linux-kernel - Re: MD/RAID time out writing superblock

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Date:	Fri, 18 Sep 2009 00:47:44 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Mark Lord <liml@....ca>
CC:	Chris Webb <chris@...chsys.com>, Ric Wheeler <rwheeler@...hat.com>,
	Andrei Tanas <andrei@...as.ca>, NeilBrown <neilb@...e.de>,
	linux-kernel@...r.kernel.org,
	IDE/ATA development list <linux-ide@...r.kernel.org>,
	linux-scsi@...r.kernel.org, Jeff Garzik <jgarzik@...hat.com>,
	Mark Lord <mlord@...ox.com>
Subject: Re: MD/RAID time out writing superblock

Hello,

Mark Lord wrote:
>> That's interesting.  One important side effect of issuing IDENTIFY is
>> that they will serialize command streams as they are not NCQ commands
>> and thus could change command patterns significantly.
> ..
> 
> SMART is the opcode that is most frequently implicated here, not IDENTIFY.

Yeap, any non-NCQ commands would do it.

> Note that even a barrier FLUSH CACHE is non NCQ and will serialize
> the stream.

Yeah, I was just thinking that issuing non-NCQ commands mixed with NCQ
commands would make the command stream fluctuate more.  Modern drives
are pretty good at lowering power consumption while idle so being more
fluctuative (is it a word?) might have something to do with the
problem.  Just a wild speculation tho.

Thanks.

-- 
tejun
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