[CentOS] Very slow disk performance on CentOS 4.5 [solved]

[Alfred von Campe]

> Yes, I was going to say this too, make sure the SATA settings in the
> BIOS are all set to SATA operation and not "legacy", then you should
> see /dev/sda and /dev/sdb and all DMA, IO size, NCQ and multiple  
> sector
> settings will be properly negotiated at start-up.

I bit the bullet and rebooted my system.  I couldn't find anything in  
the BIOS regarding this.  However, while the system was down, I  
simply swapped the SATA connections of one of the optical drives with  
the slow hard disk.  Upon reboot, the slow disk formerly at /dev/hda  
disk was now /dev/sda (and the former /dev/sda was now /dev/sdb) and  
the two optical drives are /dev/hda and /dev/hdb.  Moreover, the disk  
drive now performs as before.

So why does a simple swap like this cause such a performance  
difference (almost an order of magnitude!)?

> To set these via hdarpm:
>
> hdparm -c 1 /dev/hda (for 32-bit)
> hdparm -d 1 /dev/hda (for DMA)
> hdparm -m 16 /dev/hda (multiple sector IO = 16 sectors)
>
> These can be combined to: hdparm -c 1 -d 1 -m 16 /dev/hda

Now that the drive is /dev/sda, hdparm doesn't work.  Is there a  
similar utility to change these settings on SATA drives that show up  
as /dev/sdx?  Is this even needed?  In my case, I'm happy to get back  
the previous performance, but if there is a way to increase it even  
further, I'd like to know.

Alfred