max_backups_io_thread_pool_free_size
If the number of idle threads in the Backups IO Thread pool exceedsmax_backup_io_thread_pool_free_size, ClickHouse will release resources occupied by idling threads and decrease the pool size. Threads can be created again if necessary.
max_backups_io_thread_pool_size
ClickHouse uses threads from the Backups IO Thread pool to do S3 backup IO operations.max_backups_io_thread_pool_size limits the maximum number of threads in the pool.