Re: SATA Performance with Intel ICH6

[Alan <alan@lxorguk.ukuu.org.uk>]

On Fri, 24 Nov 2006 15:10:11 +0100
"Martin A. Fink" <fink@mpe.mpg.de> wrote:

> Well this seems to be independend from the file system. I tried to write 
> directly to the raw device, but nevertheless the cpu time was 20% (sys time).

sys time is not neccessarily CPU time.
 
> This is an interessting point. The specification say that I can handle around 
> 120 to 150 MB/s each of the 4 S-ATA ports. With ICH6 the S-ATA ports seem to 

At once ?

> be directly connected to the Southbridge, and the Southbridge is directly 
> connected to Northbridge via PCI Express. So it should be possible to get 150 
> MB/s from north to south and from there in packages of 55 MB/s to the 
> disks ?!

Ask the vendor.
 
> <6>scsi1 : ata_piix
> <5>  Vendor: ATA       Model: WDC WD740GD-75FL  Rev: 21.0
> <5>  Type:   Direct-Access                      ANSI SCSI revision: 05

The PIIX interface needs CPU intervention each command, so in practice
about every 64K or so, and the CPU gets stalled waiting for the disk
during the setup of each I/O. The newer kernels support AHCI which does
not have this overhead, but it is only present on the newest intel
controllers.

> and strace dd... gives among other information
> 6.84s 1004calls syscall: write
> 
> So I spend 45s of 52s within the kernel. Why so long?

Waiting for the disk I would imagine.

Alan