What would happen if /tmp is full? Would the server stop working? Only the applications running on it? Can /tmp be filled by processes in memory?

Sorry for such basic question

more or less yes. Probably will not be stopped, just slow down.

Any process that needs space in /tmp will have to work around this failure. If the process was written with the assumption that /tmp always has space (which would be bad code), it might just abort or raise other error conditions with misleading messages.

Stricly speaking, “the server” would not stop working, but it depends on what you mean by “the server”. For example, you might be unable to log on, or run any programs. The kernel would not panic.
What do you mean by that? Processes are in memory when they run. A process that is not in memory, i.e. swapped out, can’t run and therefore not fill /tmp.
Not that basic, actually. It’s not clear at all which parts of your system would stop working.

Out of interest, why are you asking this question? Are you trying to figure out how much disk space to allocate to /tmp, or are you experiencing issues where /tmp is fairly full?

Thanks. I see the layers of complexity building up. I’m experiencing issues with /tmp getting filled up. When I check individuall files they don’t amount to the size of the file system. My server is a Hp with centos on it. Yesterday I got a close call and notice a core dump in the messages log file. I moved the largest file on /tmp to another file system and nothing happened after some time then the usage dropped to 40%.

Let's see how your system is setup. Run this in a terminal and post the output.

Filesystem Type Size Used Avail Use% Mounted on
/dev/mapper/centos-root xfs 38G 3.7G 34G 10% /
devtmpfs devtmpfs 1.9G 0 1.9G 0% /dev
tmpfs tmpfs 1.9G 0 1.9G 0% /dev/shm
tmpfs tmpfs 1.9G 9.4M 1.9G 1% /run
tmpfs tmpfs 1.9G 0 1.9G 0% /sys/fs/cgroup
/dev/sda1 xfs 1014M 178M 837M 18% /boot
/dev/mapper/centos-home xfs 19G 43M 19G 1% /home
tmpfs tmpfs 379M 12K 379M 1% /run/user/42
tmpfs tmpfs 379M 0 379M 0% /run/user/1001

I don’t see /tmp in the list of mounted filesystems. This means you should have 34G free space.

Often, temporary files are unlinked immediately after their creation. As a result, the ls command can’t see them, but they still exist and use up space as long as the corresponding file descriptor is not closed. This would explain why your file sizes and disk usage don’t correlate.

The OP opened a duplicate thread - now closed - that has more relevant information.If indeed /tmp did fill, that is most likely merely an after-effect of whatever caused the fail-over.
Talk to your sysadmin.

Thanks. He's looking into this.