linux-ext4 - Re: [PATCH 0/5] Enable per-file/directory DAX operations

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Message-ID: <20191023221332.GE2044@dread.disaster.area>
Date:   Thu, 24 Oct 2019 09:13:32 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Boaz Harrosh <boaz@...xistor.com>
Cc:     ira.weiny@...el.com, linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 0/5] Enable per-file/directory DAX operations

On Wed, Oct 23, 2019 at 04:09:50PM +0300, Boaz Harrosh wrote:
> On 22/10/2019 14:21, Boaz Harrosh wrote:
> > On 20/10/2019 18:59, ira.weiny@...el.com wrote:
> Please explain the use case behind your model?

No application changes needed to control whether they use DAX or
not. It allows the admin to control the application behaviour
completely, so they can turn off DAX if necessary. Applications are
unaware of constraints that may prevent DAX from being used, and so
admins need a mechanism to prevent DAX aware application from
actually using DAX if the capability is present.

e.g. given how slow some PMEM devices are when it comes to writing
data, especially under extremely high concurrency, DAX is not
necessarily a performance win for every application. Admins need a
guaranteed method of turning off DAX in these situations - apps may
not provide such a knob, or even be aware of a thing called DAX...

e.g. the data set being accessed by the application is mapped and
modified by RDMA applications, so those files must not be accessed
using DAX by any application because DAX+RDMA are currently
incompatible. Hence you can have RDMA on pmem devices co-exist
within the same filesystem as other applications using DAX to access
the pmem...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com