Re: Plan: journalling fixes for WAPBL

[<Paul.Koning%dell.com@localhost>]

> On Sep 23, 2016, at 10:51 AM, Warner Losh <imp%bsdimp.com@localhost> wrote:
> 
> 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