The AXUtil Optimize function reindexes the Model database, but also run a shrink of the database. As the model database is in the main database in AX 2012 RTM, the whole database is shrinked.
In a large database example 1 TB database, this shrink can take extreme long time and block the production system for normal usage.
If you are running in AX 2012 RTM, you need to plan when you run the AXUtil Optimize or alter the XU_Shrink stored procedure, so does not run.
In Dynamics AX 2012 R2 we have moved the model database out of the main database, so here you will not be affected by the issue.
Author: Kim Truelsen