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

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Message-ID: <20150622154056.GB7952@lst.de>
Date:	Mon, 22 Jun 2015 17:40:56 +0200
From:	Christoph Hellwig <hch@....de>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Christoph Hellwig <hch@....de>, 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 12:39:34AM -0700, Dan Williams wrote:
> Now I'm confused, you *don't* want the raw device to be hidden *and*
> you want to kill the stacking?  Something got crossed.  The current
> implementation hides nothing, you get to see the entire stacked
> composition.  I'd much prefer to avoid hiding anything.

You see it, but you can't actually use it.  The proper way to expose it
would be to the devices visible in the low-level bus sysfs enumeration
logic but only one ULD attach to it.

> The sinister aspect of sector tearing is that most applications don't
> know they have this dependency.  At least today's disk's rarely ever
> tear sectors and if they do you almost certainly get a CRC error on
> access.  NVDIMMs will always tear and always silently.  I think not
> merging BTT at all to see what happens is simply wrong.

So now you leave your users with a choice between a rock and a hard
place, that is using BTT to introduce non-significant overhead and not
supporting DAX or just use it as-is.
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