Re: Exposing FUA as alternative to DIOCCACHESYNC for WAPBL

[Thor Lancelot Simon <tls%panix.com@localhost>]

On Fri, Mar 31, 2017 at 02:16:44PM +0200, Edgar Fu? wrote:
> Oh well.
> 
> TLS> If the answer is that you're running with WCE on in the mode pages, then
> TLS> don't do that:
> EF> I don't get that. If you turn off the write cache, you need neither cache 
> EF> flushes nor ordering, no?
> MB> You still need ordering. With tagged queuing, you have multiple commands
> MB> running at the same time (up to 256, maybe more fore newer scsi) and the 
> MB> drive is free to complete them in any order.  Unless one of them is an 
> MB> ORDERED command, in which case comamnds queued before have to complete 
> MB> before.
> 
> I guess we are talking past each other. I should have phrased that ``If you
> don't use any tagging and turn off the write cache, ...''.

But that doesn't make sense.  Why would our SCSI layer not use tagging?

The problem is that it does not always use SIMPLE and ORDERED tags in a
way that would facilitate the use of ORDERED tags to enforce barriers.

Also, that we may not know enough about the behavior of our filesystems
in the real world to be 100% sure it's safe to set the other mode page
bits that allow the drive to arbitrarily reorder SIMPLE commands (which
under some conditions is necessary to match the performance of running
with WCE set).

When SCSI tagged queueing is used properly, it is not necessary to set WCE
to get good write performance, and doing so is in fact harmful, since it
allows the drive to return ORDERED commands as complete before any of the
data for those or prior commands have actually been committed to stable
storage.

Thor