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By mistake I noticed that in /tmp directory are continuously created some files then immediately deleted. Using a succession of ls -l /tmp I managed to catch the created files:

-rw------- 1 root root 0 Apr 2 19:37 YlOmPA069G
-rw------- 1 root root 0 Apr 2 19:37 l74jZzbcs6

or another example:

-rw------- 1 root root 0 Apr 2 19:44 AwVhWakvQ_
-rw------- 1 root root 0 Apr 2 19:44 RpRGl__cIM
-rw------- 1 root root 0 Apr 2 19:44 S0e72nkpBl
-rw------- 1 root root 0 Apr 2 19:44 emxIQQMSy2

It's about Ubuntu 18.10 with 4.18.0-16-generic. This is an almost fresh install: I added some server software (nginx, mysql, php7.2-fpm) but even with those closed the problem persists.

What are the files created and why?
How would I stop this behaviour? a very undesirable one on a SSD

Thank you!

UPDATE

The question is about when not having /tmp in RAM (no tmpfs).

The guilty software is x2goserver.service otherwise a must have one.

I suggest installing and running fnotifystat to detect the process that is creating these files:

sudo apt-get install fnotifystat
sudo fnotifystat -i /tmp

You will see process that is doing the open/close/read/write activity something like the following:

Total Open Close Read Write PID Process Pathname
 3.0 1.0 1.0 0.0 1.0 5748 firefox /tmp/cubeb-shm-5748-input (deleted)
 2.0 0.0 1.0 0.0 1.0 18135 firefox /tmp/cubeb-shm-5748-output (deleted)
 1.0 1.0 0.0 0.0 0.0 5748 firefox /tmp/cubeb-shm-5748-output (deleted)

Determine which program/process is touching files

You can use tools such as lsof to determine which processes and binaries are touching/opening which files. This could become troublesome if the files change frequently, so you can instead set up a watch to notify you:

$ sudo fnotifystat -i /tmp

Sometimes, simply looking at the user or group owner gives you a good hint (ie: ls -lsha).

Put /tmp into RAM instead of disk

If you desire, you can put your /tmp directory into RAM. You will have to determine if this is a smart move based on available RAM, as well as the size and frequency of read/writes.

$ sudo vim /etc/fstab

...
# tmpfs in RAM
tmpfs /tmp tmpfs defaults,noatime,mode=1777 0 0
...

$ sudo mount /tmp
$ mount | grep tmp # Check /tmp is in RAM
tmpfs on /tmp type tmpfs (rw,noatime)

If you have enough RAM, this can be considered a very good thing to do for both the longevity of your SSD, as well as the speed of your system. You can even accomplish this with smaller amounts of RAM if you tweak tmpreaper (sometimes tmpwatch) to be more aggressive.

very undesirable one on a SSD

You tagged your question with tmpfs, so it is not quite clear to me how this relates to SSD at all. Tmpfs is an in-memory (or more precisely, in-block-cache) filesystem, so it will never hit a physical disk.

Furthermore, even if you had a physical backing store for your /tmp filesystem, unless you have a system with only a couple of kilobytes of RAM, those short-lived files will never hit the disk, all operations will happen in the cache.

So, in other words, there is nothing to worry about since you are using tmpfs, and if you weren't, there still would be nothing to worry about.

People worry too much about SSD write endurance. Assuming that creating and deleting an empty file writes 24 kB every second, and using the 150 TBW spec for the popular Samsung 860 EVO 250 GB, wear-out takes 193 years!

(150 * 10 ^ 12) / ((2 * 3 * 4 * 1024) * 60 * 60 * 24 * 365.25) = 193

For ext4 filesystems, use "tune2fs -l" to find Lifetime writes. Or, use "smartctl -a" and look for Total_LBAs_Written. I always find the SSD has lots of life left.

You were using the wrong /dev/nvme0... name:

$ sudo tune2fs -l /dev/nvme0n1
tune2fs 1.42.13 (17-May-2015)
tune2fs: Bad magic number in super-block while trying to open /dev/nvme0n1
Couldn't find valid filesystem superblock.

The right format is:

$ sudo tune2fs -l /dev/nvme0n1p6
tune2fs 1.42.13 (17-May-2015)
Filesystem volume name: New_Ubuntu_16.04
Last mounted on: /
Filesystem UUID: b40b3925-70ef-447f-923e-1b05467c00e7
Filesystem magic number: 0xEF53
Filesystem revision #: 1 (dynamic)
Filesystem features: has_journal ext_attr resize_inode dir_index filetype needs_recovery extent flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize
Filesystem flags: signed_directory_hash 
Default mount options: user_xattr acl
Filesystem state: clean
Errors behavior: Continue
Filesystem OS type: Linux
Inode count: 2953920
Block count: 11829504
Reserved block count: 534012
Free blocks: 6883701
Free inodes: 2277641
First block: 0
Block size: 4096
Fragment size: 4096
Reserved GDT blocks: 1021
Blocks per group: 32768
Fragments per group: 32768
Inodes per group: 8160
Inode blocks per group: 510
Flex block group size: 16
Filesystem created: Thu Aug 2 20:14:59 2018
Last mount time: Thu Apr 4 21:05:29 2019
Last write time: Thu Feb 14 21:36:27 2019
Mount count: 377
Maximum mount count: -1
Last checked: Thu Aug 2 20:14:59 2018
Check interval: 0 (<none>)
Lifetime writes: 4920 GB
Reserved blocks uid: 0 (user root)
Reserved blocks gid: 0 (group root)
First inode: 11
Inode size: 256
Required extra isize: 28
Desired extra isize: 28
Journal inode: 8
First orphan inode: 1308352
Default directory hash: half_md4
Directory Hash Seed: a179d56c-6c68-468c-8070-ffa5bb7cd973
Journal backup: inode blocks

As far as lifetime of NVMe SSD goes:

$ sudo nvme smart-log /dev/nvme0
Smart Log for NVME device:nvme0 namespace-id:ffffffff
critical_warning : 0
temperature : 38 C
available_spare : 100%
available_spare_threshold : 10%
percentage_used : 0%
data_units_read : 22,351,778
data_units_written : 14,667,833
host_read_commands : 379,349,109
host_write_commands : 127,359,479
controller_busy_time : 952
power_cycles : 1,925
power_on_hours : 1,016
unsafe_shutdowns : 113
media_errors : 0
num_err_log_entries : 598
Warning Temperature Time : 0
Critical Composite Temperature Time : 0
Temperature Sensor 1 : 38 C
Temperature Sensor 2 : 49 C
Temperature Sensor 3 : 0 C
Temperature Sensor 4 : 0 C
Temperature Sensor 5 : 0 C
Temperature Sensor 6 : 0 C
Temperature Sensor 7 : 0 C
Temperature Sensor 8 : 0 C

The key line here is:

percentage_used : 0%

After 18 months of use the SSD percentage use is 0%. If after 3 years of use it hits 1% then I know the SSD will last 300 years.

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