linux-kernel - Re: [PATCH 14/15] libnvdimm: support read-only btt backing devices

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Message-ID: <20150622072844.GA31263@lst.de>
Date:	Mon, 22 Jun 2015 09:28:44 +0200
From:	Christoph Hellwig <hch@....de>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Jens Axboe <axboe@...nel.dk>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Boaz Harrosh <boaz@...xistor.com>,
	"Kani, Toshimitsu" <toshi.kani@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux ACPI <linux-acpi@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 14/15] libnvdimm: support read-only btt backing devices

On Mon, Jun 22, 2015 at 12:17:29AM -0700, Dan Williams wrote:
> To be fair the namespace was initially envisioned to be btt enabled or
> not, and hide the raw media device.

What's the fascination with hiding one access mode just because
another one is available?

> There's no guarantee that these drivers are only ever paired with
> XFSv5.

There's not guarantee for anything.   Note that anything not following
my criteria earlier would need some form of atomic sector updates,
which is a lot more.  But then again for most of those setups you
wouldn't take advantage of pmem anyway.

Sounds like we simply shouldn't merge btt at all for now and wait for
a real use case, which would simplify the whole issue a lot.
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