Re: Plan: journalling fixes for WAPBL

[Warner Losh <imp%bsdimp.com@localhost>]

On Fri, Sep 23, 2016 at 11:54 AM, Warner Losh <imp%bsdimp.com@localhost> wrote:
> On Fri, Sep 23, 2016 at 11:20 AM, Thor Lancelot Simon <tls%panix.com@localhost> wrote:
>> On Fri, Sep 23, 2016 at 05:15:16PM +0000, Eric Haszlakiewicz wrote:
>>> On September 23, 2016 10:51:30 AM EDT, Warner Losh <imp%bsdimp.com@localhost> wrote:
>>> >All NCQ gives you is the ability to schedule multiple requests and
>>> >to get notification of their completion (perhaps out of order). There's
>>> >no coherency features are all in NCQ.
>>>
>>> This seems like the key thing needed to avoid FUA: to implement fsync() you just wait for notifications of completion to be received, and once you have those for all requests pending when fsync was called, or started as part of the fsync, then you're done.
>>
>> The other key point is that -- unless SATA NCQ is radically different from
>> SCSI tagged queuing in a particularly stupid way -- the rules require all
>> "simple" tags to be completed before any "ordered" tag is completed.  That is,
>> ordered tags are barriers against all simple tags.
>
> SATA NCQ doesn't have ordered tags. There's just 32 slots to send
> requests into. Don't allow the word 'tag' to confuse you into thinking
> it is anything at all like SCSI tags. You get ordering by not
> scheduling anything until after the queue has drained when you send
> your "ordered" command. It is that stupid.

And it can be even worse, since if the 'ordered' item must complete
after all before it, you have to drain the queue before you can even
send it to the drive. Depends on what the ordering guarantees you want
are...

Warner