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

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Message-ID: <20150622165717.GA9803@lst.de>
Date:	Mon, 22 Jun 2015 18:57:17 +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 09:54:51AM -0700, Dan Williams wrote:
> > I don't see why you're comparing with MD and DM here.  MD and DM
> > sit cleanly ontop of any block device.  If btt was independent of
> > libnvdimm and just used ->rw_bytes we could see it as this.
> >
> > But it's all a giant entangled mess, where btt for example is probed
> > by libnvdimm.  At the same time pmem.c isn't really a true block
> > driver, it's really just a trivial shim between the block API
> > and pmem-style memcpy.  Especially with the proper pmem API btt
> > would become cleaner just calling that directly.
> 
> The pmem api does nothing to fix torn sectors, there's no extra
> atomicity guarantees that come from those instructions.

Of course not.  And neither does pmem.c help with you in any way.

That's the point:  btt should be a peer to pmem.c, not on top of it
as there's no value add in pmem.c for it, and they are logically peers.

> Well, let's start with per-disk btt and see where that gets us, we can
> always ramp up complexity later.  I'd just as soon make the default
> opt-in/out a Kconfig toggle with a sysfs override.

Kconfig or sysfs are both utterly horrible choices.  It's a disk format
choice so it needs to be persisted.
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