Previously, we did not use the path specified by the tmpdir. From Percona XtraDB Cluster 5.6.35-26.20-3 and 5.7.17-29.20, we use the tmpdir option to specify the location of temporary files used by the SST (on the DONOR this may be very large since the backup logs may be stored here).
Specifying the tmpdir is as expected. You supply the directory path, as in the following example:
We look for the tmpdir in the [sst], [xtrabackup], and [mysqld] sections, in that order. If left unspecified, “mktemp” follows the default behavior (on Linux distributions this will typically create the directory in $TMPDIR, followed by /tmp).
Normal security settings and permissions apply here. The directory must already exist and be readable and writable by MySQL otherwise a fatal error will be generated by the SST script.