Re: AHCI maximum transfer size

Gionatan Danti
2014-09-16T08:55:26Z

> Hi all,
> I searched the list and the internet for this question, but I did not
> find a definitive answer.
>
> I wonder what is the maximum AHCI transfer size for kernel 2.6.32.x
> Let me explain: using a command as "dd if=/dev/zero of=/dev/sda bs=2M" I
> can see, via iostat, that request-size is at 2M (as expected). However
> it seems to me that the effective transfer size is way lower: disabling
> writeback cache on the target disk, I see very low performance even for
> big-chunk transfers.
>
> Googling, I found this post on stackoverflow stating that maximum
> observerd AHCI transfer size (with a SATA protocol analyzer) is 128KB:
> http://stackoverflow.com/questions/24053662/how-to-enlarge-sata-transfer-size-in-linux-ahci-driver.
>
> The stackoverflow post seems to agree with this old LWN.net article:
> http://lwn.net/Articles/77981/
>
> Anyone has any idea of how to find (and eventually change) the used AHCI
> transfer size without touching the code?
>
> NOTE: please CC me directly.
> Regards.
>

Hi all,
any idea on the subject?

Regards.