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Date:   Sat, 15 Apr 2023 22:54:49 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     "Darrick J. Wong" <djwong@kernel.org>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Bernd Schubert <bschubert@ddn.com>, axboe@kernel.dk,
        io-uring@vger.kernel.org, linux-ext4@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org,
        dsingh@ddn.com
Subject: Re: [PATCH 1/2] fs: add FMODE_DIO_PARALLEL_WRITE flag
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On Fri, Apr 14, 2023 at 08:36:12AM -0700, Darrick J. Wong wrote:
> IIUC uring wants to avoid the situation where someone sends 300 writes
> to the same file, all of which end up in background workers, and all of
> which then contend on exclusive i_rwsem.  Hence it has some hashing
> scheme that executes io requests serially if they hash to the same value
> (which iirc is the inode number?) to prevent resource waste.
> 
> This flag turns off that hashing behavior on the assumption that each of
> those 300 writes won't serialize on the other 299 writes, hence it's ok
> to start up 300 workers.
> 
> (apologies for precoffee garbled response)

It might be useful if someone (Jens?) could clearly document the
assumptions for this flag.