Re: READ CAPACITY 16

[Boaz Harrosh <bharrosh@panasas.com>]

James Bottomley wrote:
> On Thu, 2008-12-18 at 16:38 +0200, Boaz Harrosh wrote:
>> OK Then I say D, go to T10, while white list the (0) devices that currently
>> report !SCSI_3 but do support UNMAP. These are only USB right?
>>
>> Your tested devices report SCSI_3? Do all devices that are scsi_level > SCSI_2
>> suppose to support RC16?
> 
> The problem isn't whether they support it or not.  A proper standards
> compliant SCSI device can be sent READ CAPACITY(16) and just return
> ILLEGAL REQUEST sense quite normally.  If those were all the devices in
> the world, we'd send 16 first and fall back to 10.
> 
> The problem is that there are devices (USB devices) that go haywire when
> sent a READ CAPACITY 16 command (or, indeed, any other SCSI command not
> in their vocabulary).  It's for these devices that we do the 10->16
> dance the way we do in sd.c
> 
> Our problem is to identify devices that could reliably receive (and this
> doesn't mean process it just means return a standards compliant response
> without crashing or going out to lunch) READ CAPACITY 16 because the
> current Thin Provisioning draft requires this to indicate thin
> provisioning support.
> 
> My take is still that TP devices have to be SCSI-3 SBC-3 or higher, so
> we just check this and for them do READ CAPACITY 16 with fallback to 10
> on ILLEGAL REQUEST return.  USB has to whitelist the TP compliant
> devices and not mangle the inquiry version field down to SCSI_2 for them
> and the world will just work.
> 
>> My point is make the standard, which is still a draft, crystal clear
>> in a backward compatible way. All new, supporting, devices can be easily
>> identified, and the very few devices that do support the new fixture but
>> were released prior to the finalization of the draft be white-listed.
>> And in any event don't let the standard be broken like that.
> 
> James
> 
> 

OK Jams Mathew thanks that makes sense.

All these emulation layers being in HW, USB, or SW, libata will have to attempt
an higher-then-SCSI_2 if they want RC16 stuff, full stop. That sounds safe to
me, and the market will win.

Boaz