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Comment by flanfly | Hacker News Reader

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.

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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)

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.

I know and I agree with you. It doesn't seem that smart.

And you can tell by the fact that the filler data is called "burp.mp3" and things like that.

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.

Interesting, makes sense, seems to be bad precedent if everyone follows suit.

It does, but for critical apps (that might have some awful security holes) it guarantees them space.

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.

If I recall correctly:

    dd if=/dev/urandom of=/home/myrandomfile bs=1 count=N

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.

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

Almost all CPUs have AES native instructions so you'll be able to produce pseudorandom junk really fast. Even my old system will produce it at about 3Gb/s. Much faster than urandom can go.

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 > foo

Fwiw you can also do this with

    head -c 1G /dev/urandom > /home/myrandomfile

And not have to remember dd's bizarre snowflake command syntax.

I 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.

Interesting! I did `fallocate -l 1G myfile` - very fast. Its all zeros but probably won't be compressed by the filesystem since its created with the fallocate() system call.

My choice has always been `shred`:

  $ sudo truncate --size 1G /emergency-space
  $ sudo shred /emergency-space

I find it widely available, even in tiny distros.

Very cool. Reading the man page I see, by default, shred does 3 iterations. Adding --iterations=1 makes it random enough for this purpose and faster.

Terrific, thank you!

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

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

If you add conv=sparse to the dd command with a smaller block size it will sparsify what you copy too, use the wrong cp command flags and they will explode.

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.

A good way to do this is to create a swap file, both because then you can use it as a swap file until you need to delete it and because swap files are required to not be sparse.

I'm not at a machine, linux doesn't zero the swap "file" does it? if i set vm.swappiness = 0, will your "trick" work if i never hit memory pressure?

If you create the file with 'mkswap --file' it allocates the blocks. Trying to use 'swapon' with an existing sparse file won't remove the holes for you but does notice them and then refuse to use it.

No, just use LVM or other dynamic volume management.

Shit like that just wastes space that SSD could use for wear levelling...

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.

But even ignoring the wear-levelling issue, the spare space still fulfils a need in providing the ballast space which is the main thing we are talking about here. Of course there are other ways to manage that issue¹ but a bit of spare space in the volume group is the one I go for.

In fact since enlarging live ext* filesystems has been very reliable² for quite some time and is quick, I tend to leave a lot of space initially and grow volumes as needed. There used to be a potential problem with that in fragmenting filesystems over the breadth of a traditional drive's head seek meaning slower performance, but the amount of difference is barely detectable in almost all cases³ and with solid state drives this is even more a non-issue.

> And most importantly 10% […] nowadays it's 50-100GB at least.

It doesn't have to be 10%. And the space isn't lost: it can be quickly brought into service when needed, that is the point, and if there is more than one volume in the group then I'm not allocating space separately to every filesystem as would be needed with the files approach. It is all relative. My /home at home isn't nearly 50GB in total⁴, nor is / anywhere I'm responsible for even if /var/log and friends are kept in the same filesystem, but if I'm close to as little as 50GB free on a volume hosting media files then I consider it very full, and I either need to cull some content or think about enlarging the volume, or the whole array if there isn't much slack space available, very soon.

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[1] The root-only-reserved blocks on ext* filesystems, though that doesn't help if a root process has overrun, or files as already mentioned above.

[2] Reducing them is still a process I'd handle with care, it can be resource intensive, has to move a lot more around so there is more that could go wrong, and I've just not done it enough to be as comfortable with the process as I am with enlarging.

[3] You'd have to work hard to spread things far and randomly enough to make a significant difference.

[4] though it might be if I wasn't storing 3d print files on the media array instead of in /home

I do that + script to auto resize within sane limits. So for most servers the partitions will automatically fit the usage while still leaving some spare space.

Usually something like "expand if there is less than 5% left, with monitoring triggering when there is 4% free space left", so there is still warning when the automatic resize is on limit

carving space per PV like that is pointless

> 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.

YMMV but most distros set up a cron/timer that does fstrim monthly. So it shouldn't be needed, as any free space will be returned to SSD.

> [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.

other options is telling LVM this LV is striped (so it uses space from both drives equally), or manually allocating from drive with more free space when expanding/adding LV

> 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.

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).

Please explain!

ZFS supports instant resizing of datasets. When that dataset is the virtual disk for a VM you can just increase its size on the hypervisor, then it's a simple growfs operation for the VM to see the increased size. On LXC the dataset is usually mounted directly so the resize operation is reflected immediately.

I use Proxmox as the hypervisor, and the ZFS resize part is supported on the GUI and it's trivial to use. Let me know if you need more details.

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...

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.

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.

ncdu is a lifesaver...

This is why I never empty the Rubbish Bin/trash Can on my Linux laptop until the disk fills.

That's not a great idea for three different reasons: Filesystems have to do ugly things when they're almost full like split files into many small blocks and store more metadata to keep track of them all, SSDs get slower and have compromised wear leveling when they're almost full, and it makes you more likely to subject yourself to perils of fully running out which can cause random non-temporary problems even if it only happens temporarily.

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.

Let's get crazy

1. swapoff

2. drop disk cache (1)

3. panik!?!

hrm, seems ok

Actually there should be a 'sync' as step 2 :)

it has been 3 days since esseph has nuked prod

"That's fine let's just restore from the backups."

"..."

"Oh no, restore failed"

(I feel like this is about 9 out of 10 companies these days)

Reminder to regularly test backup restoration!

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

https://gist.github.com/klaushardt/9a5f6b0b078d28a23fd968f75...

Make sure your /tmp is on disk and not a tmpfs, like in recent Linux distrubitions.

Surely a 50% warning alarm on disk usage covers this without manual intervention?

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.

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.

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.

Yep. That is why doing both can be beneficial. Alerts are more proactive if acted upon, but often too easy to ignore meaning ballast is more fail-safe in that respect.

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.)

A + B would be best. Warn at 200, file to reserve the last 100 (or 50 or whatever). That way if the fill is too fast to react to in time, you still have a quick way to temporarily gain disk space, if needed to solve the problem.

I like that idea. Belt and suspenders.

Alerting on an unexpectedly high rate-of-change, as some others have suggested, also seems good for some workloads.

How does the ballast file prevent extreme runaway? You ain't gonna notice and delete it that quickly.

It doesn't prevent it. It gives you a way to potentially recover after the disk fills. Many operations become impossible once the disk is full, so this buys you some temporary breathing room to solve the problem.

Sounds like something straight out of Dilbert

I did this recently, aka, docker images prune. Can confirm, saved the day.

Just use LVM and don't allocate all of it to LV.

We have a script that basically slowly expands volume when demand grows, up to a limit. So we don't have to think on stuff like "does the logs partition need to be 1 or 10GB", it will expand to the sane limit, and if it hits that we get disk usage alert before it finishes so we can either see what's going on (app shat in logs), or take a look for the one in the 10 apps that need some special tuning there

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.

Love the simplicity and pragmatism of this solution

Some filesystems can be unable to delete a file if full. Something to be a bit worried about.

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!

> and can't look up the steps because you can't launch a browser

My early days of computing got easier when I had a second computer to look up the issues of the first computer.

Instead of deleting the ballast file you can just truncate it. That works on ZFS when you fill the pool and delete starts failing.

under what circumstances does deleting files fail on a full pool? i have one that fills up semiregularly and i've never had issues that required me to truncate files

Also good for stopping phone-home auto firmware updates

Such a simple trick, but effective!

ext2/ext3/ext4 all automatically reserve an amount of space on a partition. 5% iirc

Great idea, thanks!

> 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.

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