Higher ATA-Mode -> lower speed

Emanuel Strobl emanuel.strobl at gmx.net
Thu Jan 20 11:00:48 PST 2005

Am Donnerstag, 20. Januar 2005 19:48 schrieb Emanuel Strobl:
> Dear ata interested guys,
> I observed a strange behaviour which seems to explain my often noticed
> "16MB/s hard-limit"
> I have a UDMA133 drive (MAXTOR 6L060J3) which saturates at 16MB/s when I
> dump anything to it, regardless of the block size. This transfer rate is
> reached with bs=4k and doesn't increase any more even not with bs=64k.
> Now, when I set the mode to UDMA100 I get well over 40MB/s!!!!
> Can anybody confirm that for different hw? Especially people like fandino
> who already discussed poor ata performance on -current (~16 Oct. 04).

I can confirm that for my workstation here. i815 chipset and seagate 
When set to UDMA100 (the maximum) the following dump gives 16MB/s 'dd 
if=/dev/zero of=/usr/testfile bs=16k count=2000).
When I limit the mode to UDMA66 I get 53MB/s !!!!!!!!!!!!!!!!

It seems that's independent of UDMA133 instead it's when mode is set to the 
maximum the chipset can handle!

This really hurts! But it's _the_ eplanation for all the "poor ata 
performance" reports.


> Why does UDMA133 mode limit the transfer speed so badly? And why do I get
> significantly slower transfer rates (32MB/s insted of 42MB/s) when I set
> the mode to UDMA66 (compared to UDMA100 but twice the speed of UDMA133)?
> There's only one device on the channel, so UDMA66 should be fine for
> 42MB/s. The controller is a HPT372.
> Best regards,
> -Harry
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