Re: I/O issues, iowait problems, 2.4 v 2.6

[Andrew Morton <akpm@osdl.org>]

Paul Venezia <pvenezia@jpj.net> wrote:
>
>  > As next steps I'd suggest that you log into the server and do
>  > 
>  > 	time (dd if=/dev/zero of=x bs=1M count=2048 ; sync)
>  > 
>  > and
>  > 
>  > 	time (dd if=x of=/dev/null bs=1M count=2048 ; sync)
>  > 
>  > (this assumes that the machine has less that 2G of memory, to avoid caching
>  > effects).
> 
>  The raw file read/write is the ticket. The box tightens right up at 100% iowait.

Well that's nice and simple.  Could you please run `vmstat 1' during that
big `dd'?  Wait for everything to achieve steady state, send us twenty
lines of the vmstat trace?

> 
>  I'd done bonnie++ i/o tests already, and except for an apparent NPTL issue on the per char,
>  the block i/o numbers were fine; no abnormal results whatsoever. In fact, block r/w
>  numbers were improved compared to 2.4.22. Now that I'm looking for it, however, I 
>  do note extremely elevated iowait numbers during a bonnie++ run. Something in the MPT
>  modules?

Greater than 90% I/O wait is to be expected in these tests.  What is of
interest is the overall bandwidth.  2.5 megabytes per second is very
broken.  I have a 53c1030 box here which uses the MPT fusion driver and it
happily does 50MB/sec to a single disk, but I guess that's a different
setup.