wsrep_sst_method = mariabackup very slow when reading sparse files

when page compression is enabled SST using mariabackup is only reading/transferring data with 200 Mbit for .ibd files with PAGE_COMPRESSED=1 , while running at about 5Gbit for uncompressed files.

So, I assume sparse files are not handled correct. Time for adding a node to a cluster with a 20GB database file took >15 minutes. For a 1TB database file size it will therefore take >> 12 hours instead of less than 1h, making a rotation of nodes during maintenance a task for multiple days, if not running into timeouts in between. Basically unusable for production.