Re: no merit from using DMA-66?
Hello!
That has something to do with your hard disk cache. Because, when the data
requested by the PC happens to be in the internal cache (typically
512k-2megs), it can be transferrred to the mainboard at full bus speed rate!
That's an improvement of UDMA! And, the interfaces are always a little bit
faster, then the disks are getting faster, and then they make a new
interface standard again. That's how it always is, because, it is better to
not to have problems with a slow interface.
So, either your benchmark doesn't manage to switch off the cache for its
measure or it only shows you the highest bus speed rate. But, 33MB/66MB is
definitely not a data rate any recent IDE HDD can achieve.
Kind Regards,
Stephan Hachinger
----- Original Message -----
From: Vachirasuk <vachi@pop07.odn.ne.jp>
To: <Stephan.Hachinger@gmx.de>
Cc: <debian-user@lists.debian.org>
Sent: Sunday, March 19, 2000 3:36 PM
Subject: Re: no merit from using DMA-66?
> Hi,
>
> I think I am trying to measure data transfer rate from harddisk. I
> have read UDMA-mini-HOWTO on LDP site and it also said "UDMA drives
> will give you between 10 and 15 MB/s using UDMA mode 2 (33 MB/s) or 4
> (66 MB/s) enabled". If the DTR never gets higher than 33MB/sec then
> why use DMA-66, since there is no performance improvement? How come
> benchmark under Win98 gave me DTR of DMA-66 drive 2 times of normal
> DMA-33 drive? Am I misunderstanding something here?
>
> Confused
>
> Vachi
>
> -----
> Vachirasuk Setalaphruk
> ISE, Osaka University
>
> From: "Stephan Hachinger" <Stephan.Hachinger@gmx.de>
> Subject: Re: no merit from using DMA-66?
> Date: Sat, 18 Mar 2000 21:35:53 +0100
>
> > Hello!
> >
> > Are you really sure you're measuring the maximal bus speed?
> >
> > Because the bus speed (UDMA33=33MB/sec; UDMA66=66MB/sec) is only the
maximum
> > data rate which can be transferred. However, the magnetic disk can never
be
> > that fast, so that data rates of 16MB/sec are quite normal and not bad.
> > 25MB/sec must be a very good hard disk. So, just don't worry, this high
data
> > rates can only be reached if the data comes from the HDD cache over the
bus.
> >
> > Kind Regards,
> >
> > Stephan Hachinger
> >
>
Reply to: