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

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Message-ID: <20200820064525.lae7fnnaiz7rkqvd@MacBook-Pro.localdomain>
Date:   Thu, 20 Aug 2020 08:45:25 +0200
From:   Javier Gonzalez <javier@...igon.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Damien Le Moal <Damien.LeMoal@....com>,
        Kanchan Joshi <joshi.k@...sung.com>,
        "kbusch@...nel.org" <kbusch@...nel.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "sagi@...mberg.me" <sagi@...mberg.me>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Johannes Thumshirn <Johannes.Thumshirn@....com>,
        Nitesh Shetty <nj.shetty@...sung.com>,
        SelvaKumar S <selvakuma.s1@...sung.com>
Subject: Re: [PATCH 2/2] nvme: add emulation for zone-append

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.