Date: Wed, 16 Jan 2008 00:15:03 +0000
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: "Daniel Phillips" <phillips@google.com>
Cc: "Pavel Machek" <pavel@ucw.cz>, "David Chinner" <dgc@sgi.com>,
       "Theodore Tso" <tytso@mit.edu>, "Al Boldi" <a1426z@gawab.com>,
       "Valerie Henson" <val.henson@gmail.com>,
       "Rik van Riel" <riel@redhat.com>, linux-fsdevel@vger.kernel.org,
       linux-kernel@vger.kernel.org
Subject: Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental
 fsck)
Message-ID: <20080116001503.3c0c97cf@lxorguk.ukuu.org.uk>
In-Reply-To: <4d47a5d10801151544k3bc50223ob69c25d8732e3f12@mail.gmail.com>
References: <200801090022.55589.a1426z@gawab.com>
	<200801090740.12989.a1426z@gawab.com>
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	<200801091452.14890.a1426z@gawab.com>
	<20080112145140.GB6751@mit.edu>
	<20080113171916.GB4132@ucw.cz>
	<20080113174125.5f39ac64@lxorguk.ukuu.org.uk>
	<20080115201653.GA5639@elf.ucw.cz>
	<20080115214325.GN155407@sgi.com>
	<20080115230714.GC3573@elf.ucw.cz>
	<4d47a5d10801151544k3bc50223ob69c25d8732e3f12@mail.gmail.com>
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> 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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