Bob Friesenhahn <[EMAIL PROTECTED]> wrote:
> On Thu, 2 Oct 2008, Joerg Schilling wrote:
> >
> > I am not going to accept blanket numbers... If you claim that a 15k drive
> > offers more than 2x the IOPS/s of a 7.2k drive you would need to show your
> > computation. SAS and SATA use the same cable and in case you buy server
> > grade
> > SATA disks, you also get tagged command queuing.
>
> I sense some confusion here on your part related to basic physics.
> Even with identical seek times, if the drive spins 2X as fast then it
> is able to return the data in 1/2 the time already. But the best SAS
> drives have seek times 2-3X faster than SATA drives. In order to
> return data, the drive has to seek the head, and then wait for the
> data to start to arrive, which is entirely dependent on rotation rate.
> These are reasons why enterprise SAS drives offer far more IOPS than
> SATA drives.
You seem to missunderstand drive physics.
With modern drives, seek times are not a dominating factor. It is the latency
time that is rather important and this is indeed 1/rotanilnal-speed.
On the other side you did missunderstand another important fact in drive
physics:
The sustained transfer speed of a drive is proportional to the linear data
density on the medium.
The third mistake you make is to see that confuse the effects from the
drive interface type with the effects from different drive geometry. The only
coincidence here is that the drive geometry is typically updated more
frequently for SATA drives than it is for SAS drives. This way, you benefit
from the higher data density of a recent SATA drive and get a higher sustained
data rate.
BTW: I am not saying it makes no sense to buy SAS drives but it makes sense to
look at _all_ important parameters. Power consumption is a really important
issue here and the reduced MTBF from using more disks is another one.
Jörg
--
EMail:[EMAIL PROTECTED] (home) Jörg Schilling D-13353 Berlin
[EMAIL PROTECTED] (uni)
[EMAIL PROTECTED] (work) Blog: http://schily.blogspot.com/
URL: http://cdrecord.berlios.de/private/ ftp://ftp.berlios.de/pub/schily
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