How many connections/s can I expect between PHP and MySQL on separate server?

Question

Trying to separate out my LAMP application into two servers, one for php and one for mysql. So far the application connects locally through a file socket and works fine.

I'm worried about the number connections I can establish if it is over the network. I have been testing tcp connections on unix for benchmark purposes and I know that you cannot exceed a certain amount of connections per second otherwise it halts due to the lack of resources (be it sockets, or file handles or whatever). I also understand that php does not implement connection pooling so for each page load a new connection over the network must be made. I also looked into pconnect for php and it seems to bring more problems.

I know this is a very very common setup (php+mysql), can anyone provide some typical usage and statistics they get out of their servers? Thanks!

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The problem is not related to running out of connections allowed my MySQL. The main problem is that unix cannot very quickly create and tear down tcp connections. Sockets end up in TIME_WAIT and you have to wait for a period before you free up more sockets to connect again. These two screenshots clearly shows this pattern. MySQL does work up to a certain point and then pauses because the web server ran out of sockets. After certain amount of time passed, the web server was able to make new connections.

alt text http://img35.imageshack.us/img35/3809/picture4k.png

alt text http://img35.imageshack.us/img35/4580/picture2uyw.png

Answer 1

I think the limit is at 65535. So you'd have to have 65535 connections at the same time to hit that limit since a regular mysql connection closes automatically.

mysql_connect()

Note: The link to the server will be closed as soon as the execution of the script ends, unless it's closed earlier by explicitly calling mysql_close().

But if you're using a persistent mysql connection, then you can run into trouble.

Using persistent connections can require a bit of tuning of your Apache and MySQL configurations to ensure that you do not exceed the number of connections allowed by MySQL.