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Date: Mon, 31 Aug 2009 09:21:39 -0400
From: Christoph Hellwig <hch@infradead.org>
To: Mark Lord <lkml@rtr.ca>
Cc: Christoph Hellwig <hch@infradead.org>, Ric Wheeler <rwheeler@redhat.com>,
       Michael Tokarev <mjt@tls.msk.ru>, david@lang.hm,
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Subject: Re: raid is dangerous but that's secret (was Re: [patch] ext2/3:
	document conditions when reliable operation is possible)
Message-ID: <20090831132139.GA5425@infradead.org>
References: <20090828064449.GA27528@elf.ucw.cz> <20090828120854.GA8153@mit.edu> <20090830075135.GA1874@ucw.cz> <alpine.DEB.2.00.0908300550320.6822@asgard.lang.hm> <4A9A88B6.9050902@redhat.com> <4A9A9034.8000703@msgid.tls.msk.ru> <20090830163513.GA25899@infradead.org> <4A9BCCEF.7010402@redhat.com> <20090831131626.GA17325@infradead.org> <4A9BCDFE.50008@rtr.ca>
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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.