[K12OSN] Are fast disks really that important and why?

[John Lucas]

On Thursday 31 May 2007 15:30, john wrote:
> Hello all,
>
>  I am trying to get THE definitive take on a question I have regarding
> the necessity of using a fast disk array for LTSP. I touched on this
> in a previous email and got some good advice. I want to probe a little
> bit more if you don't mind. I hope I don't sound too dumb as I ask
> these questions:

> However a unix guru I work with responded to me:
>
> "But--at least in all the UNIXen I ever worked on--you can and do implement
> this in software. And not in hardware."
>

--- snip ---

>  To prove his point we did the following:
>
> We tested our installation in a lab with 30 thin clients attached to a
> single commodity workstation running ubuntu/ltsp4.2 with 2G ram, 3Ghz
> processor. We used  vmstat to monitor CPU,memory usage and disk i/o.
> We found that disk I/O was relatively light after application startup
> and that slow downs began to happen as processes queued up behind the
> CPU waiting for their time slice to be handled. In short client speed
> fell off as the CPU got busier, and not because of busy disks.
>
> Were we looking at this in the wrong way? Was there a better way to
> determine Disk IO?
>

The difference is that SCSI/SAS can offload the CPU to a greater extent than 
IDE/SATA. With SCSI more of the "intelligence" is in the drives than with 
SATA. So for each I/O operation, less CPU is taken up with the SCSI system 
vs. the SATA system.

The availability of higher platter speeds with SCSI (15K rpm) vs. SATA (7200 
rpm with smaller drives available at 10K rpm) has the advantage of higher 
sustained transfer rates (for sequential reads, like application loading) as 
well as being "quicker" to place any given sector under the read head (random 
seeks). These are marginal considerations (for me).

If my budget accomodates SCSI, I would prefer it over SATA. As part of a 
server purchase amortized over 4 - 5 years (typically), the cost difference 
isn't that great. Only when every dollar is being scrutinized by some bean 
counter would I consider cutting this particular corner. Having said that, 
either system works acceptably and I wouldn't spend too many restless nights 
worrying if you got the absolute fastest solution. Reliability of the 
hardware and the worth of the warrantee (vendor) are more important.


-- 
        "History doesn't repeat itself; at best it rhymes."
                        - Mark Twain

| John Lucas                          MrJohnLucas at gmail.com               |
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