Re: Why is deleting (or reading) files not counted as IO-Wait in top?

[David Chinner <dgc@sgi.com>]

On Wed, Jan 02, 2008 at 08:35:03PM +0100, Matthias Schniedermeyer wrote:
> Hi
> 
> 
> Currently i'm deleting about 500.000 files on a XFS-filesystem which 
> takes a few minutes, as i had a top open i saw that 'wa' is shown as 
> 0.0% (Nothing else running currently) and everything except 'id' is near 
> the bottom too. Kernel is 2.6.23.11.

Simply because the only I/O that XFS does during a delete is
to the log and the log does async I/O and hence the process
never blocks in I/O.

Instead, it blocks in a far more complex space reservation that
may or may not be related to I/O wait....

> So, as 'rm -rf' is essentially a IO (or seek, to be more correct)-bound 
> task, shouldn't that count as "Waiting for IO"?

rm -rf is not seek bound on XFS - it's generally determined by
the sequential write speed of the block device or how fast your
CPU is....

> The man-page of top says:
> 'Amount of time the CPU has been waiting for I/O to complete.'

Async I/O means that typically your CPU does not get held up
waiting for I/O to complete....

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group