From mboxrd@z Thu Jan  1 00:00:00 1970
From: Vladislav Bolkhovitin <vst@vlnb.net>
Subject: Re: [RFC] relaxed barrier semantics
Date: Wed, 28 Jul 2010 20:17:49 +0400
Message-ID: <4C50582D.7050304@vlnb.net>
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Cc: James Bottomley <James.Bottomley@suse.de>,
	Vivek Goyal <vgoyal@redhat.com>, Jan Kara <jack@suse.cz>,
	Christoph Hellwig <hch@lst.de>, jaxboe@fusionio.com,
	linux-fsdevel@vger.kernel.org, linux-scsi@vger.kernel.org,
	tytso@mit.edu, chris.mason@oracle.com, swhiteho@redhat.com,
	konishi.ryusuke@lab.ntt.co.jp
To: Tejun Heo <teheo@suse.de>
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Tejun Heo, on 07/28/2010 06:44 PM wrote:
> Hello,
>
> On 07/28/2010 04:37 PM, James Bottomley wrote:
>>> I don't remember all the details now but IIRC what was necessary was
>>> earlier write failure failing all commands scheduled as ordered.  Does
>>> ACA / UA_INTLCK or whatever allow that?
>>
>> No.  That requires support for QErr ... which is in the same mode page.
>
> I see.
>
>> The real reason we have difficulty is that BUSY/QUEUE_FULL can cause
>> reordering in the issue queue, which is a driver problem and not in the
>> SCSI standards.
>
> Ah yeah right.  ISTR discussions about this years ago.  But one way or
> the other, given the limited amount of ordering information available
> under the block layer, I doubt the benefit of doing would be anything
> significant.  If it can be done w/o too much complexity, sure, but
> otherwise...

Hmm, this thread was started from the need to avoid queue draining, 
because it is a big performance hit. The use of ordered commands allows 
to _completely_ eliminate queue draining _at all_. It looks to be a 
significant benefit worth some additional complexity.

Vlad