Disk I/O as a bottleneck?

[guy keren]

On Sat, 2011-05-07 at 16:19 +0300, Dima (Dan) Yasny wrote:
> On Sat, May 7, 2011 at 4:06 PM, guy keren <choo at actcom.co.il> wrote:
> >
> > 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 ;)
> 
> Would probably be cheaper to get a bunch of SATAs into a raid array -
> spindle count matters after all.
> 
> My home machine is not too new, but it definitely took wing after I
> replaced one large SATA disk with 6 smaller ones in a raid5 (I'm not
> risky enough for raid0)
> 

you are, of-course, quite right. provided that a hardware RAID
controller is being used.

--guy


> >
> > --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
> >>
> >
> >
> >
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