Disk I/O as a bottleneck?

[guy keren]

you are stepping into "never-never" land ;)

"iostat -x -k 1" is your friend - just make sure you open a very wide
terminal in which to look at it.

disks are notoriously slow, regardless of error cases. it is enough if
an applications perform a lot of random I/O - to make them work very
slow.

i'd refer you to the slides of the "linux I/O" lecture, at:

http://haifux.org/lectures/254/alice_and_bob_in_io_land/

read them through. there are also some links to pages that discuss disk
I/O tweaking.

as for the elevator - you could try using the "deadline" elevator and
see if this gives you any remedy.

if you eventually decide that it is indeed disk I/O that slows you down,
and if you have a lot of money to spend - you could consider buying an
enterprise-grade SSD (e.g. from fusion I/O or from OCZ - although for
your use-case, some of the cheaper SSDs will do) and use it instead of
the hard disks. they only cost thousands of dollars for a 600GB SSD ;)

--guy

On Sat, 2011-05-07 at 15:29 +0300, Omer Zak wrote:
> 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
>