linux-kernel - Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)

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Message-ID: <20080116001503.3c0c97cf@lxorguk.ukuu.org.uk>
Date:	Wed, 16 Jan 2008 00:15:03 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	"Daniel Phillips" <phillips@...gle.com>
Cc:	"Pavel Machek" <pavel@....cz>, "David Chinner" <dgc@....com>,
	"Theodore Tso" <tytso@....edu>, "Al Boldi" <a1426z@...ab.com>,
	"Valerie Henson" <val.henson@...il.com>,
	"Rik van Riel" <riel@...hat.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental
 fsck)

> Writeback cache on disk in iteself is not bad, it only gets bad if the
> disk is not engineered to save all its dirty cache on power loss,
> using the disk motor as a generator or alternatively a small battery.
> It would be awfully nice to know which brands fail here, if any,
> because writeback cache is a big performance booster.

AFAIK no drive saves the cache. The worst case cache flush for drives is
several seconds with no retries and a couple of minutes if something
really bad happens.

This is why the kernel has some knowledge of barriers and uses them to
issue flushes when needed.
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