linux-kernel - Re: [PATCH 13/19] btrfs: avoid sync IO prioritization on checksum in HMZONED mode

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Date:   Thu, 13 Jun 2019 10:17:07 -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 13/19] btrfs: avoid sync IO prioritization on checksum in
 HMZONED mode

On Fri, Jun 07, 2019 at 10:10:19PM +0900, Naohiro Aota wrote:
> Btrfs prioritize sync I/Os to be handled by async checksum worker earlier.
> As a result, checksumming sync I/Os to larger logical extent address can
> finish faster than checksumming non-sync I/Os to smaller logical extent
> address.
> 
> Since we have upper limit of number of checksum worker, it is possible that
> sync I/Os to wait forever for non-starting checksum of I/Os for smaller
> address.
> 
> This situation can be reproduced by e.g. fstests btrfs/073.
> 
> To avoid such disordering, disable sync IO prioritization for now. Note
> that sync I/Os anyway must wait for I/Os to smaller address to finish. So,
> actually prioritization have no benefit in HMZONED mode.
> 

This stuff is going away once we finish the io.weight work anyway, I wouldn't
worry about this.  Thanks,

Josef