By default, xtrabackup-v2/mariabackup SSTs log to $datadir/innobackup.backup.log on the donor and $datadir/.sst/innobackup.prepare.log and $datadir/.sst/innobackup.move.log on the joiner. These SST logs are overwritten by each subsequent SST. This makes it very difficult to track down issues with previous SSTs unless you manually save these logs after every SST.

You can enable syslog logging for the SST by setting the following in your MariaDB configuration file:

[sst]

sst-syslog=1

I think the default behavior should be changed, so that diagnostic information about SSTs is not deleted every time.

Maybe syslog logging for the SST should be enabled by default?

Or maybe the SST script should archive old copies of innobackup.backup.log, innobackup.prepare.log, and innobackup.move.log, so that they can be fetched when needed?