备份时出现下面的提示
Unknown redo log format (4). Please follow the instructions at http://dev.mysql.com/doc/refman/8.0/en/ upgrading-downgrading.html.
MySQL8.0.20的Release Note中有下面的提示：
InnoDB: Redo log records for modifications to undo tablespaces increased in size in MySQL 8.0 due to a change in undo tablespace ID values, which required additional bytes. The change in redo log record size caused a performance regression in workloads with heavy write I/O. To address this issue, the redo log format was modified to reduce redo log record size for modifications to undo tablespaces. (Bug #29536710)
Xtrabackup8.0.11还不支持备份MySQL8.0.20
MySQL修改redo log格式版本的原因是在MySQL 8.0中，由于undo表空间ID值的变化,undo表空间修改的重做日志记录的大小增加了,导致需要额外的字节存储,redo log记录大小的变化导致写I/O较多的工作负载的性能下降(官方记录是10%),为了解决这个问题，对redo log日志格式进行了修改(优化)
mysql降级到8.19后解决。
