Re: Tiobench results LOWER with more threads

[Jakob Oestergaard <jakob@unthought.net>]

On Thu, Oct 17, 2002 at 07:31:37PM -0700, Gregory Leblanc wrote:
> On Thu, Oct 17, 2002 at 11:50:14PM +0200, Jakob Oestergaard wrote:
> > On Tue, Oct 15, 2002 at 10:20:55AM +0200, Vladimir Milovanovic wrote:
> > > OK, just joined the list and rad the faq, and something caught my eye. 
> > > Tiobench results are apparently supposed to INCREASE when there are more 
> > > threads.
> > 
> > No, what gave you that idea?
> > 
> > It is so much easier for the kernel to handle one sequential stream of
> > I/O, instead of many streams.
> > 
> > If you have more than one stream, you need to seek. Seeking is bad. One
> > sequential I/O is almost always (with the notable exception of RAID-1
> > reads) faster in total sustained throughput, and always (as in really
> > always) faster in per-thread sustained throughput.
> 
> Err, so are you saying that a single sequential I/O is slower on RAID
> 1 when compared with a single disk?  That doesn't make a lot of
> sense.  There -are- instances where parallel I/O is required.  In
> these cases, any RAID-1 should be much faster than a single disk, as
> should RAID 10.  I'm not sure that RAID 5 should give a similar
> benefit, but given the cost of disks, I don't care about RAID 5.

The exception I made was:

"One sequential I/O is almost always (with the notable exception of
RAID-1 reads) faster in total sustained throughput,..."

What I wanted to say was: One sequential reader is usually faster than N
readers are in total.  Except on an N-disk RAID-1, where up to N readers
will be faster in total than one reader.

An N-disk RAID-1 will scale with up to N readers.

There's some fuzz on these measurements, because a sequential read on a
filesystem is not a sequential read on the disk (because of fs
metadata), so therefore often a single threaded read on an N-disk RAID-1
will be faster than on a single disk - and maybe RAID-1 will not scale
up to N readers, but only N-1, but that all depends on the fs - and
let's not go too deep into that for now  :)

-- 
................................................................
:   jakob@unthought.net   : And I see the elder races,         :
:.........................: putrid forms of man                :
:   Jakob Østergaard      : See him rise and claim the earth,  :
:        OZ9ABN           : his downfall is at hand.           :
:.........................:............{Konkhra}...............:
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