From: Ric Wheeler <rwheeler@redhat.com>
Subject: Re: wishful thinking about atomic, multi-sector or full MD stripe
 width, writes in storage
Date: Thu, 03 Sep 2009 20:39:36 -0400
Message-ID: <4AA061C8.9080600@redhat.com>
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Cc: Christoph Hellwig <hch@infradead.org>, Mark Lord <lkml@rtr.ca>,
	Michael Tokarev <mjt@tls.msk.ru>, david@lang.hm,
	Pavel Machek <pavel@ucw.cz>, Theodore Tso <tytso@mit.edu>,
	NeilBrown <neilb@suse.de>, Rob Landley <rob@landley.net>,
	Florian Weimer <fweimer@bfk.de>,
	Goswin von Brederlow <goswin-v-b@web.de>,
	kernel list <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@osdl.org>, mtk.manpages@gmail.com,
	rdunlap@xenotime.net, linux-doc@vger.kernel.org,
	linux-ext4@vger.kernel.org, corbet@lwn.net
To: Krzysztof Halasa <khc@pm.waw.pl>
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On 09/03/2009 07:50 PM, Krzysztof Halasa wrote:
> Ric Wheeler<rwheeler@redhat.com>  writes:
>
>    
>> The whole thread above is about software MD using commodity drives
>> (S-ATA or SAS) without battery backed write cache.
>>      
> Yes. However, you mentioned external RAID arrays disable disk caches.
> That's why I asked if they are using SATA or SCSI/etc. disks, and if
> they have battery-backed cache.
>
>    

Sorry for the confusion - they disable the write caches on the component 
drives normally, but have their own write cache which is not disabled in 
most cases.

>> Also, when you enable the write cache (MD or not) you are buffering
>> multiple MB's of data that can go away on power loss. Far greater
>> (10x) the exposure that the partial RAID rewrite case worries about.
>>      
> The cache is flushed with working barriers. I guess it should be
> superior to disabled WB cache, in both performance and expected disk
> lifetime.
>    

True - barriers (especially on big, slow s-ata drives) usually give you 
an overall win. SAS drives it seems to make less of an impact, but then 
you always need to benchmark your workload on anything to get the only 
numbers that really matter :-)

ric