“If semisynchronous replication is enabled on the master side and there is at least one semisynchronous slave, a thread that performs a transaction commit on the master blocks after the commit is done and waits until at least one semisynchronous slave acknowledges that it has received all events for the transaction, *or until a timeout occurs*.”

The default timeout is 10 seconds…which seems REALLY long for a client to wait, I think a second is plenty unless your replica is on the other side of the planet.

http://dev.mysql.com/doc/refman/5.5/en/server-system-variables.html#sysvar_rpl_semi_sync_master_timeout

In this case, if you hit timeout – then there is no guarantee.

I wonder if you looked into what is practical limit of .frm files which can be synchronized without large downtime ?
I wonder what technologies could be used to reduce the downtime. Can we sync .frm files with no lock first and when do
second round with lock when most likely no files will need to be transferred as well as it is more likely they are in cache.

In the recent version of MySQL Enterprise Backup they announced “lockless” backup of .frm files for InnoDB tables.
I am not sure how this is done, I guess the individual table is still gets locked on short period of time ( datadict->mutex or something) and frm files are copied together with .ibd.

We can look how we can make the same in XtraBackup.

What I mentioned is rather rync like method to copy files and when copy them again under lock if they are changed. It would not reduce locking but can reduce locking time significantly. As I understand fuzzy copy might be possible by reconstructing .frm updates from records in transaction log

Congratulations to everyone for launching this! This will be a game changer in the MySQL world.