Re: T10 WCE interpretation in Linux & device level access

[Paolo Bonzini <pbonzini@redhat.com>]

Il 24/04/2013 14:07, Hannes Reinecke ha scritto:
> On 04/24/2013 01:17 PM, Paolo Bonzini wrote:
>> Il 23/04/2013 22:07, James Bottomley ha scritto:
>>> On Tue, 2013-04-23 at 15:41 -0400, Ric Wheeler wrote:
>>>> For many years, we have used WCE as an indication that a device has a volatile 
>>>> write cache (not just a write cache) and used this as a trigger to send down 
>>>> SYNCHRONIZE_CACHE commands as needed.
>>>>
>>>> Some arrays with non-volatile cache seem to have WCE set and simply ignore the 
>>>> command.
>>>
>>> I bet they don't; they probably obey the spec.  There's a SYNC_NV bit
>>> which if unset (which it is in our implementation) means only sync your
>>> non-NV cache.  For a device with all NV, that equates to nop.
>>
>> Isn't it the other way round?
>>
>> SYNC_NV = 0 means "sync all your caches to the medium", and it's what we do.
>>
>> SYNC_NV = 1 means "sync volatile to non-volatile", and it's what Ric wants.
>>
>> So we should set SYNC_NV=1 if NV_SUP is set, perhaps only if the medium
>> is non-removable just to err on the safe side.
> 
> Or use 'WRITE_AND_VERIFY' here; that's guaranteed to hit the disk.
> Plus it even has a guarantee about data consistency on the disk,
> which the normal WRITE command has not.

The point is to _avoid_ hitting the disk. :)

Paolo