linux-kernel - Re: [Patch] SCSI: Retrieve Cache Mode Using SG_ATA

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Date:	Tue, 11 Oct 2011 09:05:26 -0500
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Amit Sahrawat <amit.sahrawat83@...il.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Douglas Gilbert <dgilbert@...erlog.com>,
	linux-usb@...r.kernel.org, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	NamJae Jeon <linkinjeon@...il.com>
Subject: Re: [Patch] SCSI: Retrieve Cache Mode Using SG_ATA_16 if normal
 routine fails

On Tue, 2011-10-11 at 13:56 +0530, Amit Sahrawat wrote:
> SCSI: Retrieve Cache Mode Using SG_ATA_16 if normal routine fails
> 
> It has been observed that a number of USB HDD's do not respond correctly
> to SCSI mode sense command(retrieve caching pages) which results in their
> Write Cache being discarded by queue requests i.e., WCE if left set to
> '0'(disabled). So, in order to identify the devices correctly - give it
> a last try using SG_ATA_16 after failure from normal routine.

This is a non-starter, as I've said before.  Apart from the layering
violation of trying to make sd ATA aware, ATA_16 is known to crash some
USB devices (the bug reports are mostly about smartctl which can be made
to use ATA_16 failing).

The correct way to implement this is to have a user visible WCE variable
that can be written to in the scsi_disk class sysfs files so the user
can alter the caching type on the fly.

James


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