Linux And High I/o Wait

December 11th, 2009 |

When you look at the CPU activity of your computer, one of the parameters is the iowait. This value shows how much time your CPU wastes while it is waiting for I/O operations for complete. These include disk read/write operations, network, IPC, etc. Is this behavior a problem and, if so, what causes it and how to fix it? One one of the popular Unix-related forums one “genius” wrote:

The iowait “problem” is funny. It’s like when people complain that Linux is “using all my memory”. Yeah, no shit. You should be upset if you are copying files and your computer is /not/ in 100% iowait.

In reality, 100% iowait indicates that there is a problem and in most cases – a big problem that may even lead to data loss. Essentially, there is a bottleneck somewhere in the system. Maybe one of your disks is getting ready to die; or, perhaps, the NIC firmware is having problems with the latest kernel upgrade you installed. The troubleshooting process starts with the potentially more serious possibility: bad disk.

Take a quick look at /etc/messages, /etc/dmesg, /etc/boot.log and any other system log files. You are looking for disk I/O errors, failed read/write operations, bad sectors – anything that indicates a hardware problem with a disk. If you don’t find anything, look for IRQ and disk controller errors. Also look for memory errors and kernel panics. The three most likely culprits of high iowait are: bad disk, faulty memory and network problems.