linux-kernel - Re: ATA 4 KiB sector issues.

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Message-ID: <yq1bpeya5fh.fsf@sermon.lab.mkp.net>
Date:	Mon, 08 Mar 2010 16:07:14 -0500
From:	"Martin K. Petersen" <martin.petersen@...cle.com>
To:	Cláudio Martins <ctpm@....utl.pt>
Cc:	James Bottomley <James.Bottomley@...e.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Tejun Heo <tj@...nel.org>,
	"linux-ide\@vger.kernel.org" <linux-ide@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Daniel Taylor <Daniel.Taylor@....com>,
	Jeff Garzik <jeff@...zik.org>, Mark Lord <kernel@...savvy.com>,
	tytso@....edu, hirofumi@...l.parknet.co.jp,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>, irtiger@...il.com,
	Matthew Wilcox <matthew@....cx>, aschnell@...e.de,
	knikanth@...e.de, jdelvare@...e.de
Subject: Re: ATA 4 KiB sector issues.

>>>>> "Cláudio" == Cláudio Martins <ctpm@....utl.pt> writes:

Cláudio> So the question is: what are hard drive makers guaranteeing (if
Cláudio> anything at all)?

No guarantees.  Nothing that you can get in writing, anyway.

Cláudio> Was a 512B sector write really atomic?

Sometimes.

Cláudio> Is a 4k one?  

Sometimes, maybe.

The problem with 4KB physical blocks is that if you do a partial or
misaligned write you'll end up having to do read-modify-write.  And that
introduces are scenario where a subsequent write error will affect
logical blocks that were not part of the I/O request.

However, you also have that with regular drives because they often write
more than the actual block undergoing I/O.  For instance to reduce
hotspot bleed to adjacent sectors.

There have been several unsuccessful attempts at nudging the drive
vendors into giving us real guarantees (supercapacitors, NVRAM or
flash-backed write cache).  No luck so far.  So people that care use
arrays with non-volatile caches.

-- 
Martin K. Petersen	Oracle Linux Engineering
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