ionice does not have any effect on un-synced writes (i.e. normal writes)?

When I dump a large MySQL database (its dump weighs around 10GB)
– it appears on the disk almost immediately, but then, later, when the kernel
decides to flush it to the disk, the server almost stalls and other IO requests
take a lot more time to complete even though mysqldump is run with ionice -c3,
so the use of ionice has no real effect.