From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755388AbaIKOiA (ORCPT ); Thu, 11 Sep 2014 10:38:00 -0400 Received: from michael.mail.tiscali.it ([213.205.33.246]:42471 "EHLO michael.mail.tiscali.it" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751013AbaIKOh5 (ORCPT ); Thu, 11 Sep 2014 10:37:57 -0400 X-Greylist: delayed 348 seconds by postgrey-1.27 at vger.kernel.org; Thu, 11 Sep 2014 10:37:57 EDT To: linux-kernel@vger.kernel.org Subject: AHCI maximum transfer size X-PHP-Originating-Script: 0:rcmail.php MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Date: Thu, 11 Sep 2014 16:32:00 +0200 From: Gionatan Danti Cc: g.danti@assyoma.it Organization: Assyoma s.r.l. Message-ID: User-Agent: Roundcube Webmail/0.9.5 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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