Partitioning database across different servers

I am trying to partition our database across 4 separate servers.

When testing on our development site I had the tables split into four databases but on the same server. This worked.

However, when it came to splitting the databases across different servers the site crashed.

I’ve tried a number of different configs. Including:

$wpdb->add_database(array(
	'host'     => 'xxx1',     // If port is other than 3306, use host:port.
	'user'     => DB_USER,
	'password' => DB_PASSWORD,
	'name'     => 'db1name',
));

$wpdb->add_database(array(
	'host'     => 'xxx2',     // If port is other than 3306, use host:port.
	'user'     => DB_USER2,
	'password' => DB_PASSWORD2,
	'name'     => 'db2name',
	'write'    => 1,
	'read'     => 1,
	'dataset'  => 'news',
	'timeout'  => 0.2,
));

$wpdb->add_table( 'news', 'tablename' );

I have also tried using :

$wpdb->add_callback('my_db_callback');
function my_db_callback($query, $wpdb) {
	// Multisite blog tables are "{$base_prefix}{$blog_id}_*"
	if ( preg_match("/tablename/", $wpdb->table) )
		return 'news';

Can anyone tell me what i’m missing? we ideally want to split the tables over several servers but currently i have settled for a master/slave environment.

Any help would be grand…

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