atomic write & T10 standards

[Ric Wheeler <rwheeler@redhat.com>]

On 07/03/2013 11:00 AM, James Bottomley wrote:
> On Wed, 2013-07-03 at 10:56 -0400, Ric Wheeler wrote:
>> On 07/03/2013 10:38 AM, Chris Mason wrote:
>>> Quoting Ric Wheeler (2013-07-03 10:34:04)
>>>> As I was out walking Skeeter this morning, I was thinking a bit about the new
>>>> T10 atomic write proposal that Chris spoke about some time back.
>>>>
>>>> Specifically, I think that we would see a value only if the atomic write was
>>>> also durable - if not, we need to always issue a SYNCHRONIZE_CACHE command which
>>>> would mean it really is not effectively more useful than a normal write?
>>>>
>>>> Did I understand the proposal correctly?  If I did, should we poke the usual T10
>>>> posse to nudge them (David Black, Fred Knight, etc?)...
>>> I don't think the atomic writes should be a special case here.  We've
>>> already got the cache flush and fua machinery and should just apply it
>>> on top of the atomic constructs...
>>>
>>> -chris
>>>
>> I should have sent this to the linux-scsi list I suppose, but wanted clarity
>> before embarrassing myself :)
> Yes, it is a better to have a wider audience

Adding in linux-scsi....

>
>> If we have to use fua/flush after an atomic write, what makes it atomic?  Why
>> not just use a normal write?
>>
>> It does not seem to add anything that write + flush/fua does?
> It adds the all or nothing that we can use to commit journal entries
> without having to worry about atomicity.  The guarantee is that
> everything makes it or nothing does.

I still don't see the difference in write + SYNC_CACHE versus atomic write + 
SYNC_CACHE.

If the write is atomic and not durable, it is not really usable as a hard 
promise until after we flush it somehow.
>
> In theory, if we got ordered tags working to ensure transaction vs data
> ordering, this would mean we wouldn't have to flush at all because the
> disk image would always be journal consistent ... a bit like the old
> soft update scheme.
>
> James
>

Why not have the atomic write actually imply that it is atomic and durable for 
just that command?

Ric