I manage a PXC 5.6 cluster and noticed that the tmpdir variable in /etc/my.cnf isn’t set.
I was thinking to set it to /tmp (which is tmpfs) to explicitly make the cluster store temporary files in RAM. Without this setting, MySQL could choose /var/tmp or /usr/tmp (which on my system, RHEL7, are on-disk) to store temporary files.
Is there any drawback to do this on PXC? I ask because on a MySQL replication slave one must not use a RAM disk to store temporary files, since the slave will need some of the files when the machine restarts.
Is this really going to improve performances, or it looks like a unnecessary twiddling? The InnoDB buffer size is already large enough, and 95% of temporary tables are stocked in memory.
Thanks in advance.