SQL memory and update usage

A clients servers were having performance issues. As part of the background information we were told that the hosting provider had been having troubles with the underlying virtual hosts in their virtual environment but everything was now resolved. When we looked at the servers, there was 32GB of memory allocated to the server. SQL Server memory was set to a minimum of 20GB and a maximum of 28GB. But the whole server was only using 10GB of memory. We decided to do a dbbcc updateusage on all databases on the server. When this was completed the memory usage for SQL Server was now 20GB and increasing as the load on the server increased. We are unsure what happened with the virtual machine, but until the DBCC was run the memory was not being correctly allocated.

Once the memory allocated was back to normal, performance went back to normal levels. Very strange indeed.