AHCI maximum transfer size

AHCI maximum transfer size

[Gionatan Danti]

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.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8

Re: AHCI maximum transfer size

[Gionatan Danti]

> 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.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8

Re: AHCI maximum transfer size

[Gionatan Danti]

On 16/09/2014 10:56, Gionatan Danti wrote:
>> 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.

Hi guys,
anyone with a suggestion?

Thank you.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8