Re: Exposing FUA as alternative to DIOCCACHESYNC for WAPBL

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

On Sat, Apr 01, 2017 at 08:54:40AM +0000, Michael van Elst wrote:
> tls%panix.com@localhost (Thor Lancelot Simon) writes:
> 
> >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.
> 
> Do you think that real world disks agree? WCE is often necessary to
> get any decent performance and yes, data is not committed to stable

I don't agree.  What's sometimes necessary is to adjust the other mode
page bits that allow the drive to arbitrarily reorder SIMPLE commands,
but with an I/O subsystem that can put enough data in flight at once,
there's no performance reason to use WCE and considerable reliability
reason not to.

That said, very high-latency transports like iSCSI require a lot more
data than we can put into flight at once.  We just don't have enough
parallelism in our I/O subsystem (and most applications can't supply
enough).

Thor