Disk I/O as a bottleneck?

[Omer Zak]

I have a PC with powerful processor, lots of RAM and SATA hard disk.
Nevertheless I noticed that sometimes applications (evolution E-mail
software and Firefox[iceweasel] Web browser) have the sluggish feel of a
busy system (command line response time remains crisp, however, because
the processor is 4x2 core one [4 cores, each multithreads as 2]).

I run the gnome-system-monitor all the time.

I notice that even when those applications feel sluggish, only one or at
most two CPUs have high utilization, and there is plenty of free RAM (no
swap space is used at all).

Disk I/O is not monitored by gnome-system-monitor.
So I suspect that the system is slowed down by disk I/O.  I would like
to eliminate it as a possible cause for the applications' sluggish feel.

I ran smartctl tests on the hard disk, and they gave it clean bill of
health.  Therefore I/O error recovery should not be the reason for
performance degradation.

I am asking Collective Wisdom for advice about how to do:
1. Monitoring disk I/O load (counting I/O requests is not sufficient, as
each request takes different time to complete due for example to disk
head seeks or platter rotation time).
2. Disk scheduler fine-tuning possibilities to optimize disk I/O
handling.
3. If smartctl is not sufficient to ensure that no I/O error overhead is
incurred, how to better assess the hard disk's health?

Thanks,
--- Omer

-- 
My Commodore 64 is suffering from slowness and insufficiency of memory;
and its display device is grievously short of pixels.  Can anyone help?
My own blog is at http://www.zak.co.il/tddpirate/

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