linux-kernel - Re: [PATCH 11/19] btrfs: introduce submit buffer

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Date:   Thu, 13 Jun 2019 10:14:58 -0400
From:   Josef Bacik <josef@...icpanda.com>
To:     Naohiro Aota <naohiro.aota@....com>
Cc:     linux-btrfs@...r.kernel.org, David Sterba <dsterba@...e.com>,
        Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        Qu Wenruo <wqu@...e.com>, Nikolay Borisov <nborisov@...e.com>,
        linux-kernel@...r.kernel.org, Hannes Reinecke <hare@...e.com>,
        linux-fsdevel@...r.kernel.org,
        Damien Le Moal <damien.lemoal@....com>,
        Matias Bjørling <mb@...htnvm.io>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Bart Van Assche <bvanassche@....org>
Subject: Re: [PATCH 11/19] btrfs: introduce submit buffer

On Fri, Jun 07, 2019 at 10:10:17PM +0900, Naohiro Aota wrote:
> Sequential allocation is not enough to maintain sequential delivery of
> write IOs to the device. Various features (async compress, async checksum,
> ...) of btrfs affect ordering of the IOs. This patch introduces submit
> buffer to sort WRITE bios belonging to a block group and sort them out
> sequentially in increasing block address to achieve sequential write
> sequences with __btrfs_map_bio().
> 
> Signed-off-by: Naohiro Aota <naohiro.aota@....com>

I hate everything about this.  Can't we just use the plugging infrastructure for
this and then make sure it re-orders the bios before submitting them?  Also
what's to prevent the block layer scheduler from re-arranging these io's?
Thanks,

Josef