Questions[database-connection](server)

I have a domain with around 90 PC with the domain controller running by Windows Server 2012 R2 and the PCs Windows 7. Every PC has a mapped drive from the server and a database interface that's linked to the database in the server. Everything was working perfectly until a couple days ago when one PC couldn't reach the server (mapped drive is disconnected and database access couldn't be reached)

I checked the PCs connection to the network and it was able to reach other PCs and surf the internet with no problems. So I restarted the PC and that problem was fixed. The day after, almost every PC in the domain had the same problem. And restarting the PC always fixes the problem (Some PCs will reconnect to server after 2 to 3 attempts of restarts)

I tried restarting the server but that just made all PCs that were reconnected by restarting to lose connection again..

If you know anything about this problem please advice..

Scenario

1. Two servers: ApplicationServer1 and MySQLServer1
2. MySQL remote database server is connected to ApplicationServer1 through an SSH tunnel (kept alive using autossh)

The problem

Since I separated the database from the application, the latter slowed down a lot. SSH tunnel is used in the server stack to increase security. Setting up a VPN (such as Tinc) is an alternative, but it seems to slow down much more than the SSH tunnel.

So...

How can I speed up the connection between ApplicationServer1 and MySQLServer1?

Could Redis or Memcached, installed on ApplicationServer1, be a solution?

Thank you all.

I've opened too many connections to MySQL and now I can't access it. It says "Too many connections". How can I close all of these connections? I also don't know how to restart it, which may resolve the problem.

A client is using PHP to connect to MySQL. The PHP scripts and the MySQL database are located on 2 different Linux servers. He complained that database connections were being dropped or timed out and asked me to take a look.

Is there any place in MySQL that can show me what and how many connections have been dropped or timed out? I looked into slow query log and didn't see anything.

Any suggestions on how to diagnose this dropped/timed out database connection problem?

Thanks

EDIT:

Slow query log is enabled in my.cnf:

log-slow-queries=/var/log/mysql-slow-queries.log

And when I do a

mysql> show global status;

I got:

| Slow_queries                      | 11402347     | 

So there are a lot of slow queries. But the file /var/log/mysql-slow-queries.log doesn't exist. Why is that?

I have a replicated MySQL environments were the master db gets a continuous "Aborted_connects". How can I find out what IP and what user are causing the aborted connections. There are about 500 connections created by 25 applications. I only have access to the database servers.