Re: T10 WCE interpretation in Linux & device level access

[Ric Wheeler <rwheeler@redhat.com>]

On 04/24/2013 06:09 PM, James Bottomley wrote:
> On Wed, 2013-04-24 at 23:54 +0200, Paolo Bonzini wrote:
>> Il 24/04/2013 23:02, James Bottomley ha scritto:
>>> That just leaves us with random standards behaviour.  Lets permit the
>>> deterministic thing instead for the distros.  It kills two birds with
>>> one stone because we can set WCE for the stupid UAS devices that clear
>>> it wrongly as well.
>>>
>>> For those who don't read code well, you add a temporary prefix to the
>>> cache set in
>>>
>>> echo xxx > /sys/class/scsi_disk/<disk>/cache_type
>>>
>>> and it will set the flags for the lifetime of the current kernel, but
>>> won't try to do a mode select to make them permanent.
>> Having the knob is useful indeed.  I don't like the "temporary" name
>> though, because "temporary write-through" doesn't sound like it can eat
>> data on a power loss.  What about "force" or "assume"?
> I'm fairly ambivalent, except not force.  The default behaviour is to do
> the mode select, so force seems to imply that as well, except it won't.
> I don't see a difference between assume and temporary.
>
>> Also, this would be in addition to my patch (when tested), right?
> Not really ... given T10s deprecation I don't think we want to touch
> anything to do with SYNC_NV because it just adds to the uncertainty
> about what will actually happen.  Giving the ability to control WCE (and
> RCD) fixes all the problems raised so far.
>
> James
>
>

Why are we turning off the RCD bit in this? Not sure it matters, but we only 
should care about WCE (and the dirty write cache data)?

Thanks!

Ric