linux-ext4 - Re: raid is dangerous but that's secret (was Re: [patch] ext2/3: document conditions when reliable operation is possible)

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Date:	Mon, 31 Aug 2009 11:14:06 -0400
From:	jim owens <jowens@...com>
To:	Christoph Hellwig <hch@...radead.org>
CC:	Mark Lord <lkml@....ca>, Ric Wheeler <rwheeler@...hat.com>,
	Michael Tokarev <mjt@....msk.ru>, david@...g.hm,
	Pavel Machek <pavel@....cz>, Theodore Tso <tytso@....edu>,
	NeilBrown <neilb@...e.de>, Rob Landley <rob@...dley.net>,
	Florian Weimer <fweimer@....de>,
	Goswin von Brederlow <goswin-v-b@....de>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, mtk.manpages@...il.com,
	rdunlap@...otime.net, linux-doc@...r.kernel.org,
	linux-ext4@...r.kernel.org, corbet@....net
Subject: Re: raid is dangerous but that's secret (was Re: [patch] ext2/3:
 document conditions when reliable operation is possible)

Christoph Hellwig wrote:
> On Mon, Aug 31, 2009 at 09:19:58AM -0400, Mark Lord wrote:
>>> In my opinion even that is too weak.  We know how to control the cache
>>> settings on all common disks (that is scsi and ata), so we should always
>>> disable the write cache unless we know that the whole stack (filesystem,
>>> raid, volume managers) supports barriers.  And even then we should make
>>> sure the filesystems does actually use barriers everywhere that's needed
>>> which failed at for years.
>> ..
>>
>> That stack does not know that my MD device has full battery backup,
>> so it bloody well better NOT prevent me from enabling the write caches.
> 
> No one is going to prevent you from doing it.  That question is one of
> sane defaults.  And always safe, but slower if you have advanced
> equipment is a much better default than usafe by default on most of
> the install base.

I've always agreed with "be safe first" and have worked where
we always shut write cache off unless we knew it had battery.

But before we make disabling cache the default, this is the impact:

- users will see it as a performance regression

- trashy OS vendors who never disable cache will benchmark
   better than "out of the box" linux.

Because as we all know, users don't read release notes.

Been there, done that, felt the pain.

jim
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