Re: READ CAPACITY 16

Boaz Harrosh
2008-12-18T09:05:58Z

Matthew Wilcox wrote:
> On Wed, Dec 17, 2008 at 09:50:52AM -0800, Grant Grundler wrote:
>>> Algorithm A (a perfect world):
>>>
>>>
>>> Algorithm B:
>>>
>>> Issue RC10
>>> Issue RC16
>>> -> If it succeeds, use its results in preference to those from RC10
>>> -> If it fails, carry on with the results from RC10
>>> -> If it times out, reset the device, carry on with the results from RC10
>> I fail to see an effective difference between Algo A and B.
>
> Whether to issue an RC10 before issuing an RC16 or not. It matches what
> we currently do better (we currently issue an RC10 and then issue an
> RC16 if RC10 reports we have 0xffffffff LBAs).
>

Sorry to barge in but I think this is the most practical solution and the one
to go to T10 with.

If a (new) device supports RC16 it should return LBAs==0xffffffff for RC10 even
if it's capacity is smaller, to indicate an RC16 request.
If LBAs!=0xffffffff and !SCSI_3 then do not risk RC16 unless a white list
or load parameter.

Since you are going to T10 with this the white list should be, as you said
in other mail, zero length.

>> The question really is one you already asked:
>>> ...The question is what to do about devices that either
>>> hang or take a long time to respond to an RC16 command.
>> A few ideas:
>> 1) maintain a blacklist
>
> Which is obviously what we're trying to avoid doing.
>

If you are going to T10 with this a white list should be much shorter

>> 2) anything in RC10 or IDENTIFY that would clue us about RC16 functionality?
>> If so, then something like B or C would make sense.
>
> RC10 only returns number of LBAs and how many bytes per LBA. I don't
> see anything in the INQUIRY data (other than the protection bit, which
> we already use to know that RC16 is supported). We could maybe key off
> scsi_level > SCSI_2 like scsi_device_protection() does. This would work
> for ATA SSDs because libata reports SCSI ANSI revision 05, but it won't
> work for USB devices because they get mangled down to SCSI_2, no matter
> what they support.
>
<snip>

This is certainly a bug in the standard, draft as you say. It must be fixed in
a backward compatible way. Practical matters aside, the standard can not stay
as it is.

Thanks
Boaz