I/O throughput analysis

[Lennart Sorensen]

On Sat, Jan 22, 2011 at 02:28:06PM -0500, William Muriithi wrote:
> A workmate and I had an argument that we could not seem to agree on
> yesterday. That mean we did not learn anything from the discussion and
> I would like to hear your opinion on it. It could help make sense of
> each other arguments
> 
> Which of these two systems would have a better input output
> performance?  Both system have the same configurations, CPU, chipset
> everything down the power supply. They differ though with the number
> of hard disk.  Both system has a RAID card with a maximum speed of 6
> GB per sec
> 
> System A  has 6 drives. Each drive has a specification of 1500 RPM,
> 300 GB size SAS interface.
> 
> System B has 8 drives.  Each drive is 146GB in size, 1500 RPM and also
> with a SAS interface.

I imagine you meant 15000 RPM in both cases.

> SAS interface can comfortably handle 3 GB per second.
> 
> So, in your opinion, is the RAID card a potential bottleneck on both systems?

It could be.

> Which of the two system would offer a better throughput ?

The one with the larger disks, because the higher bit density gives
higher throughput.  The access time is almost certainly identical on
the two since they are the same RPM.

The 8 drive does have more heads to spread the load, so it could do a
bit more I/Os per second, but the transfer rate would be lower due to
the lower media density.

So for I/Os per second, more disks at higher RPMs always help.
For bandwidth, higher density disks are what you want.

-- 
Len Sorensen
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