On Sat, Apr 01, 2017 at 08:54:40AM +0000, Michael van Elst wrote: > t...@panix.com (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