innobackupex without --no-lock runs FLUSH TABLES WITH READ LOCK:
Quote:
With a READ LOCK, other threads can only read the data but not modify it. Then the MYI and MYD files are copied, then the lock gets released, then ibdata1 gets copied. So, the lock should be released quickly, before copying the ibdata1 file.
With --no-lock I am surprised you still see locking occur.
Maybe use pt-online-schema-change to convert the memory table to innodb (which is also held in memory if you have sufficient memory, but with additional writes to the harddisk to ensure consistency even after a crash).