Can a filled up swap partition cause file locking issues on linux

Question

I have a machine at home running elementary OS 0.2 with very little RAM, the swap is always allmost full (I'll upgrade the RAM as soon as I can.) It has an OpenSSH server set up with public key authentication. When loging in today, it asked me for a password. I re-cat:ed my public key to the remote authorized_keys file and the public key authentication started working again. My guess is there was a lock on the file that got lifted when I wrote to it.

Am I interpreting this situation correctly? Is the likely cause of the issue a lock on the authorized_keys file? And, can a full swap partition cause file locking issues?

Answer 1

Unfortunately, your question is application specific, and scenario/situation specific. The short answer is, simply, no (or rather it shouldn't).

Your swap partition should be seen as an extension of your RAM. To prevent your SWAP getting full, you might (prior to getting more RAM) try to make your SWAP partition larger. But do get more RAM as well, SWAP can be 1000X slower than RAM.

So, to answer your question fully, ask the following question: What happens to a PC (and it's running applications) when it runs out of RAM? Or RAM and SWAP?

It would refuse to start up new applications... It would refuse to supply new memory blocks to applications requesting memory (if these applications can recover from such a refusal, they will log an error and continue to run, most applications will crash, or exit gracefully as they cannot fulfil their purpose if they cannot allocate memory).

So if your key file was indeed locked (which I don't dispute could have happened), it was most likely due to something SSH/SSL related, like the daemon hanging because it couldn't allocate memory, or the daemon restarting for the above-mentioned reason.

If using openssh-server, change your /etc/ssh/sshd_conf to not allow password authentication, and try to recreate a scenario where you would have zero RAM and zero SWAP space left. I really suspect sshd will just crash, to be honest.