From mboxrd@z Thu Jan 1 00:00:00 1970 From: "H. Peter Anvin" Subject: Re: ATA 4 KiB sector issues. Date: Mon, 08 Mar 2010 11:11:49 -0800 Message-ID: <4B954BF5.2050506@zytor.com> References: <4B947393.2050002@kernel.org> <1268031640.4389.11.camel@mulgrave.site> <4B9546E6.6050006@zytor.com> <1268074705.10660.23.camel@mulgrave.site> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Return-path: Received: from terminus.zytor.com ([198.137.202.10]:36954 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753576Ab0CHTMs (ORCPT ); Mon, 8 Mar 2010 14:12:48 -0500 In-Reply-To: <1268074705.10660.23.camel@mulgrave.site> Sender: linux-ide-owner@vger.kernel.org List-Id: linux-ide@vger.kernel.org To: James Bottomley Cc: "Martin K. Petersen" , Tejun Heo , "linux-ide@vger.kernel.org" , lkml , Daniel Taylor , Jeff Garzik , Mark Lord , tytso@mit.edu, hirofumi@mail.parknet.co.jp, Andrew Morton , Alan Cox , irtiger@gmail.com, Matthew Wilcox , aschnell@suse.de, knikanth@suse.de, jdelvare@suse.de On 03/08/2010 10:58 AM, James Bottomley wrote: >> >> On the flipside, though, there really is very little net benefit to 4K >> as opposed to 512 byte logical sectors: the additional protocol overhead >> is relatively minimal, and as long as writes are aligned full blocks, >> there shouldn't be any additional overhead on either the OS or the drive >> side. On the plus side, you get full compatibility with the existing >> software stack. The equation really seems rather simple. > > There's another problem that afflicts 4k drives emulating 512b: they > have to do a read modify write for any isolated 512b write ... that > leads to potential corruption of adjacent 512b blocks if power is lost > at the moment the write is being done. Since most Linux filesystems are > 4k sectors, misalignment really hammers this, plus most journal writes > seem to be done in 512 byte increments. I suppose for USB this could be > regarded as flakey as usual, though. > Misalignment sucks in general. This is nothing new - the RAID and flash people have had these problems for a long time now. It's clear we need to align our filesystems, period. As to the read-modify-write issue: to some degree there is very little you can do about it other than a big enough capacitor. If you can't write a sector atomically and have it stick, you're screwed no matter what. -hpa