mjhjmtu

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.

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).

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.

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

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