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On a Linux VM I would like to TEST the NAGIOS monitoring more deeply than just switching off the VM or disconnecting the virtual NIC; I would like to test or "enforce a disk space alarm" through occupying several % of free space for a short period of time.

I know that I could just use a

dd if=/dev/zero of=/tmp/hd-fillup.zeros bs=1G count=50

or something like that... but this takes time and loads the system and requires again time when removing the test files with rm.

Is there a quick (almost instant) way to fill up a partition that does not load down the system and takes a lot of time ? im thinking about something that allocates space, but does not "fill" it.

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  • sorry, forgot to mention that its a >> ext3 filesystem. Jun 29, 2016 at 12:17
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    Zip bomb always works
    – galois
    Jun 29, 2016 at 18:53
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    @jaska Make it an answer. It was the very first idea I got when reading the title...
    – Crowley
    Jun 30, 2016 at 9:35
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    @AxelWerner From en.wikipedia.org/wiki//dev/full : "In Linux /dev/full or the always full device is a special file that always returns the error code ENOSPC (meaning "No space left on device") on writing [...]". Basically, it will always say that there's no space left, when you try to write to it. Jun 30, 2016 at 16:08
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    the question was enough of an answer for what I needed Feb 15, 2021 at 12:04

4 Answers 4

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The fastest way to create a file in a Linux system is using fallocate:

fallocate -l 50G file 

From man:

fallocate is used to manipulate the allocated disk space for a file, either to deallocate or preallocate it.
For filesystems which support the fallocate system call, preallocation is done quickly by allocating blocks and marking them as uninitialized, requiring no IO to the data blocks. This is much faster than creating a file by filling it with zeros.
Supported for XFS (since Linux 2.6.38), ext4 (since Linux 3.0), Btrfs (since Linux 3.7) and tmpfs (since Linux 3.5).

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    Why are you running it through sudo?
    – gerrit
    Jun 29, 2016 at 23:31
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    @gerrit Added that point to the answer. Jun 30, 2016 at 6:53
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    "fallocate needs root privileges" Not on my system (Linux Mint 17.3, downstream of Ubuntu, thus Debian). (ext4 file system) Jun 30, 2016 at 7:48
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    +1 although the OP explicitly mentioned that his filesystem is ext3. Jun 30, 2016 at 7:59
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    @RuiFRibeiro, thanks! for sles11sp4 ive been able to create a file, format it with ext4, but where unable to mount it in RW mode. later i found a kernel message in /var/log/messages that said, ext4 is supported only as read-only. :/ Jul 1, 2016 at 11:40
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Other alternatives include:

  1. to change the alarm thresholds to something near or below the current usage, or
  2. to create a very small test partition with limited inodes, size, or other attributes.

Being able to test things such as running into the root reserved percentage, if any, may also be handy.

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  • root reserved percentage normally is 10% unless you tweak it - it ends up a too big waste of system in big partitions/modern disks. When defining alarms, you´d better already take it in account. Jun 29, 2016 at 16:59
  • +1 for the first thing. a hundred times true. why the hell should I actually create something on a machine disk? what if something (like coredump, batch job generating big temporary files, ...) happens at the time of my testing and whole disk gets accidentally eaten up?
    – Fiisch
    Jun 29, 2016 at 18:30
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    @Fisch - Why? To make sure your alerting threshold is correct and that you're not doing something like accidentally setting the inode free percentage instead of the disk space free percentage (which I've seen done before). If something fails because you filled up a disk to the alerting threshold, then your alerting threshold is too low - the whole point of alerting is that it's supposed to alert you before things start to break.
    – Johnny
    Jun 29, 2016 at 20:06
  • Cat, good point. But no solution for me. I dont have controll over the VM configuration (cant alter partitions or virtual disks), nor have controll over the NAGIOS Server. Jun 30, 2016 at 6:37
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    @AxelWerner Can you loopback-mount a file as "fake" partitiion? That still would allow you to test without seriously affecting anything. Format it with one of the supported filesystems and and you can play around with fallocate too.
    – Tonny
    Jun 30, 2016 at 14:09
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  1. fallocate -l 50G big_file

  2. truncate -s 50G big_file

  3. dd of=bigfile bs=1 seek=50G count=0

As those three ways can all fill up a partition quickly.

If you like use dd, usually you can try it with seek. Just set seek=file_size_what_you_need and set count=0. That will tell the system there is a file, and its size is what you set, but the system will not create it actually. And used this way, you can create a file which is bigger than the partition size.


Example, on an ext4 partition with less than 3G available. Use dd to create a 5T file which exists as metadata -- requiring virtually no block space.

df -h . ; dd of=biggerfile bs=1 seek=5000G count=0 ; ls -log biggerfile ; df -h .

Output:

Filesystem      Size  Used Avail Use% Mounted on
/dev/sda9        42G   37G  2.8G  94% /home
0+0 records in
0+0 records out
0 bytes copied, 4.9296e-05 s, 0.0 kB/s
-rw-rw-r-- 1 5368709120000 Jun 29 13:13 biggerfile
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda9        42G   37G  2.8G  94% /home
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    can you add some more information to your answer?
    – cat
    Jun 29, 2016 at 15:18
  • I just add more thinking in a finished question for people which search the same question other way. ignore it what if you are not.
    – Se ven
    Jun 29, 2016 at 15:50
  • This count=0 method is quite interesting, I've added an example.
    – agc
    Jun 29, 2016 at 16:47
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    Note that the dd example above may well allocate a sparse file. In that case the file size is 50G, it's actually only using a block (or not even) and so the disk is not getting full. YMMV.
    – MAP
    Jun 29, 2016 at 19:49
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    I tested your suggestion on my ext3 filesystem. it did not work as expected. truncate and dd did create a file with a large file size, but "df -h" did not recognize it. still shows the same free hd space. Jun 30, 2016 at 6:53
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You could also take advantage of the stress-ng tool that is supported on a wide number of Linux-based systems:

stress-ng --fallocate 4 --fallocate-bytes 70% --timeout 1m --metrics --verify --times
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  • Exactly what I was looking for to test autoscaling functionality. Being able to allocate disk-space dependent on the total available size was a must-have feature. Nov 27, 2021 at 11:39

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