4.3.5.4. I/O resources

While I/O limitations are a major cause of stress for system admins, the Linux system offers rather poor utilities to measure I/O performance. The ps, vmstat and top tools give some indication about how many programs are waiting for I/O; netstat displays network interface statistics, but there are virtually no tools available to measure the I/O response to system load, and the iostat command gives a brief overview of general I/O usage. Various graphical front-ends exist to put the output of these commands in a humanly understandable form.

Each device has its own problems, but the bandwidth available to network interfaces and the bandwidth available to disks are the two primary causes of bottlenecks in I/O performance.

Network I/O problems:

• Network overload:

  The amount of data transported over the network is larger than the network's capacity, resulting in slow execution of every network related task for all users. They can be solved by cleaning up the network (which mainly involves disabling protocols and services that you don't need) or by reconfiguring the network (for example use of subnets, replacing hubs with switches, upgrading interfaces and equipment).

• Network integrity problems:

  Occurs when data is transferred incorrectly. Solving this kind of problem can only be done by isolating the faulty element and replacing it.

Disk I/O problems:

• per-process transfer rate too low:

  Read or write speed for a single process is not sufficient.

• aggregate transfer rate too low:

  The maximum total bandwidth that the system can provide to all programs that run is not enough.

This kind of problem is more difficult to detect, and usually takes extra hardware in order to re-divide data streams over buses, controllers and disks, if overloaded hardware is cause of the problem. One solution to solve this is a RAID array configuration optimized for input and output actions. This way, you get to keep the same hardware. An upgrade to faster buses, controlers and disks is usually the other option.

If overload is not the cause, maybe your hardware is gradually failing, or not well connected to the system. Check contacts, connectors and plugs to start with.