On Fri, 30 Sep 2011, Amit Sahrawat wrote:

Adding linux-usb - to get more insight's into the problem.

On Fri, Sep 30, 2011 at 11:23 PM, Amit Sahrawat
[off-list ref] wrote:

quoted

On Fri, Sep 30, 2011 at 5:48 PM, James Bottomley
[off-list ref] wrote:

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On Fri, 2011-09-30 at 12:26 +0530, Amit Sahrawat wrote:

quoted

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.

A usbmon trace would be very helpful for understanding this.  
Instructions can be found in the kernel source file 
Documentation/usb/usbmon.txt.  Post a trace showing what happens when 
the drive is first plugged in.

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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.

That is not true.  The word "removable" refers to the storage media.  
Thus, a cdrom drive is removable because you can remove the disc from
the drive.  The same for a card reader, because you can remove the card
from the reader.  But a flash drive is not removable, because you can't
take the flash memory chip out of the device.

You are confusing "removable" with "hot-unpluggable".  All USB drives
are hot-unpluggable, but relatively few of them are removable.  Vendors 
often get this wrong, however.

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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.

This does not happen in usb-storage at all.  The SCSI core is
responsible for determining whether or not a device is removable, and
it does this by looking at second byte of the data returned by the SCSI
INQUIRY command.

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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.

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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.

This indicates those drives do not implement the SCSI MODE SENSE 
command correctly.  usbmon will help track down the problem.

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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.

Alan Stern