[PATCH 2/2] nvme: add emulation for zone-append

[Javier Gonzalez]

On 19.08.2020 12:43, Christoph Hellwig wrote:
>On Wed, Aug 19, 2020 at 09:14:13AM +0000, Damien Le Moal wrote:
>> While defining a zone append command for SCSI/ZBC is possible (using sense data
>> for returning the written offset), there is no way to define zone append for
>> SATA/ZAC without entirely breaking the ATA command model. This is why we went
>> after an emulation implementation instead of trying to standardized native
>> commands. That implementation does not have any performance impact over regular
>> writes *and* zone write locking does not in general degrade HDD write
>> performance (only a few corner cases suffer from it). Comparing things equally,
>> the same could be said of NVMe drives that do not have zone append native
>> support: performance will be essentially the same using regular writes and
>> emulated zone append. But mq-deadline and zone write locking will significantly
>> lower performance for emulated zone append compared to a native zone append
>> support by the drive.
>
>And to summarize the most important point - Zone Append doesn't exist
>in ZAC/ABC.  For people that spent the last years trying to make zoned
>storage work, the lack of such a primite has been the major pain point.
>That's why I came up with the Zone Append design in response to a
>request for such an operation from another company that is now heavily
>involved in both Linux development and hosting Linux VMs.  For ZAC and
>ZBC the best we can do is to emulate the approach in the driver, but
>for NVMe we can do it.  ZNS until just before the release had Zone
>Append mandatory, and it did so for a very good reason.  While making
>it optional allows OEMs to request drives without it, I fundamentally
>think we should not support that in Linux and request vendors do
>implement writes to zones the right way.

Ok. We will just pursue Linux support for the ZNS following the append
model.

>
>And just as some OEMs can request certain TPs or optional features to
>be implemented, so can Linux.  Just to give an example from the zone
>world - Linux requires uniform and power of two zone sizes, which in
>ZAC and ZBC are not required.