Disk I/O as a bottleneck?

[Dima (Dan) Yasny]

On Sat, May 7, 2011 at 4:41 PM, guy keren <choo at actcom.co.il> wrote:
> 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.
>

I've seen performance increases even using fakeraid (the feared intel
matrix 8.x in my machine) and mdadm. Especially in a machine that has
a UPS (to step in instead of a BBU) and, like mentioned above - lots
of crunchpower

> --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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>> >
>
>
>