nixCraft

Hai,

Anyone know what is I/O wait .. many commands related to CPU and Disk use the term I/O wait under Linux or UNIX oses.

Especially when I use ps command talks about i/o wait… It talks about
Uninterruptible sleep
Rnning or runnabl
Interruptible sleep
And so on…

I/O waits are associated with process states. For example state D indicates Uninterruptible sleep generally related to IO (indicated by the STAT column in top command).

SO I/O wait is related to process Uninterruptible sleep. For example if process waiting for disk I/O or any other I/O, process is put in in D state.

Hope this helps.

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Can you provide elaborate info

Ok my friend had given me some info. I am sharing same with all of you:
Generally, a process is put into D state when waiting for some sort of I/O to complete. The process has made a call into the kernel and is waiting for the result. During this period, it is unable to be interrupted, as doing so might jeopardize the state of the driver and hardware.

It is important to note that from a CPU standpoint, I/O wait is equivalent to idle time. A process in I/O wait will not block a process that is ready to run (R state).

High I/O does not mean you have a problem.

If you are running MySQL or Oracle server or I/O intensive apps, then you should see high I/O load. In other words, you should expect to see high I/O for I/O bound applications :P

Good info about I/O wait.

Just in case if anyone need PROCESS STATE CODES (from man p:
Here are the different values that the s, stat and state output specifiers
(header "STAT" or "S") will display to describe the state of a process:

D : Uninterruptible sleep (usually IO)
R : Running or runnable (on run queue)
S : Interruptible sleep (waiting for an event to complete)
T : Stopped, either by a job control signal or because it is being traced.
W : paging (not valid since the 2.6.xx kernel)
X : dead (should never be seen)
Z : Defunct ("zombie") process, terminated but not reaped by its parent.

For BSD formats and when the stat keyword is used, additional characters may
be displayed:
< : high-priority (not nice to other user
N : low-priority (nice to other user
L : has pages locked into memory (for real-time and custom IO)
s : is a session leader
l : is multi-threaded (using CLONE_THREAD, like NPTL pthreads do)
+ : is in the foreground process group

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