had great packing ratio!
Personally I just keep the file on a ramdisk so you can avoid having to fetch it from slow storage
Neat! I optimized for my own case, and I'm storing my ramdisk on SSD to gain persistence.
Interesting strategy, can't believe I've never heard of this one before.
Would it be more pragmatic to allocate a swap file instead? Something that provides a theoretical benefit in the short term vs a static reservation.
Because adding swap file is instantaneous, removing one that is in use can take a longtime unless you reboot the OS so you can't just nuke it quickly.
This is why I never empty the Rubbish Bin/trash Can on my Linux laptop until the disk fills.
This is my snippet i used alot. In doubt when even rm wont work just reboot.
Disc Space Insurance File
fallocate -l 8G /tmp/DELETE_IF_OUT_OF_SPACE.img
Surely a 50% warning alarm on disk usage covers this without manual intervention?
Depends. A Kubernetes container might have only a few megabytes of disk space, because it shouldn't need it.
Except that one time when .NET decides that the incoming POST is over some magic limit and it doesn't do the processing in-memory like before, but instead has to write it to disk, crashing the whole pod. Fun times.
Also my Unraid NAS has two drives in "WARNING! 98% USED" alert state. One has 200GB of free space, the other 330GB. Percentages in integers don't work when the starting number is too big :)
The "ballast file" idea doesn't really change that spill-to-disk crash, as far as I can tell. You have to delete it manually; it already crashed by the time you realize it.
Seems like the sort of thing that only makes sense in a "I know my cheapskate boss won't have larger drives ready to go (or be willing to pay to expand it in a cloud scenario), and he insists that the alarm not go off until 95%, but it'll be my fault if we have a bad incident we can't recover quickly from, so I'm gonna give myself some headroom by padding things a bit" extra-paranoid scenario.
You don't want an alarm on a usage threshold, you want a linear regression that predicts when utilization will cross a threshold. Then you set your alarms for "How long does it take me to remediate this condition?"
That's far more complicated and fragile. Where are you storing this log of disk usage? If you already have some external time series database then this is already a solved problem. But for a single server, desktop, or embedded device you'll need a database or text log, a cron job to measure it, and another script to parse, make predictions, and then raise alerts.
And a single large dump to disk, like some daemon suddenly bugging out and writing incessantly to logs, will render all that moot anyway.
> Surely a 50% warning alarm on disk usage covers this without manual intervention?
surely you don't need a fire extinguisher in your kitchen, if you have a smoke detector?
a "warning alarm" is a terrible concept, in general. it's a perfect way to lead to alert fatigue.
over time, you're likely to have someone silence the alarm because there's some host sitting at 57% disk usage for totally normal reasons and they're tired of getting spammed about it.
even well-tuned alert rules (ones that predict growth over time rather than only looking at the current value) tend to be targeted towards catching relatively "slow" leaks of disk usage.
there is always the possibility for a "fast" disk space consumer to fill up the disk more quickly than your alerting system can bring it to your attention and you can fix it. at the extreme end, for example, a standard EBS volume has a throughput of 125mb/sec. something that saturates that limit will fill up 10gb of free space in 80 seconds.
50% is probably unrealistic. Nobody really wants to diminish their storage by 50%.
Let's set a fixed threshold -- 100GB, say -- and play out both methods.
Method A: One or more ballast files are created, totalling 100GB. The machine runs out of storage and grinds to a halt. Hopefully someone notices soon or gets a generic alert that it has ceased, remembers that there's ballast files, and deletes one or more of them. They then poke it with a stick and get it going again, and set forth to resolve whatever was causing the no-storage condition (adding disk, cleaning trash, or whatever).
Method B: A specific alert that triggers with <100GB of free space. Someone sees this alert, understands what it means (because it is descriptive instead of generic), and logs in to resolve the low-storage condition (however that is done -- same as Method A). There is no stick-poking.
Method C: The control. We do nothing, and run out of space. Panic ensues. Articles are written.
---
Both A and B methods have an equal number of alerts for each low-disk condition (<100GB). Both methods work, in that they can form the impetus to free up some space.
But Method A relies on a system to crash, while Method B does not rely upon a crash at all.
I think that the lack of crash makes Method B rather superior all on its own.
(Method C sucks.)
How does the ballast file prevent extreme runaway? You ain't gonna notice and delete it that quickly.
If the alarms are reliably configured, confirmed to be working, low noise enough to be actioned, etc etc.
And of course there's nothing to say that both of these things can't be done simultaneously.
If the alarm works. And it actioned not just snoozed too much or just dismissed entirely.
Defence in depth is a good idea: proper alarms, and a secondary measure in case they don't have the intended effect.
Alarms are great, but when something goes wrong SSDs can fill up amazingly fast!
Surely there are pitfalls either way. A ballast file can be deleted too readily, or someone could forget to re-add it.
Similar to the old game development trick of hiding some memory away and then freeing it up near the end of development when the budget starts getting tight.
Some filesystems can be unable to delete a file if full. Something to be a bit worried about.
Instead of deleting the ballast file you can just truncate it. That works on ZFS when you fill the pool and delete starts failing.
Please name and shame those filesystems so that we will all be forewarned.
Any Copy-on-Write filesystem can run into this. There's always some way around it, but it can be problematic if you only have one device, can't remember the steps to fix a full filesystem, and can't look up the steps because you can't launch a browser without it trying to make some files!
> A neat trick I was told is to always have sleep statements in your code. Just a few sleep statements that you can delete in cases like this. This won't fix the problem, but will buy you time and free up latency for stuff like slow algorithms so you can get faster code.
FTFY ;)
Would another way be to drop the reserved space (typically 1% to 5% on an ext file system)?
Reserved space doesn't protect you against root, who is often the user to blame for the last used MB.
It can be difficult to reason about seemingly innocuous things at scale. I have definitely fallen into the trap of increasing file size from 8 KB to 10 KB and having it cause massive problems when multiplied across all customers at once.
If you run nginx anyway, why not serve static files from nginx? No need for temporary files, no extra disk space.
The authorization can probably be done somehow in nginx as well.
Even if your authorization is so sophisticated that nginx cannot do it, a common pattern I’ve seen is to support a special HTTP response header for the reverse proxy to read directly from disk after your custom authorization code completes. This trick dates back to at least 2010. The nginx version of this seemed to be called X-Accel-Redirect from a quick search.
Yeah it's a bit odd to use a Haskell server to serve a static file which nginx then needs to buffer. You'd do much much better just serving the file out of nginx. You could authenticate requests using the very simple auth_request module:
https://nginx.org/en/docs/http/ngx_http_auth_request_module....
> I rushed to run du -sh on everything I could, as that’s as good as I could manage.
I recently came across gdu (1) and have installed/used it on every machine since then.
gdu is really nice but ncdu, though slower, is very useful and is usually available on distro repos.
I have an alias named usage with this in it:
du -hs -- * .??* 2> /dev/null | sort -h | tail -$LINES
I use dust for this, but gdu looks nice, I'll give it a try. Thanks for sharing.
I also discovered gdu recently. It's really good. It saves me running du -h --max-depth=1 | sort -h a million times trying to find where the space has gone while you're stressing about production being down.
Putting limits on folders where information may be added (with partitions or project quotas) is a proactive way to avoid that something misbehaves and fills the whole disk. Filling that partition or quota may still cause some problems, depending on the applications writing there, but the impact may be lower and easier to fix than running out of space for everything.
I appreciate the last line
> Note: this was written fully by me, human.
My fav line from the post is one above it. There is something very blunt and funny about it.
> It’s difficult to reason under pressure. Experience, that I didn’t have here, would have helped.
I'm not sure that his problems are really over if a LOT of people were downloading a 2GB file. It would depend on the plan. Especially if his server is in the US.
But maybe the European Hetzner servers still have really big limits even for small ones.
But still, if people keep downloading, that could add up.
I was thinking the same thing - wouldn't blob storage or a CDN help?
European Hetzner VPSes have at least 20 TB of bandwidth, and US ones have at least 1 TB.
I don't think there is a cheaper CDN.
I've run into that "process still has deleted files open" situation a few times. df shows disk full, but du can't account for all of it, that's your clue to run lsof and look for "deleted" files that are open.
Even more confusing can be cases where a file is opened, deleted or renamed without being closed, and then a different file is created under the orginal path. To quote the man page, "lsof reports only the path by which the file was opened, not its possibly different final path."
I remember a story of an Oracle Database customer who had production broken for days until an Oracle support escalation led to identifying the problem as mere "No disk space left".
Or NTP, if something is not working df -h and date are the first commands I input.
It's always lupu... I mean NTP or disk space.
You missed out point five.
5. Implement infrastructure monitoring.
Assuming you're on something like Ubuntu, the monit program is brilliant.
It's open source and self hosted, configured using plain text files, and can run scripts when thresholds are met.
I personally have it configured to hit a Slack webhook for a monitoring channel. Instant notifications for free!
> Plausible Analytics, with a 8.5GB (clickhouse) database
And this is why I tried Plausible once and never looked back.
To get basic but effective analytics, use GoAccess and point it at the Caddy or Nginx logs. It’s written in C and thus barely uses memory. With a few hundreds visits per day, the logs are currently 10 MB per day. Caddy will automatically truncate if logs go above 100 MB.
Why not implement x send file ?
Came here to say this
X-Accel-Redirect (Nginx sendfile), if supported by Haskell is the way, it is zero copy and will dramatically help in many cases.
If you are modifying the body is one of the cases where it doesn’t work.
Didn't root used to have some reserved space (and a bunch of inodes) on file systems just for occasions like this?
Never partition 100%. Simple solution here really and should be standard for every sysadmin. Like never worked with one that needed to be told this...
Wait until you run out of inodes!
Old war story: I had an old Sun 4/260 with 2 1G drives - I had SunOS on 1 and Gentoo on the other - my initial Gentoo install worked for a while but then the portage directory used all the configured iNodes - really weird errors and I could not figure it out at the time; error msgs maybe should mention inodes? I had to do #gentoo-sun IRC and someone suggested df -i which was indeed the issues (solve: you can configure extN filesystems to have more iNodes)
That happened to me exactly once in my 20-year career. It was on a web server (maybe even NGINX) that had too many cached files.
Even though it only happened once, I still set up monitoring for inode exhaustion.
Anonymous
[flagged]
[dead]
[dead]
[dead]