ahci problems with sata disk.

[kenneth johansson <ken@kenjo.org>]

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I changed my bios setting for SATA from IDE to AHCI.

This resulted in some "interesting" read throughput. 

plots can be found at http://kenjo.org/~ken/sata/
The plots was done on a live disk so some noise is expected but in the
ahci mode the throughput get stuck at 17 MB way to much.

The read was done with dd on the device and plot data was from "vmstat
1"

kernel version is  "2.6.20-5-generic" from ubuntu feisty (unstable)
running in 32 bit mode on a core 2 duo

---
00:1f.2 IDE interface: Intel Corporation 82801H (ICH8 Family) 4 port
SATA IDE Controller (rev 02) (prog-if 8f [Master SecP SecO PriP PriO])
        Subsystem: ASUSTeK Computer Inc. Unknown device 821a
        Flags: bus master, 66MHz, medium devsel, latency 0, IRQ 21
        I/O ports at ec00 [size=8]
        I/O ports at e880 [size=4]
        I/O ports at e800 [size=8]
        I/O ports at e480 [size=4]
        I/O ports at e400 [size=16]
        I/O ports at e080 [size=16]
        Capabilities: <access denied>
---
Handle 0x0002, DMI type 2, 15 bytes
Base Board Information
        Manufacturer: ASUSTeK Computer INC.
        Product Name: P5B-V
        Version: Rev 1.xx
 ---


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