Re: I/O wait problem with hardware raid

["Eric S. Johansson" <esj@harvee.org>]

Bill Davidsen wrote:

> iowait means that there is a program waiting for I/O. That's all. 

I was under the impression that I/O wait created a blocking condition.

> Of
> course when you do a copy (regardless of software) the CPU is waiting
> for disk transfers. I'm not sure what you think you should debug, i/o
> takes time, and if the program is blocked until the next input comes in
> it will enter the waitio state. If there is no other process to use the
> available CPU it becomes waitio, which is essentially available CPU
> cycles similar to idle.
> 
> What exactly do you think is wrong?

As I run rsync which increases the I/O wait state, the first thing I notice is
that IMAP starts getting slow, users start experiencing failures in sending
e-mail, and the initial exchange for ssh gets significantly longer.

All of these problems have both networking and file I/O in common and I'm trying
to separate out where the problem is coming from.  I have run netcat which has
shown that the network throughput is not wonderful but that's a different
problem for me to solve.  When I run netcat, there is no degradation of ssh,
IMAP or SMTP response times. the problem shows up if I run CP or rsync internal
source and target.   the problem becomes the worst when I'm doing rsync within
the local filesystem and another rsync to an external rsync server.  At that
point, the system becomes very close to unusable.

Of course, I can throttle back rsync and regain some usability but I'm backing
up a couple terabytes of information and it's a time-consuming process even with
rsync and would like  it to run as quickly as possible. I should probably point
out that the disk array is a relatively small raid five set up with six 1 TB
drives.  Never did like raid five especially when it's on a bit of firmware.
Can't wait for ZFS (or its equivalent) on linux to reach production quality.

from where I stand right now, this might be "it sucks but it's perfectly
normal".  In a situation with heavy disk I/O, I would expect anything that
accesses the disc to run slowly and in a more naïve moment, I thought that the
GUI wouldn't be hurt by heavy disk I/O and then I remembered that gnome and its
kindred have lots of configuration files to read every time you move the mouse.  :-)

Any case, the people that sign my check aren't happy because they spent all his
money on an HP server and performs no better than an ordinary PC.  I'm hoping I
can learn enough to give them a cogent explanation if I can't give them a solution.


I appreciate the help.


---eric
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html