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SubjectRe: BUG in kernel: Wrong Handling of USB HDD’s in scsiglue(slave configure) and scsi/sd(sd read cache type)
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Adding linux-usb - to get more insight's into the problem.

On Fri, Sep 30, 2011 at 11:23 PM, Amit Sahrawat
<amit.sahrawat83@gmail.com> wrote:
> On Fri, Sep 30, 2011 at 5:48 PM, James Bottomley
> <James.Bottomley@hansenpartnership.com> wrote:
>> On Fri, 2011-09-30 at 12:26 +0530, Amit Sahrawat wrote:
>>> Now, for the USB HDD which do have write cache - sginfo is showing
>>> them to Write Cache Enabled as false.
>>> Why do the result of hdparm identification and sginfo varies- (I know
>>> they have different interface to work with and hdparm takes care of
>>> that by using SG_IO interface from it's code)? hdparm showed me
>>> correct results - that lead me to digging in the kernel code and
>>> checking the performance for USB HDD with Write cache enabled/disabled
>>> - which also showed that QUEUE ordering chosen for USB HDD is not
>>> correct.
>>
>> Well, what all this means is the SATL in the USB device is implemented
>> wrongly.  Since USB devices only preset SCSI interfaces, that's what we
>> have to believe.
>>
>> hdparm when used correctly sends an ATA inquiry command wrapped in an
>> ATA_12 or ATA_16 SCSI command.  A large number of legacy SATLs are known
>> to crash on these commands.
>>
>> Are you sure the ATA command is reporting correctly?  A write back cache
>> is a remarkably silly thing to enable for a USB device because they're
>> highly likely to be surprise ejected which powers the device down.
>>
> In addition to the problem reported - there is one more thing I have
> noticed with USB HDD - they should be shown as 'removable' but the
> removable is marked only for USB PEN Drives. This seems to be a bit of
> confusing, any mass storage media connected on USB port should be
> recognized as removable.
> So, for handling the issue, I would consider adding the handling in
> slave_configure()(usb/storage/scsiglue) which marks the HDD/pen drives
> as removable also signifying them to be USB based.
> Then, as part of sd_revalidation – how about adding the ATA_IDENTIFY
> command part if the device is USB HDD? As far as the result of
> ATA_IDENTIFY is concerned – they return proper ‘256’ bytes - response
> and the Words – 82, 85 used for feature supported and enabled/disabled
> returns proper values for the USB HDD’s I have seen. In case of USB
> pen drives – they return failure – I did not see any crash – maybe I
> don’t have one of the legacy SATL based disk.
> Since, I am new to this – I will check more on this to get a viable
> solution. Please add your opinion. Can you share the name of the
> device which causes crash with these ATA commands, If I am able to get
> one I can try on that also.
>
>>> I have a large number of USB HDD's - with different vendors, and for
>>> all of them - it is showing Write Cache Enabled as false.
>>> The code works only for the Pen Drives or the USB HDD which do not
>>> have internal cache.
>>>
>>> Also, for journalling filesystem being used on USB HDD - it does
>>> becomes a cause of concern.
>>>
>>> Please share your opinion, I guess we need a change for mode sensing
>>> in the kernel code for USB HDD.
>>
>> Well that's a nastily complex problem.  It really needs to be
>> whitelisted in the USB stack, but if every drive is doing it, that's
>> quite a task.
>>
>> The question becomes how do we detect in a SCSI fashion that the device
>> has a write back cache if none of the standard SCSI mechanisms reports
>> it?
> As far as detecting in SCSI fashion – I wonder using that I would have
> never reached the conclusion that it is the Write Cache of USB HDD
> which is causing problem instead I would have been focusing on
> particular file system (XFS in my case –which in itself is complex) –
> there BARRIER support and also the Queue handling in the elevator with
> I/O scheduler.
> None of the sg utils is showing anything related with the Write Cache
> in USB HDD – which provide any hint that the Cache is enabled – this
> is a bit surprising because most of the high end USB mass storages
> device in the market have Write Cache in them.
>
> Thanks & Regards,
> Amit Sahrawat
>>
>> James
>>
>>
>>
>
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