Disk I/O as a bottleneck?

[Dima (Dan) Yasny]

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)

>
> --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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