On Wed, 2011-09-28 at 21:27 +0530, Amit Sahrawat wrote:

Marking mail to linux-scsi.

Thanks Christoph.

Regards,
Amit Sahrawat

On Wed, Sep 28, 2011 at 9:12 PM, Amit Sahrawat
[off-list ref] wrote:

quoted

When a USB HDD is connected to the device, it invokes slave_configure
to configure the USB HDD. In this function, whenever there is a SCSI
device of type TYPE_DISK, it sets:
       /* A number of devices have problems with MODE SENSE for
                * page x08, so we will skip it. */
               sdev->skip_ms_page_8 = 1;

Now, as a part of SCSI device probing, in the function
sd_revalidate_disk()-->sd_read_cache_type(), there is a condition
if (sdp->skip_ms_page_8)
               goto defaults;
which becomes always true for all the USB HDD’s – the net result is
that the Write Cache is never considered for USB HDD(WCE = 0) –
“Assuming drive cache: write through”

What’s more – the QUEUE ordering which is marked for WCE=0 is
QUEUE_ORDERED_DRAIN, instead of QUEUE_ORDERED_DRAIN_FLUSH
This means there is no flushing of USB HDD internal cache (although
SYNCHRONIZE_CACHE is implemented as passed as command in
sd_prepare_flush) – queue_flush()(called in function
blk_do_ordered()-->start_ordered()) does not gets called in case of
QUEUE_ORDERED_DRAIN.

This causes a serious impact on USB HDD’s.

Please let me know in case I have missed something in my observations.

This should be working in 3.0 ... what version of the kernel are you
testing.  The actual patch that relaxes the caching mode page check is
this one:

commit 0bcaa11154f07502e68375617e5650173eea8e50
Author: Luben Tuikov [off-list ref]
Date:   Thu May 19 00:00:58 2011 -0700

    [SCSI] Retrieve the Caching mode page (version 2)

James