Actually, this is already implemented.
Further, in MariaDB 10.0, there is no fsync() needed in step 3. This is because in case of a crash, XA crash recovery can repeat the step 3 using the information saved in step 1 and 2. So in 10.0, we only need one shared fsync in step 1 plus one shared fsync in step 2.
If you look in the innodb/xtradb code, you can see this. The prepare step calls trx_prepare_for_mysql() in trx/trx0trx.cc. This calls trx_prepare() which goes to trx_flush_log_if_needed_low() and calls log_write_up_to() in log/log0log.cc. And in log_write_up_to(), you will see the group commit logic. The transaction will wait for any previous fsync to complete; then if it still needs the fsync(), it will fsync not just itself, but also any other transactions that are waiting for fsync.
There is some description of the removal of fsync() in step 3 here:
However, the group commit in step 1 has been in the InnoDB code for many years, as far as I know.
yeah, I got it. InnoDB/xtradb group commit indeed reduce fsync() called times in prepare step, but it could do more than one fsync() for a group of transactions in binlog group commit to be durable in prepare step. what I think is it only to make sure that a group of transactions writing to binlog has been flushed to innodb/xtradb redolog. so how about don't flush redolog in prepare(), insteadly let leader thread to flush innodb/xtradb redolog to latest lsn just before it begin to write follower transactions and itself to binlog. that only need one fsync() for a group of transactions completed prepare step to flush to redolog. Thanks