When a solution is imported into CRM, CRM is still accessible, however various SQL locks will be applied which may block users.
There is no way to control the SQL locking (this is necessary for the import process to be able to rollback changes if the import fails), and no high availability technology will help.
The main thing to do is to minimise the size of any solution imports, if possible. I find that the main locking overhead is if you add new attributes to an entity, as this affect the physical SQL schema
Microsoft CRM MVP - http://mscrmuk.blogspot.com/ http://www.excitation.co.uk