Updating a mirror depends on whether it is synchronous or asynchronous.
Updating an asynchronous or Copy-on-Write mirror
There is no need to unmount file systems for this type of mirror. After ensuring that all the mirror relationships in the span are in a single consistency group, use your storage management interface to take or update a copy of the data. This action is usually scripted and run from a cron job.
Updating a synchronous mirror
Synchronous mirrors are automatically updated every time the server writes to primary storage. No manual procedure is required.