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Date:   Thu, 23 Mar 2023 09:27:56 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Bart Van Assche <bvanassche@acm.org>
Cc:     Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        linux-block@vger.kernel.org, Jaegeuk Kim <jaegeuk@kernel.org>,
        Jan Kara <jack@suse.cz>
Subject: Re: [PATCH 0/2] Submit split bios in LBA order
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References: <20230317195938.1745318-1-bvanassche@acm.org> <20230318062909.GB24880@lst.de> <da0c7538-1a51-61dd-6359-8c618fde6c1b@acm.org>
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On Mon, Mar 20, 2023 at 10:22:41AM -0700, Bart Van Assche wrote:
> How to achieve optimal performance with REQ_OP_ZONE_APPEND for SCSI 
> devices? My understanding of how REQ_OP_ZONE_APPEND works for SCSI devices 
> is as follows:
> * ATA devices cannot support this operation directly because there are
>   not enough bits in the ATA sense data to report where appended data
>   has been written.

ATA doesn't really have autosense in the SCSI way.  It could be handled
the same way that CDL completions are handled.  That is a complete
mess, and between CDL and Zone Append we'll probably eventually need
an extended FIS for SATA if we want to keep ATA alive.

> * T10 has not yet started with standardizing a zone append operation.

Time to get it started then!

> * The code that emulates REQ_OP_ZONE_APPEND for SCSI devices (in
>   sd_zbc.c) serializes REQ_OP_ZONE_APPEND operations (QD=1).

Because that's the only thing that actually works.

> * To achieve optimal performance, QD > 1 is required.

If you have something magic that works, this code is the place to take
advantage of it.