Message-ID: <49D0CDBA.7040702@rtr.ca>
Date: Mon, 30 Mar 2009 09:48:42 -0400
From: Mark Lord <lkml@rtr.ca>
Organization: Real-Time Remedies Inc.
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To: Ric Wheeler <rwheeler@redhat.com>
Cc: "Andreas T.Auer" <andreas.t.auer_lkml_73537@ursus.ath.cx>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>, Theodore Tso <tytso@mit.edu>,
       Stefan Richter <stefanr@s5r6.in-berlin.de>,
       Jeff Garzik <jeff@garzik.org>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Matthew Garrett <mjg59@srcf.ucam.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       David Rees <drees76@gmail.com>, Jesper Krogh <jesper@krogh.cc>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Linux 2.6.29
References: <alpine.LFD.2.00.0903271511230.3994@localhost.localdomain>	<alpine.LFD.2.00.0903271522210.3994@localhost.localdomain>	<49CD7B10.7010601@garzik.org>	<49CD891A.7030103@rtr.ca>	<49CD9047.4060500@garzik.org>	<49CE2633.2000903@s5r6.in-berlin.de>	<49CE3186.8090903@garzik.org>	<49CE35AE.1080702@s5r6.in-berlin.de>	<49CE3F74.6090103@rtr.ca>	<20090329231451.GR26138@disturbed>	<20090330003948.GA13356@mit.edu>	<49D0710A.1030805@ursus.ath.cx> <20090330100546.51907bd2@the-village.bc.nu> <49D0A3D6.4000300@ursus.ath.cx> <49D0AA4A.6020308@redhat.com>
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Ric Wheeler wrote:
>
> People keep forgetting that storage (even on your commodity s-ata class 
> of drives) has very large & volatile cache. The disk firmware can hold 
> writes in that cache as long as it wants, reorder its writes into 
> anything that makes sense and has no explicit ordering promises.
..

Hi Ric,

No, we don't forget about those drive caches.  But in practice,
for nearly everyone, they don't actually matter.

The kernel can crash, and the drives, in practice, will still
flush their caches to media by themselves.  Within a second or two.

Sure, there are cases where this might not happen (total power fail),
but those are quite rare for desktop users -- and especially for the
most common variety of desktop user:  notebook users (whose machines
have built-in UPSs).

Cheers
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