linux-kernel - Re: [PATCHv2] usb: gadget: storage: optional SCSI WRITE FUA bit

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Date:	Wed, 14 Jul 2010 16:55:33 +0300
From:	Roger Quadros <roger.quadros@...ia.com>
To:	ext Andy Shevchenko <andy.shevchenko@...il.com>
CC:	Michał Nazarewicz <m.nazarewicz@...sung.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Shevchenko Andriy (EXT-Teleca/Helsinki)" 
	<ext-andriy.shevchenko@...ia.com>,
	"Karpov Denis.2 (EXT-Teleca/Helsinki)" <ext-denis.2.karpov@...ia.com>,
	"Hunter Adrian (Nokia-MS/Helsinki)" <adrian.hunter@...ia.com>,
	Alan Stern <stern@...land.harvard.edu>,
	David Brownell <dbrownell@...rs.sourceforge.net>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>
Subject: Re: [PATCHv2] usb: gadget: storage: optional SCSI WRITE FUA bit

On 07/14/2010 04:44 PM, ext Andy Shevchenko wrote:
> 2010/7/14 Michał Nazarewicz<m.nazarewicz@...sung.com>:
>> On Wed, 14 Jul 2010 11:05:31 +0200, Andy Shevchenko
>> <andy.shevchenko@...il.com>  wrote:
>>>
>>> MS Windows mounts removable storage in "Removal optimized mode" by
>>> default. All the writes to the media are synchronous which is achieved
>>> by setting FUA (Force Unit Access) bit in SCSI WRITE(10,12) commands.
>>> This prevents I/O requests aggregation in block layer dramatically
>>> decreasing performance.
>>
>>> diff --git a/drivers/usb/gadget/file_storage.c
>>> b/drivers/usb/gadget/file_storage.c
>>> index b49d86e..45f58d9 100644
>>> --- a/drivers/usb/gadget/file_storage.c
>>> +++ b/drivers/usb/gadget/file_storage.c
>>> @@ -93,6 +93,8 @@
>>>   *     removable               Default false, boolean for removable media
>>>   *     luns=N                  Default N = number of filenames, number of
>>>   *                                     LUNs to support
>>> + *     fua=b[,b...]            Default false, booleans for ignore FUA
>>> flag
>>> + *                                     in SCSI WRITE(6,10,12) commands
>>
>> I wonder if it makes sense to make it per-LUN.  I would imagine that it's
>> great
>> to ignore FUA if the device has its own power supply in which case after
>> disconnect
>> the data won't be lost.  This is a per-device property not really per-LUN.
>>   As such
>> I'd make this option global for the gadget.
> Make sense only for removable media with one partition.
> Otherwise. why we have sync option per partition f.e., not per device?
>
by partition do you mean medium? They are different terms.
A storage medium may have 1 or more logical partitions. It is left upto the 
Host/user to decide how he wants to partition the medium.

file_storage driver does not deal with partitions. only mediums.

regards,
-roger
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