linux-kernel - Re: [PATCH] sd: do not set changed flag on all unit attention conditions

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Date:	Wed, 25 Jul 2012 14:09:20 +0200
From:	Hannes Reinecke <hare@...e.de>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org
Subject: Re: [PATCH] sd: do not set changed flag on all unit attention conditions

On 07/17/2012 11:11 AM, James Bottomley wrote:
> On Tue, 2012-07-17 at 10:54 +0200, Paolo Bonzini wrote:
>> Il 17/07/2012 10:40, James Bottomley ha scritto:
>>>>>
>>>>> It's not specific to virtio-scsi, in fact I expect that virtio-scsi will
>>>>> be almost always used with non-removable disks.
>>>>>
>>>>> However, QEMU's SCSI target is not used just for virtio-scsi (for
>>>>> example it can be used for USB storage), and it lets you mark a disk as
>>>>> removable---why? because there exists real hardware that presents itself
>>>>> as an SBC removable disk.  The only thing that is specific to
>>>>> virtualization, is support for online resizing (which generates a unit
>>>>> attention condition CAPACITY DATA HAS CHANGED).
>>> So what's the problem?  If you're doing pass through of a physical disk,
>>> we pick up removable from its inquiry string ... a physical removable
>>> device doesn't get resized.  If you have a virtual disk you want to
>>> resize, you don't set the removable flag in the inquiry data.
>>
>> In practice people will do what you said, and it's not a problem.
>>
>> However, there's nothing that prevents you from running qemu with a
>> removable SCSI disk, and then resizing it.  I would like this to work,
>> because SBC allows it and there's no reason why it shouldn't.
> 
> There's no such thing in the market today as a removable disk that's
> resizeable.  Removable disks are for things like backup cartridges and
> ageing jazz drives.  Worse: most removeable devices today are USB card
> readers whose standards compliance varies from iffy to non existent.
> Resizeable disks are currently the province of storage arrays.
> 
Ho-hum. I beg to disagree.

drivers/scsi/aacraid/aachba.c:2266

		/* Do not cache partition table for arrays */
		scsicmd->device->removable = 1;

To the extend of my knowledge aacraid does this _precisely_ to allow
for resizing; in effect every open() will trigger a device revalidation.

So I guess by just setting the 'removable' flag you should be okay.
You might need to remount it, but that's another story.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@...e.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)


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