I'm running Centos 7.4

I could not really understand the use of tmpfs. Yes, tmpfs is stored in memory instead of hard disk, then what now? What is the use of it

One more thing, devtmpfs. Correct me if I'm wrong. Is it now the one who maps the devices on my machine? I red on internet that even without udev it can map your devices.

Thanks!

It's extremely fast for a file system and is non-persistent between reboots. I've made use of the first for when I've wanted to compress large files, and the second where a process downloads decryption keys from another source and stores them in tmpfs, as this is stored in memory and not on a hard disk it is considered secure and not "at rest" for many audit processes.

tmpfs is like a traditional ramdisk but with two great advances:

1) A ramdisk has a fixed size. For safety reasons, you usually make it larger than you really need, but that extra space gets subtracted from available memory whether you use it or not. A tmpfs system grows and shrinks as required by the content.

2) A ramdisk is always in ram. So that block of ram is sterilised; you can't use it for anything else. A tmpfs can be swapped out like a program image when it isn't being used.

As for devtmpfs, that's just a filesystem that combines the properties of a devices filesystem with tmpfs.

yes, it was already well explained. Another example: programs usually create temporary files during their life. Using tmpfs (ramdisk) may allow to run those programs faster (this is especially true for compiling, where there will be a lot of intermediate files). tmpfs is (can be) at about 1000 times faster than an ssd.

Wow, thanks for the explanations. I get it now. I might use it when I'm extracting/compressing large files for it to become faster. Know I know tmpfs is awesome as it can automatically increase and decrease its space.

So the devices lookup will become fast? My devtmpfs is mounted on /dev/shm

I use tmpfs for browser cache. I used to use it for application development to speed up compiles.

Now, what I see on my machine when I use df command:

devtmpfs /dev

I don't get devtmpfs as much as tmpfs.

devtmpfs is a device directory in ram.

I don't get what's the use of it. Udev use devtmpfs?

that means it is kept in RAM, not saved anywhere (else). But it is separated from anything else, that's why it has its own fs.

Nice! Now I get it. Thank you!

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Now, be very careful when you say that it is "in RAM, and not saved anywhere else." This is virtual memory and it can be swapped-out.

Hm, interesting question. I'm not really sure about that (devtmpfs or other special tmpfs can also be swapped out).

Yes, that's the whole point of tmpfs. An old-fashioned ramdisk couldn't be swapped out; it was literally a ram disk.