systemctl enable tmp.mount

Question

Consider the practice of mounting the /tmp directory on a tmpfs memory based filesystem, as can be done with: systemctl enable tmp.mount

And consider the following:

one justification: The use of separate file systems for different paths can protect the system from failures resulting from a file system becoming full or failing.

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another justification: Some applications writing files in the /tmp directory can see huge improvements when memory is used instead of disk.

Is disk caching always in effect? By that I mean when you write to any folder (not just /tmp) you are probably writing to RAM anyway until such time it gets flushed to disk... the kernel handles all this under the hood and it is my opinion I don't need to go meddling tweaking things. So does doing systemctl enable tmp.mount has any real value, if so what?

Also (in CentOS-7.6) I am testing this to try and understand what's happening I am experiencing:

• CentOS 7.6 installed on one 500GB SSD with simple disk partitioning as
  • 1GB /dev/sda1 as /boot
  • 100MB /dev/sda2 as /boot/efi
  • 475GB /dev/sda3 as /
• PC has 8GB of DDR-4 RAM
• if I do just systemctl enable tmp.mount I then get
  • 3.9GB tmpfs as /tmp

How is this tmpfs /tmp at 3.9GB any better than the default way which would (a) first have up to ~8GB based on RAM thanks to disk caching and (b) then when disk caching at capacity based on 8GB of RAM there is > 400GB of disk available to use ?

Answer 1

Whether this configuration "adds value" depends entirely on the use-case in question. "Good", "bad", these are not valid labels for different configuration options without context.

For systems with a lot of RAM and high transaction numbers, the tmpfs filesystem would probably increase performance (example: a Type-2 hypervisor). This might be "good" use case.

Alternatively, systems with a small amount of ram but plenty of storage (example: any of the Internet-of-Things crap made these days) probably perform better with physically written /tmp storage devices, since anything in memory can be written to swap if underutilized (if swap is activated as well). This could be considered a "bad" case for tmpfs /tmp.