Re: Plan: journalling fixes for WAPBL

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

On Fri, Sep 23, 2016 at 7:38 AM, Thor Lancelot Simon <tls%panix.com@localhost> wrote:
> On Fri, Sep 23, 2016 at 11:47:24AM +0200, Manuel Bouyer wrote:
>> On Thu, Sep 22, 2016 at 09:33:18PM -0400, Thor Lancelot Simon wrote:
>> > > AFAIK ordered tags only guarantees that the write will happen in order,
>> > > but not that the writes are actually done to stable storage.
>> >
>> > The target's not allowed to report the command complete unless the data
>> > are on stable storage, except if you have write cache enable set in the
>> > relevant mode page.
>> >
>> > If you run SCSI drives like that, you're playing with fire.  Expect to get
>> > burned.  The whole point of tagged queueing is to let you *not* set that
>> > bit in the mode pages and still get good performance.
>>
>> Now I remember that I did indeed disable disk write cache when I had
>> scsi disks in production. It's been a while though.
>>
>> But anyway, from what I remember you still need the disk cache flush
>> operation for SATA, even with NCQ. It's not equivalent to the SCSI tags.

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.

> I think that's true only if you're running with write cache enabled; but
> the difference is that most ATA disks ship with it turned on by default.
>
> With an aggressive implementation of tag management on the host side,
> there should be no performance benefit from unconditionally enabling
> the write cache -- all the available cache should be used to stage
> writes for pending tags.  Sometimes it works.

You don't need to flush all the writes, but do need to take special care
if you need more coherent semantics, which often is a small minority
of the writes, so I would agree the affect can be mostly mitigated. Not
completely since any coherency point has to drain the queue completely.
The cache drain ops are non-NCQ, and to send non-NCQ requests
no NCQ requests can be pending. TRIM[*] commands are the same way.

Warner

[*] There is an NCQ version of TRIM, but it requires the AUX register
to be sent and very few sata hosts controllers support that (though
AHCI does, many of the LSI controllers don't in any performant way).