mariadb hangs on mysqldump (indefinitely), restarts on second mysqldump or strace

Nightly backups of mariadb via mariadb-dump hang randomly, and at random spots in the backup.

• full backup is about 5G

• the backup can hang at any point, 10% in, 50%, etc

• the database is mostly static, and mostly idle
  (it is a test DB staging before PROD, so it has small change, and is completely idle during backups at night)

What happens:

• cron will start a backup of the DB to a file in /tmp (same issue happens with piped output)

• at a random point, the dump will stall. It just hangs. It will never resume (even days) if left alone.

• while attempting to debug, I noticed that running a second mysqldump immediately kicks off a resume, the same is true of a strace. The dump will then complete successfully

strace just shows the standard upon attach:

113065 restart_syscall(<... resuming interrupted read ...> <unfinished ...>

and everything then just seems normal.

Again, it's not just strace. Doing another mysqldump "kicks" the first into action again.

We're not seeing this on busy servers, as I presume at this point that any sort of DB activity will kick it back into gear. Will verified next time this happens.

(It only happens every few days)