what is the preferred practice when linq2sql using (in asp.net mvc applications): to create "singleton" for DataContext like:

partial class db
{
    static db _db = new db(global::data.Properties.Settings.Default.nanocrmConnectionString, new AttributeMappingSource());

    public static db GetInstance()
    {
        return _db;
    }
}

or to retrieve new instance when it needed within using:

using (db _db = new db())
{
    ...
}

the usage of using brings some limitations on code. so I prefer to use singleton one. is it weird practice?