Thumbstick vs HD

John Geldman (jgeldman) jgeldman at lexar.com
Sat Sep 17 23:45:51 PDT 2011


Formatted message: <a href="http://www.t10.org/cgi-bin/ac.pl?t=r&f=r1109181_f.htm">HTML-formatted message</a>

I'm not sure what you want to distinguish between. 
Are you looking for hints on removability?
Are you looking for USB/SATA bridged SSD's or HDD's? 
Or to distinguish USB jumpdrives designed for use as media/copy storage
devices? Unfortunately, a USB jumpdrive can be as boot-ready as a
USB-SATA bridged device.
RMB:
In the data returned by INQUIRY, there is an RMB bit (byte 1, bit 7)
that describes a controller that supports removable media. Think of a
DVD drive with a removable disk, or a USB card reader. In these
removable media cases, the controller is around to report whether or not
the media is present. This just doesn't apply to USB jumpdrives: when a
USB device is yanked, the controller leaves too.
However, certain common OS's have used this reported bit to change the
caching and access strategy for USB storage devices. And sometimes said
OS's and their maintainers may be schizoid on what the policy changes.
That may be an opinion. Some OS's require the bit to cleared for the
device to be considered bootable.
The bottom line for RMB is that  some USB devices set the RMB bit and
some don't. Most consumer devices have it set (removable media), but
some performance devices have it cleared (non-removable media). Some
vendor's have produced vendor specific tools to change the value of this
bit.
Protocol:
Almost all USB 2.0 USB jumpdrives and most early USB 3.0 jumpdrives use
the USB protocol, Bulk-Only Transport (BOT). This is a single threaded
flow, with a USB packet wrapping the command block, a series of USB
packets wrapping the data block (in or out), followed by a USB packet
wrapping the command status, to be followed with yet another command
sequence. 
New USB 3.0 jumpdrives and potentially new USB 2.0 jumpdrives can use
UAS. UAS/UASP was defined partly in T10 and partly in USB-IF (USB Across
SCSI as defined in T10, and USB Across SCSI Protocol in USB-IF). This
enables robust full command queuing. Full command queuing is an
essential ingredient to the functionality we expect for boot storage
performance.
So, the bottom line for SSD-type performance (as opposed to media or
copy centric storage, which still has a place in the market) is to look
for the protocol. BTW, this applies to devices with native or bridged
controllers.
SAT:
Finally, some USB/SATA devices that implement SAT (SCSI ATA Translation)
support a VPD page that provides ATA's IDENTIFY DEVICE data parameters.
This will  identify a bridged device.
I hope I hit the main areas you were looking for.
Regards,
John Geldman
Director, Industry Standards
Lexar Media Inc. (a Micron Technology Inc. subsidiary) 
47300 Bayside Parkway Fremont, CA 94538
P: 510-580-8715
C: 510-449-3597
From: owner-t10 at t10.org [mailto:owner-t10 at t10.org] On Behalf Of Peter
Van Hove
Sent: Saturday, September 17, 2011 9:00 AM
To: T10 Reflector
Subject: Thumbstick vs HD
Hi,
Is there a way (and if so, what is it) to distinguish between a
(removable) thumbstick (memory stick or card) and a (removable) HD ?
I would like to recognize what is what.
Peter



More information about the T10 mailing list